{- % (c) The University of Glasgow 2006 (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 -} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE DataKinds #-} {-# OPTIONS_GHC -Wno-incomplete-uni-patterns #-} -- | Generating derived instance declarations -- -- This module is nominally ``subordinate'' to "GHC.Tc.Deriv", which is the -- ``official'' interface to deriving-related things. -- -- This is where we do all the grimy bindings' generation. module GHC.Tc.Deriv.Generate ( AuxBindSpec(..), gen_Eq_binds, gen_Ord_binds, gen_Enum_binds, gen_Bounded_binds, gen_Ix_binds, gen_Show_binds, gen_Read_binds, gen_Data_binds, gen_Lift_binds, gen_Newtype_binds, gen_Newtype_fam_insts, mkCoerceClassMethEqn, genAuxBinds, ordOpTbl, boxConTbl, litConTbl, mkRdrFunBind, mkRdrFunBindEC, mkRdrFunBindSE, error_Expr, getPossibleDataCons, DerivInstTys(..), buildDataConInstArgEnv, derivDataConInstArgTys, substDerivInstTys, zonkDerivInstTys ) where import GHC.Prelude import GHC.Tc.Utils.Monad import GHC.Hs import GHC.Types.Name.Reader import GHC.Types.Basic import GHC.Types.Fixity import GHC.Core.DataCon import GHC.Types.Name import GHC.Types.SourceText import GHC.Driver.Session import GHC.Tc.Instance.Family import GHC.Core.FamInstEnv import GHC.Builtin.Names import GHC.Builtin.Names.TH import GHC.Types.Id.Make ( coerceId ) import GHC.Builtin.PrimOps import GHC.Builtin.PrimOps.Ids (primOpId) import GHC.Types.SrcLoc import GHC.Core.TyCon import GHC.Tc.Utils.Env import GHC.Tc.Utils.TcType import GHC.Tc.Utils.Zonk import GHC.Tc.Validity ( checkValidCoAxBranch ) import GHC.Core.Coercion.Axiom ( coAxiomSingleBranch ) import GHC.Builtin.Types.Prim import GHC.Builtin.Types import GHC.Core.Type import GHC.Core.Class import GHC.Types.Unique.FM ( lookupUFM, listToUFM ) import GHC.Types.Var.Set import GHC.Types.Var.Env import GHC.Utils.Misc import GHC.Types.Var import GHC.Utils.Outputable import GHC.Utils.Panic import GHC.Utils.Panic.Plain import GHC.Utils.Lexeme import GHC.Data.FastString import GHC.Data.Pair import GHC.Data.Bag import Data.List ( find, partition, intersperse ) import GHC.Data.Maybe ( expectJust ) import GHC.Unit.Module -- | A declarative description of an auxiliary binding that should be -- generated. See @Note [Auxiliary binders]@ for a more detailed description -- of how these are used. data AuxBindSpec -- DerivTag2Con, and DerivMaxTag are used in derived Eq, Ord, -- Enum, and Ix instances. -- All these generate ZERO-BASED tag operations -- I.e first constructor has tag 0 -- | @$tag2con@: Given a tag, computes the corresponding data constructor = DerivTag2Con TyCon -- The type constructor of the data type to which the -- constructors belong RdrName -- The to-be-generated $tag2con binding's RdrName -- | @$maxtag@: The maximum possible tag value among a data type's -- constructors | DerivMaxTag TyCon -- The type constructor of the data type to which the -- constructors belong RdrName -- The to-be-generated $maxtag binding's RdrName -- DerivDataDataType and DerivDataConstr are only used in derived Data -- instances -- | @$t@: The @DataType@ representation for a @Data@ instance | DerivDataDataType TyCon -- The type constructor of the data type to be represented RdrName -- The to-be-generated $t binding's RdrName [RdrName] -- The RdrNames of the to-be-generated $c bindings for each -- data constructor. These are only used on the RHS of the -- to-be-generated $t binding. -- | @$c@: The @Constr@ representation for a @Data@ instance | DerivDataConstr DataCon -- The data constructor to be represented RdrName -- The to-be-generated $c binding's RdrName RdrName -- The RdrName of the to-be-generated $t binding for the parent -- data type. This is only used on the RHS of the -- to-be-generated $c binding. -- | Retrieve the 'RdrName' of the binding that the supplied 'AuxBindSpec' -- describes. auxBindSpecRdrName :: AuxBindSpec -> RdrName auxBindSpecRdrName (DerivTag2Con _ tag2con_RDR) = tag2con_RDR auxBindSpecRdrName (DerivMaxTag _ maxtag_RDR) = maxtag_RDR auxBindSpecRdrName (DerivDataDataType _ dataT_RDR _) = dataT_RDR auxBindSpecRdrName (DerivDataConstr _ dataC_RDR _) = dataC_RDR {- ************************************************************************ * * Eq instances * * ************************************************************************ Here are the heuristics for the code we generate for @Eq@. Let's assume we have a data type with some (possibly zero) nullary data constructors and some ordinary, non-nullary ones (the rest, also possibly zero of them). Here's an example, with both \tr{N}ullary and \tr{O}rdinary data cons. data Foo ... = N1 | N2 ... | Nn | O1 a b | O2 Int | O3 Double b b | ... * We first attempt to compare the constructor tags. If tags don't match - we immediately bail out. Otherwise, we then generate one branch per constructor comparing only the fields as we already know that the tags match. Note that it only makes sense to check the tag if there is more than one data constructor. * For the ordinary constructors (if any), we emit clauses to do The Usual Thing, e.g.,: (==) (O1 a1 b1) (O1 a2 b2) = a1 == a2 && b1 == b2 (==) (O2 a1) (O2 a2) = a1 == a2 (==) (O3 a1 b1 c1) (O3 a2 b2 c2) = a1 == a2 && b1 == b2 && c1 == c2 Note: if we're comparing unlifted things, e.g., if 'a1' and 'a2' are Float#s, then we have to generate case (a1 `eqFloat#` a2) of r -> r for that particular test. * For nullary constructors, we emit a catch-all clause that always returns True since we already know that the tags match. * So, given this data type: data T = A | B Int | C Char We roughly get: (==) a b = case dataToTag# a /= dataToTag# b of True -> False False -> case a of -- Here we already know that tags match B a1 -> case b of B b1 -> a1 == b1 -- Only one branch C a1 -> case b of C b1 -> a1 == b1 -- Only one branch _ -> True -- catch-all to match all nullary ctors An older approach preferred regular pattern matches in some cases but with dataToTag# forcing it's argument, and work on improving join points, this seems no longer necessary. * For the @(/=)@ method, we normally just use the default method. If the type is an enumeration type, we could/may/should? generate special code that calls @dataToTag#@, much like for @(==)@ shown above. We thought about doing this: If we're also deriving 'Ord' for this tycon, we generate: instance ... Eq (Foo ...) where (==) a b = case (compare a b) of { _LT -> False; _EQ -> True ; _GT -> False} (/=) a b = case (compare a b) of { _LT -> True ; _EQ -> False; _GT -> True } However, that requires that (Ord ) was put in the context for the instance decl, which it probably wasn't, so the decls produced don't get through the typechecker. -} gen_Eq_binds :: SrcSpan -> DerivInstTys -> TcM (LHsBinds GhcPs, Bag AuxBindSpec) gen_Eq_binds loc dit@(DerivInstTys{ dit_rep_tc = tycon , dit_rep_tc_args = tycon_args }) = do return (method_binds, emptyBag) where all_cons = getPossibleDataCons tycon tycon_args non_nullary_cons = filter (not . isNullarySrcDataCon) all_cons -- Generate tag check. See #17240 eq_expr_with_tag_check = nlHsCase (nlHsPar (untag_Expr [(a_RDR,ah_RDR), (b_RDR,bh_RDR)] (nlHsOpApp (nlHsVar ah_RDR) neInt_RDR (nlHsVar bh_RDR)))) [ mkHsCaseAlt (nlLitPat (HsIntPrim NoSourceText 1)) false_Expr , mkHsCaseAlt nlWildPat ( nlHsCase (nlHsVar a_RDR) -- Only one branch to match all nullary constructors -- as we already know the tags match but do not emit -- the branch if there are no nullary constructors (let non_nullary_pats = map pats_etc non_nullary_cons in if null non_nullary_cons then non_nullary_pats else non_nullary_pats ++ [mkHsCaseAlt nlWildPat true_Expr])) ] method_binds = unitBag eq_bind eq_bind = mkFunBindEC 2 loc eq_RDR (const true_Expr) binds where binds | null all_cons = [] -- Tag checking is redundant when there is only one data constructor | [data_con] <- all_cons , (as_needed, bs_needed, tys_needed) <- gen_con_fields_and_tys data_con , data_con_RDR <- getRdrName data_con , con1_pat <- nlParPat $ nlConVarPat data_con_RDR as_needed , con2_pat <- nlParPat $ nlConVarPat data_con_RDR bs_needed , eq_expr <- nested_eq_expr tys_needed as_needed bs_needed = [([con1_pat, con2_pat], eq_expr)] -- This is an enum (all constructors are nullary) - just do a simple tag check | all isNullarySrcDataCon all_cons = [([a_Pat, b_Pat], untag_Expr [(a_RDR,ah_RDR), (b_RDR,bh_RDR)] (genPrimOpApp (nlHsVar ah_RDR) eqInt_RDR (nlHsVar bh_RDR)))] | otherwise = [([a_Pat, b_Pat], eq_expr_with_tag_check)] ------------------------------------------------------------------ nested_eq_expr [] [] [] = true_Expr nested_eq_expr tys as bs = foldr1 and_Expr (zipWith3Equal "nested_eq" nested_eq tys as bs) -- Using 'foldr1' here ensures that the derived code is correctly -- associated. See #10859. where nested_eq ty a b = nlHsPar (eq_Expr ty (nlHsVar a) (nlHsVar b)) gen_con_fields_and_tys data_con | tys_needed <- derivDataConInstArgTys data_con dit , con_arity <- length tys_needed , as_needed <- take con_arity as_RDRs , bs_needed <- take con_arity bs_RDRs = (as_needed, bs_needed, tys_needed) pats_etc data_con | (as_needed, bs_needed, tys_needed) <- gen_con_fields_and_tys data_con , data_con_RDR <- getRdrName data_con , con1_pat <- nlParPat $ nlConVarPat data_con_RDR as_needed , con2_pat <- nlParPat $ nlConVarPat data_con_RDR bs_needed , fields_eq_expr <- nested_eq_expr tys_needed as_needed bs_needed = mkHsCaseAlt con1_pat (nlHsCase (nlHsVar b_RDR) [mkHsCaseAlt con2_pat fields_eq_expr]) {- ************************************************************************ * * Ord instances * * ************************************************************************ Note [Generating Ord instances] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Suppose constructors are K1..Kn, and some are nullary. The general form we generate is: * Do case on first argument case a of K1 ... -> rhs_1 K2 ... -> rhs_2 ... Kn ... -> rhs_n _ -> nullary_rhs * To make rhs_i If i = 1, 2, n-1, n, generate a single case. rhs_2 case b of K1 {} -> LT K2 ... -> ...eq_rhs(K2)... _ -> GT Otherwise do a tag compare against the bigger range (because this is the one most likely to succeed) rhs_3 case tag b of tb -> if 3 <# tg then GT else case b of K3 ... -> ...eq_rhs(K3).... _ -> LT * To make eq_rhs(K), which knows that a = K a1 .. av b = K b1 .. bv we just want to compare (a1,b1) then (a2,b2) etc. Take care on the last field to tail-call into comparing av,bv * To make nullary_rhs generate this case dataToTag# a of a# -> case dataToTag# b of -> a# `compare` b# Several special cases: * Two or fewer nullary constructors: don't generate nullary_rhs * Be careful about unlifted comparisons. When comparing unboxed values we can't call the overloaded functions. See function unliftedOrdOp Note [Game plan for deriving Ord] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ It's a bad idea to define only 'compare', and build the other binary comparisons on top of it; see #2130, #4019. Reason: we don't want to laboriously make a three-way comparison, only to extract a binary result, something like this: (>) (I# x) (I# y) = case <# x y of True -> False False -> case ==# x y of True -> False False -> True This being said, we can get away with generating full code only for 'compare' and '<' thus saving us generation of other three operators. Other operators can be cheaply expressed through '<': a <= b = not $ b < a a > b = b < a a >= b = not $ a < b So for sufficiently small types (few constructors, or all nullary) we generate all methods; for large ones we just use 'compare'. -} data OrdOp = OrdCompare | OrdLT | OrdLE | OrdGE | OrdGT ------------ ordMethRdr :: OrdOp -> RdrName ordMethRdr op = case op of OrdCompare -> compare_RDR OrdLT -> lt_RDR OrdLE -> le_RDR OrdGE -> ge_RDR OrdGT -> gt_RDR ------------ ltResult :: OrdOp -> LHsExpr GhcPs -- Knowing a LHsExpr GhcPs -- Knowing a=b, what is the result for a `op` b? eqResult OrdCompare = eqTag_Expr eqResult OrdLT = false_Expr eqResult OrdLE = true_Expr eqResult OrdGE = true_Expr eqResult OrdGT = false_Expr ------------ gtResult :: OrdOp -> LHsExpr GhcPs -- Knowing a>b, what is the result for a `op` b? gtResult OrdCompare = gtTag_Expr gtResult OrdLT = false_Expr gtResult OrdLE = false_Expr gtResult OrdGE = true_Expr gtResult OrdGT = true_Expr ------------ gen_Ord_binds :: SrcSpan -> DerivInstTys -> TcM (LHsBinds GhcPs, Bag AuxBindSpec) gen_Ord_binds loc dit@(DerivInstTys{ dit_rep_tc = tycon , dit_rep_tc_args = tycon_args }) = do return $ if null tycon_data_cons -- No data-cons => invoke bale-out case then ( unitBag $ mkFunBindEC 2 loc compare_RDR (const eqTag_Expr) [] , emptyBag) else ( unitBag (mkOrdOp OrdCompare) `unionBags` other_ops , aux_binds) where aux_binds = emptyBag -- Note [Game plan for deriving Ord] other_ops | (last_tag - first_tag) <= 2 -- 1-3 constructors || null non_nullary_cons -- Or it's an enumeration = listToBag [mkOrdOp OrdLT, lE, gT, gE] | otherwise = emptyBag negate_expr = nlHsApp (nlHsVar not_RDR) lE = mkSimpleGeneratedFunBind loc le_RDR [a_Pat, b_Pat] $ negate_expr (nlHsApp (nlHsApp (nlHsVar lt_RDR) b_Expr) a_Expr) gT = mkSimpleGeneratedFunBind loc gt_RDR [a_Pat, b_Pat] $ nlHsApp (nlHsApp (nlHsVar lt_RDR) b_Expr) a_Expr gE = mkSimpleGeneratedFunBind loc ge_RDR [a_Pat, b_Pat] $ negate_expr (nlHsApp (nlHsApp (nlHsVar lt_RDR) a_Expr) b_Expr) get_tag con = dataConTag con - fIRST_TAG -- We want *zero-based* tags, because that's what -- con2Tag returns (generated by untag_Expr)! tycon_data_cons = getPossibleDataCons tycon tycon_args single_con_type = isSingleton tycon_data_cons (first_con : _) = tycon_data_cons (last_con : _) = reverse tycon_data_cons first_tag = get_tag first_con last_tag = get_tag last_con (nullary_cons, non_nullary_cons) = partition isNullarySrcDataCon tycon_data_cons mkOrdOp :: OrdOp -> LHsBind GhcPs -- Returns a binding op a b = ... compares a and b according to op .... mkOrdOp op = mkSimpleGeneratedFunBind loc (ordMethRdr op) [a_Pat, b_Pat] (mkOrdOpRhs op) mkOrdOpRhs :: OrdOp -> LHsExpr GhcPs mkOrdOpRhs op -- RHS for comparing 'a' and 'b' according to op | nullary_cons `lengthAtMost` 2 -- Two nullary or fewer, so use cases = nlHsCase (nlHsVar a_RDR) $ map (mkOrdOpAlt op) tycon_data_cons -- i.e. case a of { C1 x y -> case b of C1 x y -> ....compare x,y... -- C2 x -> case b of C2 x -> ....comopare x.... } | null non_nullary_cons -- All nullary, so go straight to comparing tags = mkTagCmp op | otherwise -- Mixed nullary and non-nullary = nlHsCase (nlHsVar a_RDR) $ (map (mkOrdOpAlt op) non_nullary_cons ++ [mkHsCaseAlt nlWildPat (mkTagCmp op)]) mkOrdOpAlt :: OrdOp -> DataCon -> LMatch GhcPs (LHsExpr GhcPs) -- Make the alternative (Ki a1 a2 .. av -> mkOrdOpAlt op data_con = mkHsCaseAlt (nlConVarPat data_con_RDR as_needed) (mkInnerRhs op data_con) where as_needed = take (dataConSourceArity data_con) as_RDRs data_con_RDR = getRdrName data_con mkInnerRhs op data_con | single_con_type = nlHsCase (nlHsVar b_RDR) [ mkInnerEqAlt op data_con ] | tag == first_tag = nlHsCase (nlHsVar b_RDR) [ mkInnerEqAlt op data_con , mkHsCaseAlt nlWildPat (ltResult op) ] | tag == last_tag = nlHsCase (nlHsVar b_RDR) [ mkInnerEqAlt op data_con , mkHsCaseAlt nlWildPat (gtResult op) ] | tag == first_tag + 1 = nlHsCase (nlHsVar b_RDR) [ mkHsCaseAlt (nlConWildPat first_con) (gtResult op) , mkInnerEqAlt op data_con , mkHsCaseAlt nlWildPat (ltResult op) ] | tag == last_tag - 1 = nlHsCase (nlHsVar b_RDR) [ mkHsCaseAlt (nlConWildPat last_con) (ltResult op) , mkInnerEqAlt op data_con , mkHsCaseAlt nlWildPat (gtResult op) ] | tag > last_tag `div` 2 -- lower range is larger = untag_Expr [(b_RDR, bh_RDR)] $ nlHsIf (genPrimOpApp (nlHsVar bh_RDR) ltInt_RDR tag_lit) (gtResult op) $ -- Definitely GT nlHsCase (nlHsVar b_RDR) [ mkInnerEqAlt op data_con , mkHsCaseAlt nlWildPat (ltResult op) ] | otherwise -- upper range is larger = untag_Expr [(b_RDR, bh_RDR)] $ nlHsIf (genPrimOpApp (nlHsVar bh_RDR) gtInt_RDR tag_lit) (ltResult op) $ -- Definitely LT nlHsCase (nlHsVar b_RDR) [ mkInnerEqAlt op data_con , mkHsCaseAlt nlWildPat (gtResult op) ] where tag = get_tag data_con tag_lit = noLocA (HsLit noComments (HsIntPrim NoSourceText (toInteger tag))) mkInnerEqAlt :: OrdOp -> DataCon -> LMatch GhcPs (LHsExpr GhcPs) -- First argument 'a' known to be built with K -- Returns a case alternative Ki b1 b2 ... bv -> compare (a1,a2,...) with (b1,b2,...) mkInnerEqAlt op data_con = mkHsCaseAlt (nlConVarPat data_con_RDR bs_needed) $ mkCompareFields op (derivDataConInstArgTys data_con dit) where data_con_RDR = getRdrName data_con bs_needed = take (dataConSourceArity data_con) bs_RDRs mkTagCmp :: OrdOp -> LHsExpr GhcPs -- Both constructors known to be nullary -- generates (case data2Tag a of a# -> case data2Tag b of b# -> a# `op` b# mkTagCmp op = untag_Expr [(a_RDR, ah_RDR),(b_RDR, bh_RDR)] $ unliftedOrdOp intPrimTy op ah_RDR bh_RDR mkCompareFields :: OrdOp -> [Type] -> LHsExpr GhcPs -- Generates nested comparisons for (a1,a2...) against (b1,b2,...) -- where the ai,bi have the given types mkCompareFields op tys = go tys as_RDRs bs_RDRs where go [] _ _ = eqResult op go [ty] (a:_) (b:_) | isUnliftedType ty = unliftedOrdOp ty op a b | otherwise = genOpApp (nlHsVar a) (ordMethRdr op) (nlHsVar b) go (ty:tys) (a:as) (b:bs) = mk_compare ty a b (ltResult op) (go tys as bs) (gtResult op) go _ _ _ = panic "mkCompareFields" -- (mk_compare ty a b) generates -- (case (compare a b) of { LT -> ; EQ -> ; GT -> }) -- but with suitable special cases for mk_compare ty a b lt eq gt | isUnliftedType ty = unliftedCompare lt_op eq_op a_expr b_expr lt eq gt | otherwise = nlHsCase (nlHsPar (nlHsApp (nlHsApp (nlHsVar compare_RDR) a_expr) b_expr)) [mkHsCaseAlt (nlNullaryConPat ltTag_RDR) lt, mkHsCaseAlt (nlNullaryConPat eqTag_RDR) eq, mkHsCaseAlt (nlNullaryConPat gtTag_RDR) gt] where a_expr = nlHsVar a b_expr = nlHsVar b (lt_op, _, eq_op, _, _) = primOrdOps "Ord" ty unliftedOrdOp :: Type -> OrdOp -> RdrName -> RdrName -> LHsExpr GhcPs unliftedOrdOp ty op a b = case op of OrdCompare -> unliftedCompare lt_op eq_op a_expr b_expr ltTag_Expr eqTag_Expr gtTag_Expr OrdLT -> wrap lt_op OrdLE -> wrap le_op OrdGE -> wrap ge_op OrdGT -> wrap gt_op where (lt_op, le_op, eq_op, ge_op, gt_op) = primOrdOps "Ord" ty wrap prim_op = genPrimOpApp a_expr prim_op b_expr a_expr = nlHsVar a b_expr = nlHsVar b unliftedCompare :: RdrName -> RdrName -> LHsExpr GhcPs -> LHsExpr GhcPs -- What to compare -> LHsExpr GhcPs -> LHsExpr GhcPs -> LHsExpr GhcPs -- Three results -> LHsExpr GhcPs -- Return (if a < b then lt else if a == b then eq else gt) unliftedCompare lt_op eq_op a_expr b_expr lt eq gt = nlHsIf (ascribeBool $ genPrimOpApp a_expr lt_op b_expr) lt $ -- Test (<) first, not (==), because the latter -- is true less often, so putting it first would -- mean more tests (dynamically) nlHsIf (ascribeBool $ genPrimOpApp a_expr eq_op b_expr) eq gt where ascribeBool e = noLocA $ ExprWithTySig noAnn e $ mkHsWildCardBndrs $ noLocA $ mkHsImplicitSigType $ nlHsTyVar NotPromoted boolTyCon_RDR nlConWildPat :: DataCon -> LPat GhcPs -- The pattern (K {}) nlConWildPat con = noLocA $ ConPat { pat_con_ext = noAnn , pat_con = noLocA $ getRdrName con , pat_args = RecCon $ HsRecFields { rec_flds = [] , rec_dotdot = Nothing } } {- ************************************************************************ * * Enum instances * * ************************************************************************ @Enum@ can only be derived for enumeration types. For a type \begin{verbatim} data Foo ... = N1 | N2 | ... | Nn \end{verbatim} we use both dataToTag# and @tag2con_Foo@ functions, as well as a @maxtag_Foo@ variable, the later generated by @gen_tag_n_con_binds. \begin{verbatim} instance ... Enum (Foo ...) where succ x = toEnum (1 + fromEnum x) pred x = toEnum (fromEnum x - 1) toEnum i = tag2con_Foo i enumFrom a = map tag2con_Foo [dataToTag# a .. maxtag_Foo] -- or, really... enumFrom a = case dataToTag# a of a# -> map tag2con_Foo (enumFromTo (I# a#) maxtag_Foo) enumFromThen a b = map tag2con_Foo [dataToTag# a, dataToTag# b .. maxtag_Foo] -- or, really... enumFromThen a b = case dataToTag# a of { a# -> case dataToTag# b of { b# -> map tag2con_Foo (enumFromThenTo (I# a#) (I# b#) maxtag_Foo) }} \end{verbatim} For @enumFromTo@ and @enumFromThenTo@, we use the default methods. -} gen_Enum_binds :: SrcSpan -> DerivInstTys -> TcM (LHsBinds GhcPs, Bag AuxBindSpec) gen_Enum_binds loc (DerivInstTys{dit_rep_tc = tycon}) = do -- See Note [Auxiliary binders] tag2con_RDR <- new_tag2con_rdr_name loc tycon maxtag_RDR <- new_maxtag_rdr_name loc tycon return ( method_binds tag2con_RDR maxtag_RDR , aux_binds tag2con_RDR maxtag_RDR ) where method_binds tag2con_RDR maxtag_RDR = listToBag [ succ_enum tag2con_RDR maxtag_RDR , pred_enum tag2con_RDR , to_enum tag2con_RDR maxtag_RDR , enum_from tag2con_RDR maxtag_RDR -- [0 ..] , enum_from_then tag2con_RDR maxtag_RDR -- [0, 1 ..] , from_enum ] aux_binds tag2con_RDR maxtag_RDR = listToBag [ DerivTag2Con tycon tag2con_RDR , DerivMaxTag tycon maxtag_RDR ] occ_nm = getOccString tycon succ_enum tag2con_RDR maxtag_RDR = mkSimpleGeneratedFunBind loc succ_RDR [a_Pat] $ untag_Expr [(a_RDR, ah_RDR)] $ nlHsIf (nlHsApps eq_RDR [nlHsVar maxtag_RDR, nlHsVarApps intDataCon_RDR [ah_RDR]]) (illegal_Expr "succ" occ_nm "tried to take `succ' of last tag in enumeration") (nlHsApp (nlHsVar tag2con_RDR) (nlHsApps plus_RDR [nlHsVarApps intDataCon_RDR [ah_RDR], nlHsIntLit 1])) pred_enum tag2con_RDR = mkSimpleGeneratedFunBind loc pred_RDR [a_Pat] $ untag_Expr [(a_RDR, ah_RDR)] $ nlHsIf (nlHsApps eq_RDR [nlHsIntLit 0, nlHsVarApps intDataCon_RDR [ah_RDR]]) (illegal_Expr "pred" occ_nm "tried to take `pred' of first tag in enumeration") (nlHsApp (nlHsVar tag2con_RDR) (nlHsApps plus_RDR [ nlHsVarApps intDataCon_RDR [ah_RDR] , nlHsLit (HsInt noExtField (mkIntegralLit (-1 :: Int)))])) to_enum tag2con_RDR maxtag_RDR = mkSimpleGeneratedFunBind loc toEnum_RDR [a_Pat] $ nlHsIf (nlHsApps and_RDR [nlHsApps ge_RDR [nlHsVar a_RDR, nlHsIntLit 0], nlHsApps le_RDR [ nlHsVar a_RDR , nlHsVar maxtag_RDR]]) (nlHsVarApps tag2con_RDR [a_RDR]) (illegal_toEnum_tag occ_nm maxtag_RDR) enum_from tag2con_RDR maxtag_RDR = mkSimpleGeneratedFunBind loc enumFrom_RDR [a_Pat] $ untag_Expr [(a_RDR, ah_RDR)] $ nlHsApps map_RDR [nlHsVar tag2con_RDR, nlHsPar (enum_from_to_Expr (nlHsVarApps intDataCon_RDR [ah_RDR]) (nlHsVar maxtag_RDR))] enum_from_then tag2con_RDR maxtag_RDR = mkSimpleGeneratedFunBind loc enumFromThen_RDR [a_Pat, b_Pat] $ untag_Expr [(a_RDR, ah_RDR), (b_RDR, bh_RDR)] $ nlHsApp (nlHsVarApps map_RDR [tag2con_RDR]) $ nlHsPar (enum_from_then_to_Expr (nlHsVarApps intDataCon_RDR [ah_RDR]) (nlHsVarApps intDataCon_RDR [bh_RDR]) (nlHsIf (nlHsApps gt_RDR [nlHsVarApps intDataCon_RDR [ah_RDR], nlHsVarApps intDataCon_RDR [bh_RDR]]) (nlHsIntLit 0) (nlHsVar maxtag_RDR) )) from_enum = mkSimpleGeneratedFunBind loc fromEnum_RDR [a_Pat] $ untag_Expr [(a_RDR, ah_RDR)] $ (nlHsVarApps intDataCon_RDR [ah_RDR]) {- ************************************************************************ * * Bounded instances * * ************************************************************************ -} gen_Bounded_binds :: SrcSpan -> DerivInstTys -> (LHsBinds GhcPs, Bag AuxBindSpec) gen_Bounded_binds loc (DerivInstTys{dit_rep_tc = tycon}) | isEnumerationTyCon tycon = (listToBag [ min_bound_enum, max_bound_enum ], emptyBag) | otherwise = assert (isSingleton data_cons) (listToBag [ min_bound_1con, max_bound_1con ], emptyBag) where data_cons = tyConDataCons tycon ----- enum-flavored: --------------------------- min_bound_enum = mkHsVarBind loc minBound_RDR (nlHsVar data_con_1_RDR) max_bound_enum = mkHsVarBind loc maxBound_RDR (nlHsVar data_con_N_RDR) data_con_1 = head data_cons data_con_N = last data_cons data_con_1_RDR = getRdrName data_con_1 data_con_N_RDR = getRdrName data_con_N ----- single-constructor-flavored: ------------- arity = dataConSourceArity data_con_1 min_bound_1con = mkHsVarBind loc minBound_RDR $ nlHsVarApps data_con_1_RDR (replicate arity minBound_RDR) max_bound_1con = mkHsVarBind loc maxBound_RDR $ nlHsVarApps data_con_1_RDR (replicate arity maxBound_RDR) {- ************************************************************************ * * Ix instances * * ************************************************************************ Deriving @Ix@ is only possible for enumeration types and single-constructor types. We deal with them in turn. For an enumeration type, e.g., \begin{verbatim} data Foo ... = N1 | N2 | ... | Nn \end{verbatim} things go not too differently from @Enum@: \begin{verbatim} instance ... Ix (Foo ...) where range (a, b) = map tag2con_Foo [dataToTag# a .. dataToTag# b] -- or, really... range (a, b) = case (dataToTag# a) of { a# -> case (dataToTag# b) of { b# -> map tag2con_Foo (enumFromTo (I# a#) (I# b#)) }} -- Generate code for unsafeIndex, because using index leads -- to lots of redundant range tests unsafeIndex c@(a, b) d = case (dataToTag# d -# dataToTag# a) of r# -> I# r# inRange (a, b) c = let p_tag = dataToTag# c in p_tag >= dataToTag# a && p_tag <= dataToTag# b -- or, really... inRange (a, b) c = case (dataToTag# a) of { a_tag -> case (dataToTag# b) of { b_tag -> case (dataToTag# c) of { c_tag -> if (c_tag >=# a_tag) then c_tag <=# b_tag else False }}} \end{verbatim} (modulo suitable case-ification to handle the unlifted tags) For a single-constructor type (NB: this includes all tuples), e.g., \begin{verbatim} data Foo ... = MkFoo a b Int Double c c \end{verbatim} we follow the scheme given in Figure~19 of the Haskell~1.2 report (p.~147). -} gen_Ix_binds :: SrcSpan -> DerivInstTys -> TcM (LHsBinds GhcPs, Bag AuxBindSpec) gen_Ix_binds loc (DerivInstTys{dit_rep_tc = tycon}) = do -- See Note [Auxiliary binders] tag2con_RDR <- new_tag2con_rdr_name loc tycon return $ if isEnumerationTyCon tycon then (enum_ixes tag2con_RDR, listToBag [ DerivTag2Con tycon tag2con_RDR ]) else (single_con_ixes, emptyBag) where -------------------------------------------------------------- enum_ixes tag2con_RDR = listToBag [ enum_range tag2con_RDR , enum_index , enum_inRange ] enum_range tag2con_RDR = mkSimpleGeneratedFunBind loc range_RDR [nlTuplePat [a_Pat, b_Pat] Boxed] $ untag_Expr [(a_RDR, ah_RDR)] $ untag_Expr [(b_RDR, bh_RDR)] $ nlHsApp (nlHsVarApps map_RDR [tag2con_RDR]) $ nlHsPar (enum_from_to_Expr (nlHsVarApps intDataCon_RDR [ah_RDR]) (nlHsVarApps intDataCon_RDR [bh_RDR])) enum_index = mkSimpleGeneratedFunBind loc unsafeIndex_RDR [noLocA (AsPat noAnn (noLocA c_RDR) (nlTuplePat [a_Pat, nlWildPat] Boxed)), d_Pat] ( untag_Expr [(a_RDR, ah_RDR)] ( untag_Expr [(d_RDR, dh_RDR)] ( let rhs = nlHsVarApps intDataCon_RDR [c_RDR] in nlHsCase (genOpApp (nlHsVar dh_RDR) minusInt_RDR (nlHsVar ah_RDR)) [mkHsCaseAlt (nlVarPat c_RDR) rhs] )) ) -- This produces something like `(ch >= ah) && (ch <= bh)` enum_inRange = mkSimpleGeneratedFunBind loc inRange_RDR [nlTuplePat [a_Pat, b_Pat] Boxed, c_Pat] $ untag_Expr [(a_RDR, ah_RDR)] ( untag_Expr [(b_RDR, bh_RDR)] ( untag_Expr [(c_RDR, ch_RDR)] ( -- This used to use `if`, which interacts badly with RebindableSyntax. -- See #11396. nlHsApps and_RDR [ genPrimOpApp (nlHsVar ch_RDR) geInt_RDR (nlHsVar ah_RDR) , genPrimOpApp (nlHsVar ch_RDR) leInt_RDR (nlHsVar bh_RDR) ] ))) -------------------------------------------------------------- single_con_ixes = listToBag [single_con_range, single_con_index, single_con_inRange] data_con = case tyConSingleDataCon_maybe tycon of -- just checking... Nothing -> panic "get_Ix_binds" Just dc -> dc con_arity = dataConSourceArity data_con data_con_RDR = getRdrName data_con as_needed = take con_arity as_RDRs bs_needed = take con_arity bs_RDRs cs_needed = take con_arity cs_RDRs con_pat xs = nlConVarPat data_con_RDR xs con_expr = nlHsVarApps data_con_RDR cs_needed -------------------------------------------------------------- single_con_range = mkSimpleGeneratedFunBind loc range_RDR [nlTuplePat [con_pat as_needed, con_pat bs_needed] Boxed] $ noLocA (mkHsComp ListComp stmts con_expr) where stmts = zipWith3Equal "single_con_range" mk_qual as_needed bs_needed cs_needed mk_qual a b c = noLocA $ mkPsBindStmt noAnn (nlVarPat c) (nlHsApp (nlHsVar range_RDR) (mkLHsVarTuple [a,b] noAnn)) ---------------- single_con_index = mkSimpleGeneratedFunBind loc unsafeIndex_RDR [nlTuplePat [con_pat as_needed, con_pat bs_needed] Boxed, con_pat cs_needed] -- We need to reverse the order we consider the components in -- so that -- range (l,u) !! index (l,u) i == i -- when i is in range -- (from http://haskell.org/onlinereport/ix.html) holds. (mk_index (reverse $ zip3 as_needed bs_needed cs_needed)) where -- index (l1,u1) i1 + rangeSize (l1,u1) * (index (l2,u2) i2 + ...) mk_index [] = nlHsIntLit 0 mk_index [(l,u,i)] = mk_one l u i mk_index ((l,u,i) : rest) = genOpApp ( mk_one l u i ) plus_RDR ( genOpApp ( (nlHsApp (nlHsVar unsafeRangeSize_RDR) (mkLHsVarTuple [l,u] noAnn)) ) times_RDR (mk_index rest) ) mk_one l u i = nlHsApps unsafeIndex_RDR [mkLHsVarTuple [l,u] noAnn, nlHsVar i] ------------------ single_con_inRange = mkSimpleGeneratedFunBind loc inRange_RDR [nlTuplePat [con_pat as_needed, con_pat bs_needed] Boxed, con_pat cs_needed] $ if con_arity == 0 -- If the product type has no fields, inRange is trivially true -- (see #12853). then true_Expr else foldl1 and_Expr (zipWith3Equal "single_con_inRange" in_range as_needed bs_needed cs_needed) where in_range a b c = nlHsApps inRange_RDR [mkLHsVarTuple [a,b] noAnn, nlHsVar c] {- ************************************************************************ * * Read instances * * ************************************************************************ Example infix 4 %% data T = Int %% Int | T1 { f1 :: Int } | T2 T instance Read T where readPrec = parens ( prec 4 ( do x <- ReadP.step Read.readPrec expectP (Symbol "%%") y <- ReadP.step Read.readPrec return (x %% y)) +++ prec (appPrec+1) ( -- Note the "+1" part; "T2 T1 {f1=3}" should parse ok -- Record construction binds even more tightly than application do expectP (Ident "T1") expectP (Punc '{') x <- Read.readField "f1" (ReadP.reset readPrec) expectP (Punc '}') return (T1 { f1 = x })) +++ prec appPrec ( do expectP (Ident "T2") x <- ReadP.step Read.readPrec return (T2 x)) ) readListPrec = readListPrecDefault readList = readListDefault Note [Use expectP] ~~~~~~~~~~~~~~~~~~ Note that we use expectP (Ident "T1") rather than Ident "T1" <- lexP The latter desugares to inline code for matching the Ident and the string, and this can be very voluminous. The former is much more compact. Cf #7258, although that also concerned non-linearity in the occurrence analyser, a separate issue. Note [Read for empty data types] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ What should we get for this? (#7931) data Emp deriving( Read ) -- No data constructors Here we want read "[]" :: [Emp] to succeed, returning [] So we do NOT want instance Read Emp where readPrec = error "urk" Rather we want instance Read Emp where readPred = pfail -- Same as choose [] Because 'pfail' allows the parser to backtrack, but 'error' doesn't. These instances are also useful for Read (Either Int Emp), where we want to be able to parse (Left 3) just fine. -} gen_Read_binds :: (Name -> Fixity) -> SrcSpan -> DerivInstTys -> (LHsBinds GhcPs, Bag AuxBindSpec) gen_Read_binds get_fixity loc dit@(DerivInstTys{dit_rep_tc = tycon}) = (listToBag [read_prec, default_readlist, default_readlistprec], emptyBag) where ----------------------------------------------------------------------- default_readlist = mkHsVarBind loc readList_RDR (nlHsVar readListDefault_RDR) default_readlistprec = mkHsVarBind loc readListPrec_RDR (nlHsVar readListPrecDefault_RDR) ----------------------------------------------------------------------- data_cons = tyConDataCons tycon (nullary_cons, non_nullary_cons) = partition isNullarySrcDataCon data_cons read_prec = mkHsVarBind loc readPrec_RDR rhs where rhs | null data_cons -- See Note [Read for empty data types] = nlHsVar pfail_RDR | otherwise = nlHsApp (nlHsVar parens_RDR) (foldr1 mk_alt (read_nullary_cons ++ read_non_nullary_cons)) read_non_nullary_cons = map read_non_nullary_con non_nullary_cons read_nullary_cons = case nullary_cons of [] -> [] [con] -> [nlHsDo (DoExpr Nothing) (match_con con ++ [noLocA $ mkLastStmt (result_expr con [])])] _ -> [nlHsApp (nlHsVar choose_RDR) (nlList (map mk_pair nullary_cons))] -- NB For operators the parens around (:=:) are matched by the -- enclosing "parens" call, so here we must match the naked -- data_con_str con match_con con | isSym con_str = [symbol_pat con_str] | otherwise = ident_h_pat con_str where con_str = data_con_str con -- For nullary constructors we must match Ident s for normal constrs -- and Symbol s for operators mk_pair con = mkLHsTupleExpr [nlHsLit (mkHsString (data_con_str con)), result_expr con []] noAnn read_non_nullary_con data_con | is_infix = mk_parser infix_prec infix_stmts body | is_record = mk_parser record_prec record_stmts body -- Using these two lines instead allows the derived -- read for infix and record bindings to read the prefix form -- | is_infix = mk_alt prefix_parser (mk_parser infix_prec infix_stmts body) -- | is_record = mk_alt prefix_parser (mk_parser record_prec record_stmts body) | otherwise = prefix_parser where body = result_expr data_con as_needed con_str = data_con_str data_con prefix_parser = mk_parser prefix_prec prefix_stmts body read_prefix_con | isSym con_str = [read_punc "(", symbol_pat con_str, read_punc ")"] | otherwise = ident_h_pat con_str read_infix_con | isSym con_str = [symbol_pat con_str] | otherwise = [read_punc "`"] ++ ident_h_pat con_str ++ [read_punc "`"] prefix_stmts -- T a b c = read_prefix_con ++ read_args infix_stmts -- a %% b, or a `T` b = [read_a1] ++ read_infix_con ++ [read_a2] record_stmts -- T { f1 = a, f2 = b } = read_prefix_con ++ [read_punc "{"] ++ concat (intersperse [read_punc ","] field_stmts) ++ [read_punc "}"] field_stmts = zipWithEqual "lbl_stmts" read_field labels as_needed con_arity = dataConSourceArity data_con labels = map flLabel $ dataConFieldLabels data_con dc_nm = getName data_con is_infix = dataConIsInfix data_con is_record = labels `lengthExceeds` 0 as_needed = take con_arity as_RDRs read_args = zipWithEqual "gen_Read_binds" read_arg as_needed (derivDataConInstArgTys data_con dit) (read_a1:read_a2:_) = read_args prefix_prec = appPrecedence infix_prec = getPrecedence get_fixity dc_nm record_prec = appPrecedence + 1 -- Record construction binds even more tightly -- than application; e.g. T2 T1 {x=2} means T2 (T1 {x=2}) ------------------------------------------------------------------------ -- Helpers ------------------------------------------------------------------------ mk_alt e1 e2 = genOpApp e1 alt_RDR e2 -- e1 +++ e2 mk_parser p ss b = nlHsApps prec_RDR [nlHsIntLit p -- prec p (do { ss ; b }) , nlHsDo (DoExpr Nothing) (ss ++ [noLocA $ mkLastStmt b])] con_app con as = nlHsVarApps (getRdrName con) as -- con as result_expr con as = nlHsApp (nlHsVar returnM_RDR) (con_app con as) -- return (con as) -- For constructors and field labels ending in '#', we hackily -- let the lexer generate two tokens, and look for both in sequence -- Thus [Ident "I"; Symbol "#"]. See #5041 ident_h_pat s | Just (ss, '#') <- snocView s = [ ident_pat ss, symbol_pat "#" ] | otherwise = [ ident_pat s ] bindLex pat = noLocA (mkBodyStmt (nlHsApp (nlHsVar expectP_RDR) pat)) -- expectP p -- See Note [Use expectP] ident_pat s = bindLex $ nlHsApps ident_RDR [nlHsLit (mkHsString s)] -- expectP (Ident "foo") symbol_pat s = bindLex $ nlHsApps symbol_RDR [nlHsLit (mkHsString s)] -- expectP (Symbol ">>") read_punc c = bindLex $ nlHsApps punc_RDR [nlHsLit (mkHsString c)] -- expectP (Punc "<") data_con_str con = occNameString (getOccName con) read_arg a ty = assert (not (isUnliftedType ty)) $ noLocA (mkPsBindStmt noAnn (nlVarPat a) (nlHsVarApps step_RDR [readPrec_RDR])) -- When reading field labels we might encounter -- a = 3 -- _a = 3 -- or (#) = 4 -- Note the parens! read_field lbl a = [noLocA (mkPsBindStmt noAnn (nlVarPat a) (nlHsApp read_field (nlHsVarApps reset_RDR [readPrec_RDR]) ) ) ] where lbl_str = unpackFS lbl mk_read_field read_field_rdr lbl = nlHsApps read_field_rdr [nlHsLit (mkHsString lbl)] read_field | isSym lbl_str = mk_read_field readSymField_RDR lbl_str | Just (ss, '#') <- snocView lbl_str -- #14918 = mk_read_field readFieldHash_RDR ss | otherwise = mk_read_field readField_RDR lbl_str {- ************************************************************************ * * Show instances * * ************************************************************************ Example infixr 5 :^: data Tree a = Leaf a | Tree a :^: Tree a instance (Show a) => Show (Tree a) where showsPrec d (Leaf m) = showParen (d > app_prec) showStr where showStr = showString "Leaf " . showsPrec (app_prec+1) m showsPrec d (u :^: v) = showParen (d > up_prec) showStr where showStr = showsPrec (up_prec+1) u . showString " :^: " . showsPrec (up_prec+1) v -- Note: right-associativity of :^: ignored up_prec = 5 -- Precedence of :^: app_prec = 10 -- Application has precedence one more than -- the most tightly-binding operator -} gen_Show_binds :: (Name -> Fixity) -> SrcSpan -> DerivInstTys -> (LHsBinds GhcPs, Bag AuxBindSpec) gen_Show_binds get_fixity loc dit@(DerivInstTys{ dit_rep_tc = tycon , dit_rep_tc_args = tycon_args }) = (unitBag shows_prec, emptyBag) where data_cons = getPossibleDataCons tycon tycon_args shows_prec = mkFunBindEC 2 loc showsPrec_RDR id (map pats_etc data_cons) comma_space = nlHsVar showCommaSpace_RDR pats_etc data_con | nullary_con = -- skip the showParen junk... assert (null bs_needed) ([nlWildPat, con_pat], mk_showString_app op_con_str) | otherwise = ([a_Pat, con_pat], showParen_Expr (genOpApp a_Expr ge_RDR (nlHsLit (HsInt noExtField (mkIntegralLit con_prec_plus_one)))) (nlHsPar (nested_compose_Expr show_thingies))) where data_con_RDR = getRdrName data_con con_arity = dataConSourceArity data_con bs_needed = take con_arity bs_RDRs arg_tys = derivDataConInstArgTys data_con dit -- Correspond 1-1 with bs_needed con_pat = nlConVarPat data_con_RDR bs_needed nullary_con = con_arity == 0 labels = map flLabel $ dataConFieldLabels data_con lab_fields = length labels record_syntax = lab_fields > 0 dc_nm = getName data_con dc_occ_nm = getOccName data_con con_str = occNameString dc_occ_nm op_con_str = wrapOpParens con_str backquote_str = wrapOpBackquotes con_str show_thingies | is_infix = [show_arg1, mk_showString_app (" " ++ backquote_str ++ " "), show_arg2] | record_syntax = mk_showString_app (op_con_str ++ " {") : show_record_args ++ [mk_showString_app "}"] | otherwise = mk_showString_app (op_con_str ++ " ") : show_prefix_args show_label l = mk_showString_app (nm ++ " = ") -- Note the spaces around the "=" sign. If we -- don't have them then we get Foo { x=-1 } and -- the "=-" parses as a single lexeme. Only the -- space after the '=' is necessary, but it -- seems tidier to have them both sides. where nm = wrapOpParens (unpackFS l) show_args = zipWithEqual "gen_Show_binds" show_arg bs_needed arg_tys (show_arg1:show_arg2:_) = show_args show_prefix_args = intersperse (nlHsVar showSpace_RDR) show_args -- Assumption for record syntax: no of fields == no of -- labelled fields (and in same order) show_record_args = concat $ intersperse [comma_space] $ [ [show_label lbl, arg] | (lbl,arg) <- zipEqual "gen_Show_binds" labels show_args ] show_arg :: RdrName -> Type -> LHsExpr GhcPs show_arg b arg_ty | isUnliftedType arg_ty -- See Note [Deriving and unboxed types] in GHC.Tc.Deriv.Infer = with_conv $ nlHsApps compose_RDR [mk_shows_app boxed_arg, mk_showString_app postfixMod] | otherwise = mk_showsPrec_app arg_prec arg where arg = nlHsVar b boxed_arg = box "Show" arg arg_ty postfixMod = assoc_ty_id "Show" postfixModTbl arg_ty with_conv expr | (Just conv) <- assoc_ty_id_maybe primConvTbl arg_ty = nested_compose_Expr [ mk_showString_app ("(" ++ conv ++ " ") , expr , mk_showString_app ")" ] | otherwise = expr -- Fixity stuff is_infix = dataConIsInfix data_con con_prec_plus_one = 1 + getPrec is_infix get_fixity dc_nm arg_prec | record_syntax = 0 -- Record fields don't need parens | otherwise = con_prec_plus_one wrapOpParens :: String -> String wrapOpParens s | isSym s = '(' : s ++ ")" | otherwise = s wrapOpBackquotes :: String -> String wrapOpBackquotes s | isSym s = s | otherwise = '`' : s ++ "`" isSym :: String -> Bool isSym "" = False isSym (c : _) = startsVarSym c || startsConSym c -- | showString :: String -> ShowS mk_showString_app :: String -> LHsExpr GhcPs mk_showString_app str = nlHsApp (nlHsVar showString_RDR) (nlHsLit (mkHsString str)) -- | showsPrec :: Show a => Int -> a -> ShowS mk_showsPrec_app :: Integer -> LHsExpr GhcPs -> LHsExpr GhcPs mk_showsPrec_app p x = nlHsApps showsPrec_RDR [nlHsLit (HsInt noExtField (mkIntegralLit p)), x] -- | shows :: Show a => a -> ShowS mk_shows_app :: LHsExpr GhcPs -> LHsExpr GhcPs mk_shows_app x = nlHsApp (nlHsVar shows_RDR) x getPrec :: Bool -> (Name -> Fixity) -> Name -> Integer getPrec is_infix get_fixity nm | not is_infix = appPrecedence | otherwise = getPrecedence get_fixity nm appPrecedence :: Integer appPrecedence = fromIntegral maxPrecedence + 1 -- One more than the precedence of the most -- tightly-binding operator getPrecedence :: (Name -> Fixity) -> Name -> Integer getPrecedence get_fixity nm = case get_fixity nm of Fixity _ x _assoc -> fromIntegral x -- NB: the Report says that associativity is not taken -- into account for either Read or Show; hence we -- ignore associativity here {- ************************************************************************ * * Data instances * * ************************************************************************ From the data type data T a b = T1 a b | T2 we generate $cT1 = mkDataCon $dT "T1" Prefix $cT2 = mkDataCon $dT "T2" Prefix $dT = mkDataType "Module.T" [] [$con_T1, $con_T2] -- the [] is for field labels. instance (Data a, Data b) => Data (T a b) where gfoldl k z (T1 a b) = z T `k` a `k` b gfoldl k z T2 = z T2 -- ToDo: add gmapT,Q,M, gfoldr gunfold k z c = case conIndex c of I# 1# -> k (k (z T1)) I# 2# -> z T2 toConstr (T1 _ _) = $cT1 toConstr T2 = $cT2 dataTypeOf _ = $dT dataCast1 = gcast1 -- If T :: * -> * dataCast2 = gcast2 -- if T :: * -> * -> * -} gen_Data_binds :: SrcSpan -> DerivInstTys -> TcM (LHsBinds GhcPs, -- The method bindings Bag AuxBindSpec) -- Auxiliary bindings gen_Data_binds loc (DerivInstTys{dit_rep_tc = rep_tc}) = do { -- See Note [Auxiliary binders] dataT_RDR <- new_dataT_rdr_name loc rep_tc ; dataC_RDRs <- traverse (new_dataC_rdr_name loc) data_cons ; pure ( listToBag [ gfoldl_bind, gunfold_bind , toCon_bind dataC_RDRs, dataTypeOf_bind dataT_RDR ] `unionBags` gcast_binds -- Auxiliary definitions: the data type and constructors , listToBag ( DerivDataDataType rep_tc dataT_RDR dataC_RDRs : zipWith (\data_con dataC_RDR -> DerivDataConstr data_con dataC_RDR dataT_RDR) data_cons dataC_RDRs ) ) } where data_cons = tyConDataCons rep_tc n_cons = length data_cons one_constr = n_cons == 1 ------------ gfoldl gfoldl_bind = mkFunBindEC 3 loc gfoldl_RDR id (map gfoldl_eqn data_cons) gfoldl_eqn con = ([nlVarPat k_RDR, z_Pat, nlConVarPat con_name as_needed], foldl' mk_k_app (z_Expr `nlHsApp` (eta_expand_data_con con)) as_needed) where con_name :: RdrName con_name = getRdrName con as_needed = take (dataConSourceArity con) as_RDRs mk_k_app e v = nlHsPar (nlHsOpApp e k_RDR (nlHsVar v)) ------------ gunfold gunfold_bind = mkSimpleGeneratedFunBind loc gunfold_RDR [k_Pat, z_Pat, if one_constr then nlWildPat else c_Pat] gunfold_rhs gunfold_rhs | one_constr = mk_unfold_rhs (head data_cons) -- No need for case | otherwise = nlHsCase (nlHsVar conIndex_RDR `nlHsApp` c_Expr) (map gunfold_alt data_cons) gunfold_alt dc = mkHsCaseAlt (mk_unfold_pat dc) (mk_unfold_rhs dc) mk_unfold_rhs dc = foldr nlHsApp (z_Expr `nlHsApp` (eta_expand_data_con dc)) (replicate (dataConSourceArity dc) (nlHsVar k_RDR)) eta_expand_data_con dc = mkHsLam eta_expand_pats (foldl nlHsApp (nlHsVar (getRdrName dc)) eta_expand_hsvars) where eta_expand_pats = map nlVarPat eta_expand_vars eta_expand_hsvars = map nlHsVar eta_expand_vars eta_expand_vars = take (dataConSourceArity dc) as_RDRs mk_unfold_pat dc -- Last one is a wild-pat, to avoid -- redundant test, and annoying warning | tag-fIRST_TAG == n_cons-1 = nlWildPat -- Last constructor | otherwise = nlConPat intDataCon_RDR [nlLitPat (HsIntPrim NoSourceText (toInteger tag))] where tag = dataConTag dc ------------ toConstr toCon_bind dataC_RDRs = mkFunBindEC 1 loc toConstr_RDR id (zipWith to_con_eqn data_cons dataC_RDRs) to_con_eqn dc con_name = ([nlWildConPat dc], nlHsVar con_name) ------------ dataTypeOf dataTypeOf_bind dataT_RDR = mkSimpleGeneratedFunBind loc dataTypeOf_RDR [nlWildPat] (nlHsVar dataT_RDR) ------------ gcast1/2 -- Make the binding dataCast1 x = gcast1 x -- if T :: * -> * -- or dataCast2 x = gcast2 s -- if T :: * -> * -> * -- (or nothing if T has neither of these two types) -- But care is needed for data families: -- If we have data family D a -- data instance D (a,b,c) = A | B deriving( Data ) -- and we want instance ... => Data (D [(a,b,c)]) where ... -- then we need dataCast1 x = gcast1 x -- because D :: * -> * -- even though rep_tc has kind * -> * -> * -> * -- Hence looking for the kind of fam_tc not rep_tc -- See #4896 tycon_kind = case tyConFamInst_maybe rep_tc of Just (fam_tc, _) -> tyConKind fam_tc Nothing -> tyConKind rep_tc gcast_binds | tycon_kind `tcEqKind` kind1 = mk_gcast dataCast1_RDR gcast1_RDR | tycon_kind `tcEqKind` kind2 = mk_gcast dataCast2_RDR gcast2_RDR | otherwise = emptyBag mk_gcast dataCast_RDR gcast_RDR = unitBag (mkSimpleGeneratedFunBind loc dataCast_RDR [nlVarPat f_RDR] (nlHsVar gcast_RDR `nlHsApp` nlHsVar f_RDR)) kind1, kind2 :: Kind kind1 = typeToTypeKind kind2 = liftedTypeKind `mkVisFunTyMany` kind1 gfoldl_RDR, gunfold_RDR, toConstr_RDR, dataTypeOf_RDR, mkConstrTag_RDR, mkDataType_RDR, conIndex_RDR, prefix_RDR, infix_RDR, dataCast1_RDR, dataCast2_RDR, gcast1_RDR, gcast2_RDR, constr_RDR, dataType_RDR, eqChar_RDR , ltChar_RDR , geChar_RDR , gtChar_RDR , leChar_RDR , eqInt_RDR , ltInt_RDR , geInt_RDR , gtInt_RDR , leInt_RDR , neInt_RDR , eqInt8_RDR , ltInt8_RDR , geInt8_RDR , gtInt8_RDR , leInt8_RDR , eqInt16_RDR , ltInt16_RDR , geInt16_RDR , gtInt16_RDR , leInt16_RDR , eqInt32_RDR , ltInt32_RDR , geInt32_RDR , gtInt32_RDR , leInt32_RDR , eqInt64_RDR , ltInt64_RDR , geInt64_RDR , gtInt64_RDR , leInt64_RDR , eqWord_RDR , ltWord_RDR , geWord_RDR , gtWord_RDR , leWord_RDR , eqWord8_RDR , ltWord8_RDR , geWord8_RDR , gtWord8_RDR , leWord8_RDR , eqWord16_RDR, ltWord16_RDR, geWord16_RDR, gtWord16_RDR, leWord16_RDR, eqWord32_RDR, ltWord32_RDR, geWord32_RDR, gtWord32_RDR, leWord32_RDR, eqWord64_RDR, ltWord64_RDR, geWord64_RDR, gtWord64_RDR, leWord64_RDR, eqAddr_RDR , ltAddr_RDR , geAddr_RDR , gtAddr_RDR , leAddr_RDR , eqFloat_RDR , ltFloat_RDR , geFloat_RDR , gtFloat_RDR , leFloat_RDR , eqDouble_RDR, ltDouble_RDR, geDouble_RDR, gtDouble_RDR, leDouble_RDR, word8ToWord_RDR , int8ToInt_RDR , word16ToWord_RDR, int16ToInt_RDR, word32ToWord_RDR, int32ToInt_RDR :: RdrName gfoldl_RDR = varQual_RDR gENERICS (fsLit "gfoldl") gunfold_RDR = varQual_RDR gENERICS (fsLit "gunfold") toConstr_RDR = varQual_RDR gENERICS (fsLit "toConstr") dataTypeOf_RDR = varQual_RDR gENERICS (fsLit "dataTypeOf") dataCast1_RDR = varQual_RDR gENERICS (fsLit "dataCast1") dataCast2_RDR = varQual_RDR gENERICS (fsLit "dataCast2") gcast1_RDR = varQual_RDR tYPEABLE (fsLit "gcast1") gcast2_RDR = varQual_RDR tYPEABLE (fsLit "gcast2") mkConstrTag_RDR = varQual_RDR gENERICS (fsLit "mkConstrTag") constr_RDR = tcQual_RDR gENERICS (fsLit "Constr") mkDataType_RDR = varQual_RDR gENERICS (fsLit "mkDataType") dataType_RDR = tcQual_RDR gENERICS (fsLit "DataType") conIndex_RDR = varQual_RDR gENERICS (fsLit "constrIndex") prefix_RDR = dataQual_RDR gENERICS (fsLit "Prefix") infix_RDR = dataQual_RDR gENERICS (fsLit "Infix") eqChar_RDR = varQual_RDR gHC_PRIM (fsLit "eqChar#") ltChar_RDR = varQual_RDR gHC_PRIM (fsLit "ltChar#") leChar_RDR = varQual_RDR gHC_PRIM (fsLit "leChar#") gtChar_RDR = varQual_RDR gHC_PRIM (fsLit "gtChar#") geChar_RDR = varQual_RDR gHC_PRIM (fsLit "geChar#") eqInt_RDR = varQual_RDR gHC_PRIM (fsLit "==#") neInt_RDR = varQual_RDR gHC_PRIM (fsLit "/=#") ltInt_RDR = varQual_RDR gHC_PRIM (fsLit "<#" ) leInt_RDR = varQual_RDR gHC_PRIM (fsLit "<=#") gtInt_RDR = varQual_RDR gHC_PRIM (fsLit ">#" ) geInt_RDR = varQual_RDR gHC_PRIM (fsLit ">=#") eqInt8_RDR = varQual_RDR gHC_PRIM (fsLit "eqInt8#") ltInt8_RDR = varQual_RDR gHC_PRIM (fsLit "ltInt8#" ) leInt8_RDR = varQual_RDR gHC_PRIM (fsLit "leInt8#") gtInt8_RDR = varQual_RDR gHC_PRIM (fsLit "gtInt8#" ) geInt8_RDR = varQual_RDR gHC_PRIM (fsLit "geInt8#") eqInt16_RDR = varQual_RDR gHC_PRIM (fsLit "eqInt16#") ltInt16_RDR = varQual_RDR gHC_PRIM (fsLit "ltInt16#" ) leInt16_RDR = varQual_RDR gHC_PRIM (fsLit "leInt16#") gtInt16_RDR = varQual_RDR gHC_PRIM (fsLit "gtInt16#" ) geInt16_RDR = varQual_RDR gHC_PRIM (fsLit "geInt16#") eqInt32_RDR = varQual_RDR gHC_PRIM (fsLit "eqInt32#") ltInt32_RDR = varQual_RDR gHC_PRIM (fsLit "ltInt32#" ) leInt32_RDR = varQual_RDR gHC_PRIM (fsLit "leInt32#") gtInt32_RDR = varQual_RDR gHC_PRIM (fsLit "gtInt32#" ) geInt32_RDR = varQual_RDR gHC_PRIM (fsLit "geInt32#") eqInt64_RDR = varQual_RDR gHC_PRIM (fsLit "eqInt64#") ltInt64_RDR = varQual_RDR gHC_PRIM (fsLit "ltInt64#" ) leInt64_RDR = varQual_RDR gHC_PRIM (fsLit "leInt64#") gtInt64_RDR = varQual_RDR gHC_PRIM (fsLit "gtInt64#" ) geInt64_RDR = varQual_RDR gHC_PRIM (fsLit "geInt64#") eqWord_RDR = varQual_RDR gHC_PRIM (fsLit "eqWord#") ltWord_RDR = varQual_RDR gHC_PRIM (fsLit "ltWord#") leWord_RDR = varQual_RDR gHC_PRIM (fsLit "leWord#") gtWord_RDR = varQual_RDR gHC_PRIM (fsLit "gtWord#") geWord_RDR = varQual_RDR gHC_PRIM (fsLit "geWord#") eqWord8_RDR = varQual_RDR gHC_PRIM (fsLit "eqWord8#") ltWord8_RDR = varQual_RDR gHC_PRIM (fsLit "ltWord8#" ) leWord8_RDR = varQual_RDR gHC_PRIM (fsLit "leWord8#") gtWord8_RDR = varQual_RDR gHC_PRIM (fsLit "gtWord8#" ) geWord8_RDR = varQual_RDR gHC_PRIM (fsLit "geWord8#") eqWord16_RDR = varQual_RDR gHC_PRIM (fsLit "eqWord16#") ltWord16_RDR = varQual_RDR gHC_PRIM (fsLit "ltWord16#" ) leWord16_RDR = varQual_RDR gHC_PRIM (fsLit "leWord16#") gtWord16_RDR = varQual_RDR gHC_PRIM (fsLit "gtWord16#" ) geWord16_RDR = varQual_RDR gHC_PRIM (fsLit "geWord16#") eqWord32_RDR = varQual_RDR gHC_PRIM (fsLit "eqWord32#") ltWord32_RDR = varQual_RDR gHC_PRIM (fsLit "ltWord32#" ) leWord32_RDR = varQual_RDR gHC_PRIM (fsLit "leWord32#") gtWord32_RDR = varQual_RDR gHC_PRIM (fsLit "gtWord32#" ) geWord32_RDR = varQual_RDR gHC_PRIM (fsLit "geWord32#") eqWord64_RDR = varQual_RDR gHC_PRIM (fsLit "eqWord64#") ltWord64_RDR = varQual_RDR gHC_PRIM (fsLit "ltWord64#" ) leWord64_RDR = varQual_RDR gHC_PRIM (fsLit "leWord64#") gtWord64_RDR = varQual_RDR gHC_PRIM (fsLit "gtWord64#" ) geWord64_RDR = varQual_RDR gHC_PRIM (fsLit "geWord64#") eqAddr_RDR = varQual_RDR gHC_PRIM (fsLit "eqAddr#") ltAddr_RDR = varQual_RDR gHC_PRIM (fsLit "ltAddr#") leAddr_RDR = varQual_RDR gHC_PRIM (fsLit "leAddr#") gtAddr_RDR = varQual_RDR gHC_PRIM (fsLit "gtAddr#") geAddr_RDR = varQual_RDR gHC_PRIM (fsLit "geAddr#") eqFloat_RDR = varQual_RDR gHC_PRIM (fsLit "eqFloat#") ltFloat_RDR = varQual_RDR gHC_PRIM (fsLit "ltFloat#") leFloat_RDR = varQual_RDR gHC_PRIM (fsLit "leFloat#") gtFloat_RDR = varQual_RDR gHC_PRIM (fsLit "gtFloat#") geFloat_RDR = varQual_RDR gHC_PRIM (fsLit "geFloat#") eqDouble_RDR = varQual_RDR gHC_PRIM (fsLit "==##") ltDouble_RDR = varQual_RDR gHC_PRIM (fsLit "<##" ) leDouble_RDR = varQual_RDR gHC_PRIM (fsLit "<=##") gtDouble_RDR = varQual_RDR gHC_PRIM (fsLit ">##" ) geDouble_RDR = varQual_RDR gHC_PRIM (fsLit ">=##") word8ToWord_RDR = varQual_RDR gHC_PRIM (fsLit "word8ToWord#") int8ToInt_RDR = varQual_RDR gHC_PRIM (fsLit "int8ToInt#") word16ToWord_RDR = varQual_RDR gHC_PRIM (fsLit "word16ToWord#") int16ToInt_RDR = varQual_RDR gHC_PRIM (fsLit "int16ToInt#") word32ToWord_RDR = varQual_RDR gHC_PRIM (fsLit "word32ToWord#") int32ToInt_RDR = varQual_RDR gHC_PRIM (fsLit "int32ToInt#") {- ************************************************************************ * * Lift instances * * ************************************************************************ Example: data Foo a = Foo a | a :^: a deriving Lift ==> instance (Lift a) => Lift (Foo a) where lift (Foo a) = [| Foo $(lift a) |] lift ((:^:) u v) = [| (:^:) $(lift u) $(lift v) |] liftTyped (Foo a) = [|| Foo $$(liftTyped a) ||] liftTyped ((:^:) u v) = [|| (:^:) $$(liftTyped u) $$(liftTyped v) ||] Note that we use explicit splices here in order to not trigger the implicit lifting warning in derived code. (See #20688) -} gen_Lift_binds :: SrcSpan -> DerivInstTys -> (LHsBinds GhcPs, Bag AuxBindSpec) gen_Lift_binds loc (DerivInstTys{ dit_rep_tc = tycon , dit_rep_tc_args = tycon_args }) = (listToBag [lift_bind, liftTyped_bind], emptyBag) where lift_bind = mkFunBindEC 1 loc lift_RDR (nlHsApp pure_Expr) (map (pats_etc mk_untyped_bracket mk_usplice liftName) data_cons) liftTyped_bind = mkFunBindEC 1 loc liftTyped_RDR (nlHsApp unsafeCodeCoerce_Expr . nlHsApp pure_Expr) (map (pats_etc mk_typed_bracket mk_tsplice liftTypedName) data_cons) mk_untyped_bracket = HsUntypedBracket noAnn . ExpBr noExtField mk_typed_bracket = HsTypedBracket noAnn mk_usplice = HsUntypedSplice EpAnnNotUsed DollarSplice mk_tsplice = HsTypedSplice EpAnnNotUsed DollarSplice data_cons = getPossibleDataCons tycon tycon_args pats_etc mk_bracket mk_splice lift_name data_con = ([con_pat], lift_Expr) where con_pat = nlConVarPat data_con_RDR as_needed data_con_RDR = getRdrName data_con con_arity = dataConSourceArity data_con as_needed = take con_arity as_RDRs lift_Expr = noLocA (mk_bracket br_body) br_body = nlHsApps (Exact (dataConName data_con)) (map lift_var as_needed) lift_var :: RdrName -> LHsExpr (GhcPass 'Parsed) lift_var x = noLocA (HsSpliceE EpAnnNotUsed (mk_splice x (nlHsPar (mk_lift_expr x)))) mk_lift_expr :: RdrName -> LHsExpr (GhcPass 'Parsed) mk_lift_expr x = nlHsApps (Exact lift_name) [nlHsVar x] {- ************************************************************************ * * Newtype-deriving instances * * ************************************************************************ Note [Newtype-deriving instances] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We take every method in the original instance and `coerce` it to fit into the derived instance. We need type applications on the argument to `coerce` to make it obvious what instantiation of the method we're coercing from. So from, say, class C a b where op :: forall c. a -> [b] -> c -> Int newtype T x = MkT instance C a => C a (T x) where op :: forall c. a -> [T x] -> c -> Int op = coerce @(a -> [] -> c -> Int) @(a -> [T x] -> c -> Int) op In addition to the type applications, we also have an explicit type signature on the entire RHS. This brings the method-bound variable `c` into scope over the two type applications. See Note [GND and QuantifiedConstraints] for more information on why this is important. Giving 'coerce' two explicitly-visible type arguments grants us finer control over how it should be instantiated. Recall coerce :: Coercible a b => a -> b By giving it explicit type arguments we deal with the case where 'op' has a higher rank type, and so we must instantiate 'coerce' with a polytype. E.g. class C a where op :: a -> forall b. b -> b newtype T x = MkT instance C => C (T x) where op :: T x -> forall b. b -> b op = coerce @( -> forall b. b -> b) @(T x -> forall b. b -> b) op The use of type applications is crucial here. We have to instantiate both type args of (coerce :: Coercible a b => a -> b) to polytypes, and we can only do that with VTA or Quick Look. Here VTA seems more appropriate for machine generated code: it's simple and robust. However, to allow VTA with polytypes we must switch on -XImpredicativeTypes locally in GHC.Tc.Deriv.genInst. See #8503 for more discussion. Note [Newtype-deriving trickiness] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider (#12768): class C a where { op :: D a => a -> a } instance C a => C [a] where { op = opList } opList :: (C a, D [a]) => [a] -> [a] opList = ... Now suppose we try GND on this: newtype N a = MkN [a] deriving( C ) The GND is expecting to get an implementation of op for N by coercing opList, thus: instance C a => C (N a) where { op = opN } opN :: (C a, D (N a)) => N a -> N a opN = coerce @([a] -> [a]) @([N a] -> [N a] opList :: D (N a) => [N a] -> [N a] But there is no reason to suppose that (D [a]) and (D (N a)) are inter-coercible; these instances might completely different. So GHC rightly rejects this code. Note [GND and QuantifiedConstraints] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider the following example from #15290: class C m where join :: m (m a) -> m a newtype T m a = MkT (m a) deriving instance (C m, forall p q. Coercible p q => Coercible (m p) (m q)) => C (T m) The code that GHC used to generate for this was: instance (C m, forall p q. Coercible p q => Coercible (m p) (m q)) => C (T m) where join = coerce @(forall a. m (m a) -> m a) @(forall a. T m (T m a) -> T m a) join This instantiates `coerce` at a polymorphic type, a form of impredicative polymorphism, so we're already on thin ice. And in fact the ice breaks, as we'll explain: The call to `coerce` gives rise to: Coercible (forall a. m (m a) -> m a) (forall a. T m (T m a) -> T m a) And that simplified to the following implication constraint: forall a . m (T m a) ~R# m (m a) But because this constraint is under a `forall`, inside a type, we have to prove it *without computing any term evidence* (hence the ). Alas, we *must* generate a term-level evidence binding in order to instantiate the quantified constraint! In response, GHC currently chooses not to use such a quantified constraint. See Note [Instances in no-evidence implications] in GHC.Tc.Solver.Interact. But this isn't the death knell for combining QuantifiedConstraints with GND. On the contrary, if we generate GND bindings in a slightly different way, then we can avoid this situation altogether. Instead of applying `coerce` to two polymorphic types, we instead let an instance signature do the polymorphic instantiation, and omit the `forall`s in the type applications. More concretely, we generate the following code instead: instance (C m, forall p q. Coercible p q => Coercible (m p) (m q)) => C (T m) where join :: forall a. T m (T m a) -> T m a join = coerce @( m (m a) -> m a) @(T m (T m a) -> T m a) join Now the visible type arguments are both monotypes, so we don't need any of this funny quantified constraint instantiation business. While this particular example no longer uses impredicative instantiation, we still need to enable ImpredicativeTypes to typecheck GND-generated code for class methods with higher-rank types. See Note [Newtype-deriving instances]. You might think that that second @(T m (T m a) -> T m a) argument is redundant in the presence of the instance signature, but in fact leaving it off will break this example (from the T15290d test case): class C a where c :: Int -> forall b. b -> a instance C Int instance C Age where c :: Int -> forall b. b -> Age c = coerce @(Int -> forall b. b -> Int) c That is because we still need to instantiate the second argument of coerce with a polytype, and we can only do that with VTA or QuickLook. Be aware that the use of an instance signature doesn't /solve/ this problem; it just makes it less likely to occur. For example, if a class has a truly higher-rank type like so: class CProblem m where op :: (forall b. ... (m b) ...) -> Int Then the same situation will arise again. But at least it won't arise for the common case of methods with ordinary, prenex-quantified types. ----- -- Wrinkle: Use HsOuterExplicit ----- One minor complication with the plan above is that we need to ensure that the type variables from a method's instance signature properly scope over the body of the method. For example, recall: instance (C m, forall p q. Coercible p q => Coercible (m p) (m q)) => C (T m) where join :: forall a. T m (T m a) -> T m a join = coerce @( m (m a) -> m a) @(T m (T m a) -> T m a) join In the example above, it is imperative that the `a` in the instance signature for `join` scope over the body of `join` by way of ScopedTypeVariables. This might sound obvious, but note that in gen_Newtype_binds, which is responsible for generating the code above, the type in `join`'s instance signature is given as a Core type, whereas gen_Newtype_binds will eventually produce HsBinds (i.e., source Haskell) that is renamed and typechecked. We must ensure that `a` is in scope over the body of `join` during renaming or else the generated code will be rejected. In short, we need to convert the instance signature from a Core type to an HsType (i.e., a source Haskell type). Two possible options are: 1. Convert the Core type entirely to an HsType (i.e., a source Haskell type). 2. Embed the entire Core type using HsCoreTy. Neither option is quite satisfactory: 1. Converting a Core type to an HsType in full generality is surprisingly complicated. Previous versions of GHCs did this, but it was the source of numerous bugs (see #14579 and #16518, for instance). 2. While HsCoreTy is much less complicated that option (1), it's not quite what we want. In order for `a` to be in scope over the body of `join` during renaming, the `forall` must be contained in an HsOuterExplicit. (See Note [Lexically scoped type variables] in GHC.Hs.Type.) HsCoreTy bypasses HsOuterExplicit, so this won't work either. As a compromise, we adopt a combination of the two options above: * Split apart the top-level ForAllTys in the instance signature's Core type, * Convert the top-level ForAllTys to an HsOuterExplicit, and * Embed the remainder of the Core type in an HsCoreTy. This retains most of the simplicity of option (2) while still ensuring that the type variables are correctly scoped. Note that splitting apart top-level ForAllTys will expand any type synonyms in the Core type itself. This ends up being important to fix a corner case observed in #18914. Consider this example: type T f = forall a. f a class C f where m :: T f newtype N f a = MkN (f a) deriving C What code should `deriving C` generate? It will have roughly the following shape: instance C f => C (N f) where m :: T (N f) m = coerce @(...) (...) (m @f) At a minimum, we must instantiate `coerce` with `@(T f)` and `@(T (N f))`, but with the `forall`s removed in order to make them monotypes. However, the `forall` is hidden underneath the `T` type synonym, so we must first expand `T` before we can strip of the `forall`. Expanding `T`, we get `coerce @(forall a. f a) @(forall a. N f a)`, and after omitting the `forall`s, we get `coerce @(f a) @(N f a)`. We can't stop there, however, or else we would end up with this code: instance C f => C (N f) where m :: T (N f) m = coerce @(f a) @(N f a) (m @f) Notice that the type variable `a` is completely unbound. In order to make sure that `a` is in scope, we must /also/ expand the `T` in `m :: T (N f)` to get `m :: forall a. N f a`. Fortunately, we will do just that in the plan outlined above, since when we split off the top-level ForAllTys in the instance signature, we must first expand the T type synonym. Note [GND and ambiguity] ~~~~~~~~~~~~~~~~~~~~~~~~ We make an effort to make the code generated through GND be robust w.r.t. ambiguous type variables. As one example, consider the following example (from #15637): class C a where f :: String instance C () where f = "foo" newtype T = T () deriving C A naïve attempt and generating a C T instance would be: instance C T where f :: String f = coerce @String @String f This isn't going to typecheck, however, since GHC doesn't know what to instantiate the type variable `a` with in the call to `f` in the method body. (Note that `f :: forall a. String`!) To compensate for the possibility of ambiguity here, we explicitly instantiate `a` like so: instance C T where f :: String f = coerce @String @String (f @()) All better now. -} gen_Newtype_binds :: SrcSpan -> Class -- the class being derived -> [TyVar] -- the tvs in the instance head (this includes -- the tvs from both the class types and the -- newtype itself) -> [Type] -- instance head parameters (incl. newtype) -> Type -- the representation type -> (LHsBinds GhcPs, [LSig GhcPs]) -- See Note [Newtype-deriving instances] gen_Newtype_binds loc' cls inst_tvs inst_tys rhs_ty = (listToBag binds, sigs) where (binds, sigs) = mapAndUnzip mk_bind_and_sig (classMethods cls) -- Same as inst_tys, but with the last argument type replaced by the -- representation type. underlying_inst_tys :: [Type] underlying_inst_tys = changeLast inst_tys rhs_ty locn = noAnnSrcSpan loc' loca = noAnnSrcSpan loc' -- For each class method, generate its derived binding and instance -- signature. Using the first example from -- Note [Newtype-deriving instances]: -- -- class C a b where -- op :: forall c. a -> [b] -> c -> Int -- -- newtype T x = MkT -- -- Then we would generate below: -- -- instance C a => C a (T x) where -- mk_bind_and_sig :: Id -> (LHsBind GhcPs, LSig GhcPs) mk_bind_and_sig meth_id = ( -- The derived binding, e.g., -- -- op = coerce @(a -> [] -> c -> Int) -- @(a -> [T x] -> c -> Int) -- op mkRdrFunBind loc_meth_RDR [mkSimpleMatch (mkPrefixFunRhs loc_meth_RDR) [] rhs_expr] , -- The derived instance signature, e.g., -- -- op :: forall c. a -> [T x] -> c -> Int -- -- Make sure that `forall c` is in an HsOuterExplicit so that it -- scopes over the body of `op`. See "Wrinkle: Use HsOuterExplicit" in -- Note [GND and QuantifiedConstraints]. L loca $ ClassOpSig noAnn False [loc_meth_RDR] $ L loca $ mkHsExplicitSigType noAnn (map mk_hs_tvb to_tvbs) (nlHsCoreTy to_rho) ) where Pair from_ty to_ty = mkCoerceClassMethEqn cls inst_tvs inst_tys rhs_ty meth_id (_, _, from_tau) = tcSplitSigmaTy from_ty (to_tvbs, to_rho) = tcSplitForAllInvisTVBinders to_ty (_, to_tau) = tcSplitPhiTy to_rho -- The use of tcSplitForAllInvisTVBinders above expands type synonyms, -- which is important to ensure correct type variable scoping. -- See "Wrinkle: Use HsOuterExplicit" in -- Note [GND and QuantifiedConstraints]. mk_hs_tvb :: VarBndr TyVar flag -> LHsTyVarBndr flag GhcPs mk_hs_tvb (Bndr tv flag) = noLocA $ KindedTyVar noAnn flag (noLocA (getRdrName tv)) (nlHsCoreTy (tyVarKind tv)) meth_RDR = getRdrName meth_id loc_meth_RDR = L locn meth_RDR rhs_expr = nlHsVar (getRdrName coerceId) `nlHsAppType` from_tau `nlHsAppType` to_tau `nlHsApp` meth_app -- The class method, applied to all of the class instance types -- (including the representation type) to avoid potential ambiguity. -- See Note [GND and ambiguity] meth_app = foldl' nlHsAppType (nlHsVar meth_RDR) $ filterOutInferredTypes (classTyCon cls) underlying_inst_tys -- Filter out any inferred arguments, since they can't be -- applied with visible type application. gen_Newtype_fam_insts :: SrcSpan -> Class -- the class being derived -> [TyVar] -- the tvs in the instance head (this includes -- the tvs from both the class types and the -- newtype itself) -> [Type] -- instance head parameters (incl. newtype) -> Type -- the representation type -> TcM [FamInst] -- See Note [GND and associated type families] in GHC.Tc.Deriv gen_Newtype_fam_insts loc' cls inst_tvs inst_tys rhs_ty = assert (all (not . isDataFamilyTyCon) ats) $ mapM mk_atf_inst ats where -- Same as inst_tys, but with the last argument type replaced by the -- representation type. underlying_inst_tys :: [Type] underlying_inst_tys = changeLast inst_tys rhs_ty ats = classATs cls locn = noAnnSrcSpan loc' cls_tvs = classTyVars cls in_scope = mkInScopeSet $ mkVarSet inst_tvs lhs_env = zipTyEnv cls_tvs inst_tys lhs_subst = mkTvSubst in_scope lhs_env rhs_env = zipTyEnv cls_tvs underlying_inst_tys rhs_subst = mkTvSubst in_scope rhs_env mk_atf_inst :: TyCon -> TcM FamInst mk_atf_inst fam_tc = do rep_tc_name <- newFamInstTyConName (L locn (tyConName fam_tc)) rep_lhs_tys let axiom = mkSingleCoAxiom Nominal rep_tc_name rep_tvs' [] rep_cvs' fam_tc rep_lhs_tys rep_rhs_ty -- Check (c) from Note [GND and associated type families] in GHC.Tc.Deriv checkValidCoAxBranch fam_tc (coAxiomSingleBranch axiom) newFamInst SynFamilyInst axiom where fam_tvs = tyConTyVars fam_tc rep_lhs_tys = substTyVars lhs_subst fam_tvs rep_rhs_tys = substTyVars rhs_subst fam_tvs rep_rhs_ty = mkTyConApp fam_tc rep_rhs_tys rep_tcvs = tyCoVarsOfTypesList rep_lhs_tys (rep_tvs, rep_cvs) = partition isTyVar rep_tcvs rep_tvs' = scopedSort rep_tvs rep_cvs' = scopedSort rep_cvs nlHsAppType :: LHsExpr GhcPs -> Type -> LHsExpr GhcPs nlHsAppType e s = noLocA (HsAppType noSrcSpan e hs_ty) where hs_ty = mkHsWildCardBndrs $ parenthesizeHsType appPrec $ nlHsCoreTy s nlHsCoreTy :: HsCoreTy -> LHsType GhcPs nlHsCoreTy = noLocA . XHsType mkCoerceClassMethEqn :: Class -- the class being derived -> [TyVar] -- the tvs in the instance head (this includes -- the tvs from both the class types and the -- newtype itself) -> [Type] -- instance head parameters (incl. newtype) -> Type -- the representation type -> Id -- the method to look at -> Pair Type -- See Note [Newtype-deriving instances] -- See also Note [Newtype-deriving trickiness] -- The pair is the (from_type, to_type), where to_type is -- the type of the method we are trying to get mkCoerceClassMethEqn cls inst_tvs inst_tys rhs_ty id = Pair (substTy rhs_subst user_meth_ty) (substTy lhs_subst user_meth_ty) where cls_tvs = classTyVars cls in_scope = mkInScopeSet $ mkVarSet inst_tvs lhs_subst = mkTvSubst in_scope (zipTyEnv cls_tvs inst_tys) rhs_subst = mkTvSubst in_scope (zipTyEnv cls_tvs (changeLast inst_tys rhs_ty)) (_class_tvs, _class_constraint, user_meth_ty) = tcSplitMethodTy (varType id) {- ************************************************************************ * * \subsection{Generating extra binds (@tag2con@, etc.)} * * ************************************************************************ \begin{verbatim} data Foo ... = ... tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int# maxtag_Foo :: Int -- ditto (NB: not unlifted) \end{verbatim} The `tags' here start at zero, hence the @fIRST_TAG@ (currently one) fiddling around. -} -- | Generate the full code for an auxiliary binding. -- See @Note [Auxiliary binders] (Wrinkle: Reducing code duplication)@. genAuxBindSpecOriginal :: DynFlags -> SrcSpan -> AuxBindSpec -> (LHsBind GhcPs, LSig GhcPs) genAuxBindSpecOriginal dflags loc spec = (gen_bind spec, L loca (TypeSig noAnn [L locn (auxBindSpecRdrName spec)] (genAuxBindSpecSig loc spec))) where loca = noAnnSrcSpan loc locn = noAnnSrcSpan loc gen_bind :: AuxBindSpec -> LHsBind GhcPs gen_bind (DerivTag2Con _ tag2con_RDR) = mkFunBindSE 0 loc tag2con_RDR [([nlConVarPat intDataCon_RDR [a_RDR]], nlHsApp (nlHsVar tagToEnum_RDR) a_Expr)] gen_bind (DerivMaxTag tycon maxtag_RDR) = mkHsVarBind loc maxtag_RDR rhs where rhs = nlHsApp (nlHsVar intDataCon_RDR) (nlHsLit (HsIntPrim NoSourceText max_tag)) max_tag = case (tyConDataCons tycon) of data_cons -> toInteger ((length data_cons) - fIRST_TAG) gen_bind (DerivDataDataType tycon dataT_RDR dataC_RDRs) = mkHsVarBind loc dataT_RDR rhs where tc_name = tyConName tycon tc_name_string = occNameString (getOccName tc_name) definition_mod_name = moduleNameString (moduleName (expectJust "gen_bind DerivDataDataType" $ nameModule_maybe tc_name)) ctx = initDefaultSDocContext dflags rhs = nlHsVar mkDataType_RDR `nlHsApp` nlHsLit (mkHsString (showSDocOneLine ctx (text definition_mod_name <> dot <> text tc_name_string))) `nlHsApp` nlList (map nlHsVar dataC_RDRs) gen_bind (DerivDataConstr dc dataC_RDR dataT_RDR) = mkHsVarBind loc dataC_RDR rhs where rhs = nlHsApps mkConstrTag_RDR constr_args constr_args = [ nlHsVar dataT_RDR -- DataType , nlHsLit (mkHsString (occNameString dc_occ)) -- Constructor name , nlHsIntLit (toInteger (dataConTag dc)) -- Constructor tag , nlList labels -- Field labels , nlHsVar fixity ] -- Fixity labels = map (nlHsLit . mkHsString . unpackFS . flLabel) (dataConFieldLabels dc) dc_occ = getOccName dc is_infix = isDataSymOcc dc_occ fixity | is_infix = infix_RDR | otherwise = prefix_RDR -- | Generate the code for an auxiliary binding that is a duplicate of another -- auxiliary binding. -- See @Note [Auxiliary binders] (Wrinkle: Reducing code duplication)@. genAuxBindSpecDup :: SrcSpan -> RdrName -> AuxBindSpec -> (LHsBind GhcPs, LSig GhcPs) genAuxBindSpecDup loc original_rdr_name dup_spec = (mkHsVarBind loc dup_rdr_name (nlHsVar original_rdr_name), L loca (TypeSig noAnn [L locn dup_rdr_name] (genAuxBindSpecSig loc dup_spec))) where loca = noAnnSrcSpan loc locn = noAnnSrcSpan loc dup_rdr_name = auxBindSpecRdrName dup_spec -- | Generate the type signature of an auxiliary binding. -- See @Note [Auxiliary binders]@. genAuxBindSpecSig :: SrcSpan -> AuxBindSpec -> LHsSigWcType GhcPs genAuxBindSpecSig loc spec = case spec of DerivTag2Con tycon _ -> mk_sig $ L (noAnnSrcSpan loc) $ XHsType $ mkSpecForAllTys (tyConTyVars tycon) $ intTy `mkVisFunTyMany` mkParentType tycon DerivMaxTag _ _ -> mk_sig (L (noAnnSrcSpan loc) (XHsType intTy)) DerivDataDataType _ _ _ -> mk_sig (nlHsTyVar NotPromoted dataType_RDR) DerivDataConstr _ _ _ -> mk_sig (nlHsTyVar NotPromoted constr_RDR) where mk_sig = mkHsWildCardBndrs . L (noAnnSrcSpan loc) . mkHsImplicitSigType -- | Take a 'Bag' of 'AuxBindSpec's and generate the code for auxiliary -- bindings based on the declarative descriptions in the supplied -- 'AuxBindSpec's. See @Note [Auxiliary binders]@. genAuxBinds :: DynFlags -> SrcSpan -> Bag AuxBindSpec -> Bag (LHsBind GhcPs, LSig GhcPs) genAuxBinds dflags loc = snd . foldr gen_aux_bind_spec (emptyOccEnv, emptyBag) where -- Perform a CSE-like pass over the generated auxiliary bindings to avoid -- code duplication, as described in -- Note [Auxiliary binders] (Wrinkle: Reducing code duplication). -- The OccEnv remembers the first occurrence of each sort of auxiliary -- binding and maps it to the unique RdrName for that binding. gen_aux_bind_spec :: AuxBindSpec -> (OccEnv RdrName, Bag (LHsBind GhcPs, LSig GhcPs)) -> (OccEnv RdrName, Bag (LHsBind GhcPs, LSig GhcPs)) gen_aux_bind_spec spec (original_rdr_name_env, spec_bag) = case lookupOccEnv original_rdr_name_env spec_occ of Nothing -> ( extendOccEnv original_rdr_name_env spec_occ spec_rdr_name , genAuxBindSpecOriginal dflags loc spec `consBag` spec_bag ) Just original_rdr_name -> ( original_rdr_name_env , genAuxBindSpecDup loc original_rdr_name spec `consBag` spec_bag ) where spec_rdr_name = auxBindSpecRdrName spec spec_occ = rdrNameOcc spec_rdr_name mkParentType :: TyCon -> Type -- Turn the representation tycon of a family into -- a use of its family constructor mkParentType tc = case tyConFamInst_maybe tc of Nothing -> mkTyConApp tc (mkTyVarTys (tyConTyVars tc)) Just (fam_tc,tys) -> mkTyConApp fam_tc tys {- ************************************************************************ * * \subsection{Utility bits for generating bindings} * * ************************************************************************ -} -- | Make a function binding. If no equations are given, produce a function -- with the given arity that produces a stock error. mkFunBindSE :: Arity -> SrcSpan -> RdrName -> [([LPat GhcPs], LHsExpr GhcPs)] -> LHsBind GhcPs mkFunBindSE arity loc fun pats_and_exprs = mkRdrFunBindSE arity (L (noAnnSrcSpan loc) fun) matches where matches = [mkMatch (mkPrefixFunRhs (L (noAnnSrcSpan loc) fun)) (map (parenthesizePat appPrec) p) e emptyLocalBinds | (p,e) <-pats_and_exprs] mkRdrFunBind :: LocatedN RdrName -> [LMatch GhcPs (LHsExpr GhcPs)] -> LHsBind GhcPs mkRdrFunBind fun@(L loc _fun_rdr) matches = L (na2la loc) (mkFunBind Generated fun matches) -- | Make a function binding. If no equations are given, produce a function -- with the given arity that uses an empty case expression for the last -- argument that is passes to the given function to produce the right-hand -- side. mkFunBindEC :: Arity -> SrcSpan -> RdrName -> (LHsExpr GhcPs -> LHsExpr GhcPs) -> [([LPat GhcPs], LHsExpr GhcPs)] -> LHsBind GhcPs mkFunBindEC arity loc fun catch_all pats_and_exprs = mkRdrFunBindEC arity catch_all (L (noAnnSrcSpan loc) fun) matches where matches = [ mkMatch (mkPrefixFunRhs (L (noAnnSrcSpan loc) fun)) (map (parenthesizePat appPrec) p) e emptyLocalBinds | (p,e) <- pats_and_exprs ] -- | Produces a function binding. When no equations are given, it generates -- a binding of the given arity and an empty case expression -- for the last argument that it passes to the given function to produce -- the right-hand side. mkRdrFunBindEC :: Arity -> (LHsExpr GhcPs -> LHsExpr GhcPs) -> LocatedN RdrName -> [LMatch GhcPs (LHsExpr GhcPs)] -> LHsBind GhcPs mkRdrFunBindEC arity catch_all fun@(L loc _fun_rdr) matches = L (na2la loc) (mkFunBind Generated fun matches') where -- Catch-all eqn looks like -- fmap _ z = case z of {} -- or -- traverse _ z = pure (case z of) -- or -- foldMap _ z = mempty -- It's needed if there no data cons at all, -- which can happen with -XEmptyDataDecls -- See #4302 matches' = if null matches then [mkMatch (mkPrefixFunRhs fun) (replicate (arity - 1) nlWildPat ++ [z_Pat]) (catch_all $ nlHsCase z_Expr []) emptyLocalBinds] else matches -- | Produces a function binding. When there are no equations, it generates -- a binding with the given arity that produces an error based on the name of -- the type of the last argument. mkRdrFunBindSE :: Arity -> LocatedN RdrName -> [LMatch GhcPs (LHsExpr GhcPs)] -> LHsBind GhcPs mkRdrFunBindSE arity fun@(L loc fun_rdr) matches = L (na2la loc) (mkFunBind Generated fun matches') where -- Catch-all eqn looks like -- compare _ _ = error "Void compare" -- It's needed if there no data cons at all, -- which can happen with -XEmptyDataDecls -- See #4302 matches' = if null matches then [mkMatch (mkPrefixFunRhs fun) (replicate arity nlWildPat) (error_Expr str) emptyLocalBinds] else matches str = "Void " ++ occNameString (rdrNameOcc fun_rdr) box :: String -- The class involved -> LHsExpr GhcPs -- The argument -> Type -- The argument type -> LHsExpr GhcPs -- Boxed version of the arg -- See Note [Deriving and unboxed types] in GHC.Tc.Deriv.Infer box cls_str arg arg_ty = assoc_ty_id cls_str boxConTbl arg_ty arg --------------------- primOrdOps :: String -- The class involved -> Type -- The type -> (RdrName, RdrName, RdrName, RdrName, RdrName) -- (lt,le,eq,ge,gt) -- See Note [Deriving and unboxed types] in GHC.Tc.Deriv.Infer primOrdOps str ty = assoc_ty_id str ordOpTbl ty ordOpTbl :: [(Type, (RdrName, RdrName, RdrName, RdrName, RdrName))] ordOpTbl = [(charPrimTy , (ltChar_RDR , leChar_RDR , eqChar_RDR , geChar_RDR , gtChar_RDR )) ,(intPrimTy , (ltInt_RDR , leInt_RDR , eqInt_RDR , geInt_RDR , gtInt_RDR )) ,(int8PrimTy , (ltInt8_RDR , leInt8_RDR , eqInt8_RDR , geInt8_RDR , gtInt8_RDR )) ,(int16PrimTy , (ltInt16_RDR , leInt16_RDR , eqInt16_RDR , geInt16_RDR , gtInt16_RDR )) ,(int32PrimTy , (ltInt32_RDR , leInt32_RDR , eqInt32_RDR , geInt32_RDR , gtInt32_RDR )) ,(int64PrimTy , (ltInt64_RDR , leInt64_RDR , eqInt64_RDR , geInt64_RDR , gtInt64_RDR )) ,(wordPrimTy , (ltWord_RDR , leWord_RDR , eqWord_RDR , geWord_RDR , gtWord_RDR )) ,(word8PrimTy , (ltWord8_RDR , leWord8_RDR , eqWord8_RDR , geWord8_RDR , gtWord8_RDR )) ,(word16PrimTy, (ltWord16_RDR, leWord16_RDR , eqWord16_RDR, geWord16_RDR, gtWord16_RDR )) ,(word32PrimTy, (ltWord32_RDR, leWord32_RDR , eqWord32_RDR, geWord32_RDR, gtWord32_RDR )) ,(word64PrimTy, (ltWord64_RDR, leWord64_RDR , eqWord64_RDR, geWord64_RDR, gtWord64_RDR )) ,(addrPrimTy , (ltAddr_RDR , leAddr_RDR , eqAddr_RDR , geAddr_RDR , gtAddr_RDR )) ,(floatPrimTy , (ltFloat_RDR , leFloat_RDR , eqFloat_RDR , geFloat_RDR , gtFloat_RDR )) ,(doublePrimTy, (ltDouble_RDR, leDouble_RDR , eqDouble_RDR, geDouble_RDR, gtDouble_RDR)) ] -- A mapping from a primitive type to a function that constructs its boxed -- version. -- NOTE: Int8#/Word8# will become Int/Word. boxConTbl :: [(Type, LHsExpr GhcPs -> LHsExpr GhcPs)] boxConTbl = [ (charPrimTy , nlHsApp (nlHsVar $ getRdrName charDataCon)) , (intPrimTy , nlHsApp (nlHsVar $ getRdrName intDataCon)) , (wordPrimTy , nlHsApp (nlHsVar $ getRdrName wordDataCon )) , (floatPrimTy , nlHsApp (nlHsVar $ getRdrName floatDataCon )) , (doublePrimTy, nlHsApp (nlHsVar $ getRdrName doubleDataCon)) , (int8PrimTy, nlHsApp (nlHsVar $ getRdrName intDataCon) . nlHsApp (nlHsVar int8ToInt_RDR)) , (word8PrimTy, nlHsApp (nlHsVar $ getRdrName wordDataCon) . nlHsApp (nlHsVar word8ToWord_RDR)) , (int16PrimTy, nlHsApp (nlHsVar $ getRdrName intDataCon) . nlHsApp (nlHsVar int16ToInt_RDR)) , (word16PrimTy, nlHsApp (nlHsVar $ getRdrName wordDataCon) . nlHsApp (nlHsVar word16ToWord_RDR)) , (int32PrimTy, nlHsApp (nlHsVar $ getRdrName intDataCon) . nlHsApp (nlHsVar int32ToInt_RDR)) , (word32PrimTy, nlHsApp (nlHsVar $ getRdrName wordDataCon) . nlHsApp (nlHsVar word32ToWord_RDR)) ] -- | A table of postfix modifiers for unboxed values. postfixModTbl :: [(Type, String)] postfixModTbl = [(charPrimTy , "#" ) ,(intPrimTy , "#" ) ,(wordPrimTy , "##") ,(floatPrimTy , "#" ) ,(doublePrimTy, "##") ,(int8PrimTy, "#") ,(word8PrimTy, "##") ,(int16PrimTy, "#") ,(word16PrimTy, "##") ,(int32PrimTy, "#") ,(word32PrimTy, "##") ] primConvTbl :: [(Type, String)] primConvTbl = [ (int8PrimTy, "intToInt8#") , (word8PrimTy, "wordToWord8#") , (int16PrimTy, "intToInt16#") , (word16PrimTy, "wordToWord16#") , (int32PrimTy, "intToInt32#") , (word32PrimTy, "wordToWord32#") ] litConTbl :: [(Type, LHsExpr GhcPs -> LHsExpr GhcPs)] litConTbl = [(charPrimTy , nlHsApp (nlHsVar charPrimL_RDR)) ,(intPrimTy , nlHsApp (nlHsVar intPrimL_RDR) . nlHsApp (nlHsVar toInteger_RDR)) ,(wordPrimTy , nlHsApp (nlHsVar wordPrimL_RDR) . nlHsApp (nlHsVar toInteger_RDR)) ,(addrPrimTy , nlHsApp (nlHsVar stringPrimL_RDR) . nlHsApp (nlHsApp (nlHsVar map_RDR) (compose_RDR `nlHsApps` [ nlHsVar fromIntegral_RDR , nlHsVar fromEnum_RDR ]))) ,(floatPrimTy , nlHsApp (nlHsVar floatPrimL_RDR) . nlHsApp (nlHsVar toRational_RDR)) ,(doublePrimTy, nlHsApp (nlHsVar doublePrimL_RDR) . nlHsApp (nlHsVar toRational_RDR)) ] -- | Lookup `Type` in an association list. assoc_ty_id :: HasCallStack => String -- The class involved -> [(Type,a)] -- The table -> Type -- The type -> a -- The result of the lookup assoc_ty_id cls_str tbl ty | Just a <- assoc_ty_id_maybe tbl ty = a | otherwise = pprPanic "Error in deriving:" (text "Can't derive" <+> text cls_str <+> text "for primitive type" <+> ppr ty) -- | Lookup `Type` in an association list. assoc_ty_id_maybe :: [(Type, a)] -> Type -> Maybe a assoc_ty_id_maybe tbl ty = snd <$> find (\(t, _) -> t `eqType` ty) tbl ----------------------------------------------------------------------- and_Expr :: LHsExpr GhcPs -> LHsExpr GhcPs -> LHsExpr GhcPs and_Expr a b = genOpApp a and_RDR b ----------------------------------------------------------------------- eq_Expr :: Type -> LHsExpr GhcPs -> LHsExpr GhcPs -> LHsExpr GhcPs eq_Expr ty a b | not (isUnliftedType ty) = genOpApp a eq_RDR b | otherwise = genPrimOpApp a prim_eq b where (_, _, prim_eq, _, _) = primOrdOps "Eq" ty untag_Expr :: [(RdrName, RdrName)] -> LHsExpr GhcPs -> LHsExpr GhcPs untag_Expr [] expr = expr untag_Expr ((untag_this, put_tag_here) : more) expr = nlHsCase (nlHsPar (nlHsVarApps dataToTag_RDR [untag_this])) {-of-} [mkHsCaseAlt (nlVarPat put_tag_here) (untag_Expr more expr)] enum_from_to_Expr :: LHsExpr GhcPs -> LHsExpr GhcPs -> LHsExpr GhcPs enum_from_then_to_Expr :: LHsExpr GhcPs -> LHsExpr GhcPs -> LHsExpr GhcPs -> LHsExpr GhcPs enum_from_to_Expr f t2 = nlHsApp (nlHsApp (nlHsVar enumFromTo_RDR) f) t2 enum_from_then_to_Expr f t t2 = nlHsApp (nlHsApp (nlHsApp (nlHsVar enumFromThenTo_RDR) f) t) t2 showParen_Expr :: LHsExpr GhcPs -> LHsExpr GhcPs -> LHsExpr GhcPs showParen_Expr e1 e2 = nlHsApp (nlHsApp (nlHsVar showParen_RDR) e1) e2 nested_compose_Expr :: [LHsExpr GhcPs] -> LHsExpr GhcPs nested_compose_Expr [] = panic "nested_compose_expr" -- Arg is always non-empty nested_compose_Expr [e] = parenify e nested_compose_Expr (e:es) = nlHsApp (nlHsApp (nlHsVar compose_RDR) (parenify e)) (nested_compose_Expr es) -- impossible_Expr is used in case RHSs that should never happen. -- We generate these to keep the desugarer from complaining that they *might* happen! error_Expr :: String -> LHsExpr GhcPs error_Expr string = nlHsApp (nlHsVar error_RDR) (nlHsLit (mkHsString string)) -- illegal_Expr is used when signalling error conditions in the RHS of a derived -- method. It is currently only used by Enum.{succ,pred} illegal_Expr :: String -> String -> String -> LHsExpr GhcPs illegal_Expr meth tp msg = nlHsApp (nlHsVar error_RDR) (nlHsLit (mkHsString (meth ++ '{':tp ++ "}: " ++ msg))) -- illegal_toEnum_tag is an extended version of illegal_Expr, which also allows you -- to include the value of a_RDR in the error string. illegal_toEnum_tag :: String -> RdrName -> LHsExpr GhcPs illegal_toEnum_tag tp maxtag = nlHsApp (nlHsVar error_RDR) (nlHsApp (nlHsApp (nlHsVar append_RDR) (nlHsLit (mkHsString ("toEnum{" ++ tp ++ "}: tag (")))) (nlHsApp (nlHsApp (nlHsApp (nlHsVar showsPrec_RDR) (nlHsIntLit 0)) (nlHsVar a_RDR)) (nlHsApp (nlHsApp (nlHsVar append_RDR) (nlHsLit (mkHsString ") is outside of enumeration's range (0,"))) (nlHsApp (nlHsApp (nlHsApp (nlHsVar showsPrec_RDR) (nlHsIntLit 0)) (nlHsVar maxtag)) (nlHsLit (mkHsString ")")))))) parenify :: LHsExpr GhcPs -> LHsExpr GhcPs parenify e@(L _ (HsVar _ _)) = e parenify e = mkHsPar e -- genOpApp wraps brackets round the operator application, so that the -- renamer won't subsequently try to re-associate it. genOpApp :: LHsExpr GhcPs -> RdrName -> LHsExpr GhcPs -> LHsExpr GhcPs genOpApp e1 op e2 = nlHsPar (nlHsOpApp e1 op e2) genPrimOpApp :: LHsExpr GhcPs -> RdrName -> LHsExpr GhcPs -> LHsExpr GhcPs genPrimOpApp e1 op e2 = nlHsPar (nlHsApp (nlHsVar tagToEnum_RDR) (nlHsOpApp e1 op e2)) a_RDR, b_RDR, c_RDR, d_RDR, f_RDR, k_RDR, z_RDR, ah_RDR, bh_RDR, ch_RDR, dh_RDR :: RdrName a_RDR = mkVarUnqual (fsLit "a") b_RDR = mkVarUnqual (fsLit "b") c_RDR = mkVarUnqual (fsLit "c") d_RDR = mkVarUnqual (fsLit "d") f_RDR = mkVarUnqual (fsLit "f") k_RDR = mkVarUnqual (fsLit "k") z_RDR = mkVarUnqual (fsLit "z") ah_RDR = mkVarUnqual (fsLit "a#") bh_RDR = mkVarUnqual (fsLit "b#") ch_RDR = mkVarUnqual (fsLit "c#") dh_RDR = mkVarUnqual (fsLit "d#") as_RDRs, bs_RDRs, cs_RDRs :: [RdrName] as_RDRs = [ mkVarUnqual (mkFastString ("a"++show i)) | i <- [(1::Int) .. ] ] bs_RDRs = [ mkVarUnqual (mkFastString ("b"++show i)) | i <- [(1::Int) .. ] ] cs_RDRs = [ mkVarUnqual (mkFastString ("c"++show i)) | i <- [(1::Int) .. ] ] a_Expr, b_Expr, c_Expr, z_Expr, ltTag_Expr, eqTag_Expr, gtTag_Expr, false_Expr, true_Expr, pure_Expr, unsafeCodeCoerce_Expr :: LHsExpr GhcPs a_Expr = nlHsVar a_RDR b_Expr = nlHsVar b_RDR c_Expr = nlHsVar c_RDR z_Expr = nlHsVar z_RDR ltTag_Expr = nlHsVar ltTag_RDR eqTag_Expr = nlHsVar eqTag_RDR gtTag_Expr = nlHsVar gtTag_RDR false_Expr = nlHsVar false_RDR true_Expr = nlHsVar true_RDR pure_Expr = nlHsVar pure_RDR unsafeCodeCoerce_Expr = nlHsVar unsafeCodeCoerce_RDR a_Pat, b_Pat, c_Pat, d_Pat, k_Pat, z_Pat :: LPat GhcPs a_Pat = nlVarPat a_RDR b_Pat = nlVarPat b_RDR c_Pat = nlVarPat c_RDR d_Pat = nlVarPat d_RDR k_Pat = nlVarPat k_RDR z_Pat = nlVarPat z_RDR minusInt_RDR, tagToEnum_RDR :: RdrName minusInt_RDR = getRdrName (primOpId IntSubOp ) tagToEnum_RDR = getRdrName (primOpId TagToEnumOp) new_tag2con_rdr_name, new_maxtag_rdr_name :: SrcSpan -> TyCon -> TcM RdrName -- Generates Exact RdrNames, for the binding positions new_tag2con_rdr_name dflags tycon = new_tc_deriv_rdr_name dflags tycon mkTag2ConOcc new_maxtag_rdr_name dflags tycon = new_tc_deriv_rdr_name dflags tycon mkMaxTagOcc new_dataT_rdr_name :: SrcSpan -> TyCon -> TcM RdrName new_dataT_rdr_name dflags tycon = new_tc_deriv_rdr_name dflags tycon mkDataTOcc new_dataC_rdr_name :: SrcSpan -> DataCon -> TcM RdrName new_dataC_rdr_name dflags dc = new_dc_deriv_rdr_name dflags dc mkDataCOcc new_tc_deriv_rdr_name :: SrcSpan -> TyCon -> (OccName -> OccName) -> TcM RdrName new_tc_deriv_rdr_name loc tycon occ_fun = newAuxBinderRdrName loc (tyConName tycon) occ_fun new_dc_deriv_rdr_name :: SrcSpan -> DataCon -> (OccName -> OccName) -> TcM RdrName new_dc_deriv_rdr_name loc dc occ_fun = newAuxBinderRdrName loc (dataConName dc) occ_fun -- | Generate the name for an auxiliary binding, giving it a fresh 'Unique'. -- Returns an 'Exact' 'RdrName' with an underlying 'System' 'Name'. -- See @Note [Auxiliary binders]@. newAuxBinderRdrName :: SrcSpan -> Name -> (OccName -> OccName) -> TcM RdrName newAuxBinderRdrName loc parent occ_fun = do uniq <- newUnique pure $ Exact $ mkSystemNameAt uniq (occ_fun (nameOccName parent)) loc -- | @getPossibleDataCons tycon tycon_args@ returns the constructors of @tycon@ -- whose return types match when checked against @tycon_args@. -- -- See Note [Filter out impossible GADT data constructors] getPossibleDataCons :: TyCon -> [Type] -> [DataCon] getPossibleDataCons tycon tycon_args = filter isPossible $ tyConDataCons tycon where isPossible dc = not $ dataConCannotMatch (dataConInstUnivs dc tycon_args) dc -- | Information about the arguments to the class in a stock- or -- newtype-derived instance. For a @deriving@-generated instance declaration -- such as this one: -- -- @ -- instance Ctx => Cls cls_ty_1 ... cls_ty_m (TC tc_arg_1 ... tc_arg_n) where ... -- @ -- -- * 'dit_cls_tys' corresponds to @cls_ty_1 ... cls_ty_m@. -- -- * 'dit_tc' corresponds to @TC@. -- -- * 'dit_tc_args' corresponds to @tc_arg_1 ... tc_arg_n@. -- -- See @Note [DerivEnv and DerivSpecMechanism]@ in "GHC.Tc.Deriv.Utils" for a -- more in-depth explanation, including the relationship between -- 'dit_tc'/'dit_rep_tc' and 'dit_tc_args'/'dit_rep_tc_args'. -- -- A 'DerivInstTys' value can be seen as a more structured representation of -- the 'denv_inst_tys' in a 'DerivEnv', as the 'denv_inst_tys' is equal to -- @dit_cls_tys ++ ['mkTyConApp' dit_tc dit_tc_args]@. Other parts of the -- instance declaration can be found in the 'DerivEnv'. For example, the @Cls@ -- in the example above corresponds to the 'denv_cls' field of 'DerivEnv'. -- -- Similarly, the type variables that appear in a 'DerivInstTys' value are the -- same type variables as the 'denv_tvs' in the parent 'DerivEnv'. Accordingly, -- if we are inferring an instance context, the type variables will be 'TcTyVar' -- skolems. Otherwise, they will be ordinary 'TyVar's. -- See @Note [Overlap and deriving]@ in "GHC.Tc.Deriv.Infer". data DerivInstTys = DerivInstTys { dit_cls_tys :: [Type] -- ^ Other arguments to the class except the last , dit_tc :: TyCon -- ^ Type constructor for which the instance is requested -- (last arguments to the type class) , dit_tc_args :: [Type] -- ^ Arguments to the type constructor , dit_rep_tc :: TyCon -- ^ The representation tycon for 'dit_tc' -- (for data family instances). Otherwise the same as 'dit_tc'. , dit_rep_tc_args :: [Type] -- ^ The representation types for 'dit_tc_args' -- (for data family instances). Otherwise the same as 'dit_tc_args'. , dit_dc_inst_arg_env :: DataConEnv [Type] -- ^ The cached results of instantiating each data constructor's field -- types using @'dataConInstUnivs' data_con 'dit_rep_tc_args'@. -- See @Note [Instantiating field types in stock deriving]@. -- -- This field is only used for stock-derived instances and goes unused -- for newtype-derived instances. It is put here mainly for the sake of -- convenience. } instance Outputable DerivInstTys where ppr (DerivInstTys { dit_cls_tys = cls_tys, dit_tc = tc, dit_tc_args = tc_args , dit_rep_tc = rep_tc, dit_rep_tc_args = rep_tc_args , dit_dc_inst_arg_env = dc_inst_arg_env }) = hang (text "DerivInstTys") 2 (vcat [ text "dit_cls_tys" <+> ppr cls_tys , text "dit_tc" <+> ppr tc , text "dit_tc_args" <+> ppr tc_args , text "dit_rep_tc" <+> ppr rep_tc , text "dit_rep_tc_args" <+> ppr rep_tc_args , text "dit_dc_inst_arg_env" <+> ppr dc_inst_arg_env ]) -- | Look up a data constructor's instantiated field types in a 'DerivInstTys'. -- See @Note [Instantiating field types in stock deriving]@. derivDataConInstArgTys :: DataCon -> DerivInstTys -> [Type] derivDataConInstArgTys dc dit = case lookupUFM (dit_dc_inst_arg_env dit) dc of Just inst_arg_tys -> inst_arg_tys Nothing -> pprPanic "derivDataConInstArgTys" (ppr dc) -- | @'buildDataConInstArgEnv' tycon arg_tys@ constructs a cache that maps -- each of @tycon@'s data constructors to their field types, with are to be -- instantiated with @arg_tys@. -- See @Note [Instantiating field types in stock deriving]@. buildDataConInstArgEnv :: TyCon -> [Type] -> DataConEnv [Type] buildDataConInstArgEnv rep_tc rep_tc_args = listToUFM [ (dc, inst_arg_tys) | dc <- tyConDataCons rep_tc , let (_, _, inst_arg_tys) = dataConInstSig dc $ dataConInstUnivs dc rep_tc_args ] -- | Apply a substitution to all of the 'Type's contained in a 'DerivInstTys'. -- See @Note [Instantiating field types in stock deriving]@ for why we need to -- substitute into a 'DerivInstTys' in the first place. substDerivInstTys :: TCvSubst -> DerivInstTys -> DerivInstTys substDerivInstTys subst dit@(DerivInstTys { dit_cls_tys = cls_tys, dit_tc_args = tc_args , dit_rep_tc = rep_tc, dit_rep_tc_args = rep_tc_args }) | isEmptyTCvSubst subst = dit | otherwise = dit{ dit_cls_tys = cls_tys' , dit_tc_args = tc_args' , dit_rep_tc_args = rep_tc_args' , dit_dc_inst_arg_env = buildDataConInstArgEnv rep_tc rep_tc_args' } where cls_tys' = substTys subst cls_tys tc_args' = substTys subst tc_args rep_tc_args' = substTys subst rep_tc_args -- | Zonk the 'TcTyVar's in a 'DerivInstTys' value to 'TyVar's. -- See @Note [What is zonking?]@ in "GHC.Tc.Utils.TcMType". -- -- This is only used in the final zonking step when inferring -- the context for a derived instance. -- See @Note [Overlap and deriving]@ in "GHC.Tc.Deriv.Infer". zonkDerivInstTys :: ZonkEnv -> DerivInstTys -> TcM DerivInstTys zonkDerivInstTys ze dit@(DerivInstTys { dit_cls_tys = cls_tys , dit_tc_args = tc_args , dit_rep_tc = rep_tc , dit_rep_tc_args = rep_tc_args }) = do cls_tys' <- zonkTcTypesToTypesX ze cls_tys tc_args' <- zonkTcTypesToTypesX ze tc_args rep_tc_args' <- zonkTcTypesToTypesX ze rep_tc_args pure dit{ dit_cls_tys = cls_tys' , dit_tc_args = tc_args' , dit_rep_tc_args = rep_tc_args' , dit_dc_inst_arg_env = buildDataConInstArgEnv rep_tc rep_tc_args' } {- Note [Auxiliary binders] ~~~~~~~~~~~~~~~~~~~~~~~~ We often want to make top-level auxiliary bindings in derived instances. For example, derived Ix instances sometimes generate code like this: data T = ... deriving instance Ix T ==> instance Ix T where range (a, b) = map tag2con_T [dataToTag# a .. dataToTag# b] $tag2con_T :: Int -> T $tag2con_T = ...code.... Note that multiple instances of the same type might need to use the same sort of auxiliary binding. For example, $tag2con is used not only in derived Ix instances, but also in derived Enum instances: deriving instance Enum T ==> instance Enum T where toEnum i = tag2con_T i $tag2con_T :: Int -> T $tag2con_T = ...code.... How do we ensure that the two usages of $tag2con_T do not conflict with each other? We do so by generating a separate $tag2con_T definition for each instance, giving each definition an Exact RdrName with a separate Unique to avoid name clashes: instance Ix T where range (a, b) = map tag2con_T{Uniq2} [dataToTag# a .. dataToTag# b] instance Enum T where toEnum a = $tag2con_T{Uniq2} a -- $tag2con_T{Uniq1} and $tag2con_T{Uniq2} are Exact RdrNames with -- underlying System Names $tag2con_T{Uniq1} :: Int -> T $tag2con_T{Uniq1} = ...code.... $tag2con_T{Uniq2} :: Int -> T $tag2con_T{Uniq2} = ...code.... Note that: * This is /precisely/ the same mechanism that we use for Template Haskell–generated code. See Note [Binders in Template Haskell] in GHC.ThToHs. There we explain why we use a 'System' flavour of the Name we generate. * See "Wrinkle: Reducing code duplication" for how we can avoid generating lots of duplicated code in common situations. * See "Wrinkle: Why we sometimes do generated duplicate code" for why this de-duplication mechanism isn't perfect, so we fall back to CSE (which is very effective within a single module). * Note that the "_T" part of "$tag2con_T" is just for debug-printing purposes. We could call them all "$tag2con", or even just "aux". The Unique is enough to keep them separate. This is important: we might be generating an Eq instance for two completely-distinct imported type constructors T. At first glance, it might appear that this plan is infeasible, as it would require generating multiple top-level declarations with the same OccName. But what if auxiliary bindings /weren't/ top-level? Conceptually, we could imagine that auxiliary bindings are /local/ to the instance declarations in which they are used. Using some hypothetical Haskell syntax, it might look like this: let { $tag2con_T{Uniq1} :: Int -> T $tag2con_T{Uniq1} = ...code.... $tag2con_T{Uniq2} :: Int -> T $tag2con_T{Uniq2} = ...code.... } in { instance Ix T where range (a, b) = map tag2con_T{Uniq2} [dataToTag# a .. dataToTag# b] instance Enum T where toEnum a = $tag2con_T{Uniq2} a } Making auxiliary bindings local is key to making this work, since GHC will not reject local bindings with duplicate names provided that: * Each binding has a distinct unique, and * Each binding has an Exact RdrName with a System Name. Even though the hypothetical Haskell syntax above does not exist, we can accomplish the same end result through some sleight of hand in renameDeriv: we rename auxiliary bindings with rnLocalValBindsLHS. (If we had used rnTopBindsLHS instead, then GHC would spuriously reject auxiliary bindings with the same OccName as duplicates.) Luckily, no special treatment is needed to typecheck them; we can typecheck them as normal top-level bindings (using tcTopBinds) without danger. ----- -- Wrinkle: Reducing code duplication ----- While the approach of generating copies of each sort of auxiliary binder per derived instance is simpler, it can lead to code bloat if done naïvely. Consider this example: data T = ... deriving instance Eq T deriving instance Ord T ==> instance Ix T where range (a, b) = map tag2con_T{Uniq2} [dataToTag# a .. dataToTag# b] instance Enum T where toEnum a = $tag2con_T{Uniq2} a $tag2con_T{Uniq1} :: Int -> T $tag2con_T{Uniq1} = ...code.... $tag2con_T{Uniq2} :: Int -> T $tag2con_T{Uniq2} = ...code.... $tag2con_T{Uniq1} and $tag2con_T{Uniq2} are blatant duplicates of each other, which is not ideal. Surely GHC can do better than that at the very least! And indeed it does. Within the genAuxBinds function, GHC performs a small CSE-like pass to define duplicate auxiliary binders in terms of the original one. On the example above, that would look like this: $tag2con_T{Uniq1} :: Int -> T $tag2con_T{Uniq1} = ...code.... $tag2con_T{Uniq2} :: Int -> T $tag2con_T{Uniq2} = $tag2con_T{Uniq1} (Note that this pass does not cover all possible forms of code duplication. See "Wrinkle: Why we sometimes do generate duplicate code" for situations where genAuxBinds does not deduplicate code.) To start, genAuxBinds is given a list of AuxBindSpecs, which describe the sort of auxiliary bindings that must be generates along with their RdrNames. As genAuxBinds processes this list, it marks the first occurrence of each sort of auxiliary binding as the "original". For example, if genAuxBinds sees a DerivCon2Tag for the first time (with the RdrName $tag2con_T{Uniq1}), then it will generate the full code for a $tag2con binding: $tag2con_T{Uniq1} :: Int -> T $tag2con_T{Uniq1} = ...code.... Later, if genAuxBinds sees any additional DerivCon2Tag values, it will treat them as duplicates. For example, if genAuxBinds later sees a DerivCon2Tag with the RdrName $tag2con_T{Uniq2}, it will generate this code, which is much more compact: $tag2con_T{Uniq2} :: Int -> T $tag2con_T{Uniq2} = $tag2con_T{Uniq1} An alternative approach would be /not/ performing any kind of deduplication in genAuxBinds at all and simply relying on GHC's simplifier to perform this kind of CSE. But this is a more expensive analysis in general, while genAuxBinds can accomplish the same result with a simple check. ----- -- Wrinkle: Why we sometimes do generate duplicate code ----- It is worth noting that deduplicating auxiliary binders is difficult in the general case. Here are two particular examples where GHC cannot easily remove duplicate copies of an auxiliary binding: 1. When derived instances are contained in different modules, as in the following example: module A where data T = ... module B where import A deriving instance Ix T module C where import B deriving instance Enum T The derived Eq and Enum instances for T make use of $tag2con_T, and since they are defined in separate modules, each module must produce its own copy of $tag2con_T. 2. When derived instances are separated by TH splices (#18321), as in the following example: module M where data T = ... deriving instance Ix T $(pure []) deriving instance Enum T Due to the way that GHC typechecks TyClGroups, genAuxBinds will run twice in this program: once for all the declarations before the TH splice, and once again for all the declarations after the TH splice. As a result, $tag2con_T will be generated twice, since genAuxBinds will be unable to recognize the presence of duplicates. These situations are much rarer, so we do not spend any effort to deduplicate auxiliary bindings there. Instead, we focus on the common case of multiple derived instances within the same module, not separated by any TH splices. (This is the case described in "Wrinkle: Reducing code duplication".) In situation (1), we can at least fall back on GHC's simplifier to pick up genAuxBinds' slack. Note [Filter out impossible GADT data constructors] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Some stock-derivable classes will filter out impossible GADT data constructors, to rule out problematic constructors when deriving instances. e.g. ``` data Foo a where X :: Foo Int Y :: (Bool -> Bool) -> Foo Bool ``` when deriving an instance on `Foo Int`, `Y` should be treated as if it didn't exist in the first place. For instance, if we write ``` deriving instance Eq (Foo Int) ``` it should generate: ``` instance Eq (Foo Int) where X == X = True ``` Classes that filter constructors: * Eq * Ord * Show * Lift * Functor * Foldable * Traversable Classes that do not filter constructors: * Enum: doesn't make sense for GADTs in the first place * Bounded: only makes sense for GADTs with a single constructor * Ix: only makes sense for GADTs with a single constructor * Read: `Read a` returns `a` instead of consumes `a`, so filtering data constructors would make this function _more_ partial instead of less * Data: derived implementations of gunfold rely on a constructor-indexing scheme that wouldn't work if certain constructors were filtered out * Generic/Generic1: doesn't make sense for GADTs Classes that do not currently filter constructors may do so in the future, if there is a valid use-case and we have requirements for how they should work. See #16341 and the T16341.hs test case. Note [Instantiating field types in stock deriving] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Figuring out what the types of data constructor fields are in `deriving` can be surprisingly tricky. Here are some examples (adapted from #20375) to set the scene: data Ta = MkTa Int# data Tb (x :: TYPE IntRep) = MkTb x deriving instance Eq Ta -- 1. deriving instance Eq (Tb a) -- 2. deriving instance Eq (Tb Int#) -- 3. Example (1) is accepted, as `deriving Eq` has a special case for fields of type Int#. Example (2) is rejected, however, as the special case for Int# does not extend to all types of kind (TYPE IntRep). Example (3) ought to typecheck. If you instantiate the field of type `x` in MkTb to be Int#, then `deriving Eq` is capable of handling that. We must be careful, however. If we naïvely use, say, `dataConOrigArgTys` to retrieve the field types, then we would get `b`, which `deriving Eq` would reject. In order to handle `deriving Eq` (and, more generally, any stock deriving strategy) correctly, we /must/ instantiate the field types as needed. Not doing so led to #20375 and #20387. In fact, we end up needing to instantiate the field types in quite a few places: * When performing validity checks for stock deriving strategies (e.g., in GHC.Tc.Deriv.Utils.cond_stdOK) * When inferring the instance context in GHC.Tc.Deriv.Infer.inferConstraintStock * When generating code for stock-derived instances in GHC.Tc.Deriv.{Functor,Generate,Generics} Repeatedly performing these instantiations in multiple places would be wasteful, so we build a cache of data constructor field instantiations in the `dit_dc_inst_arg_env` field of DerivInstTys. Specifically: 1. When beginning to generate code for a stock-derived instance `T arg_1 ... arg_n`, the `dit_dc_inst_arg_env` field is created by taking each data constructor `dc`, instantiating its field types with `dataConInstUnivs dc [arg_1, ..., arg_n]`, and mapping `dc` to the instantiated field types in the cache. The `buildDataConInstArgEnv` function is responsible for orchestrating this. 2. When a part of the code in GHC.Tc.Deriv.* needs to look up the field types, we deliberately avoid using `dataConOrigArgTys`. Instead, we use `derivDataConInstArgTys`, which looks up a DataCon's instantiated field types in the cache. StandaloneDeriving is one way for the field types to become instantiated. Another way is by deriving Functor and related classes, as chronicled in Note [Inferring the instance context] in GHC.Tc.Deriv.Infer. Here is one such example: newtype Compose (f :: k -> Type) (g :: j -> k) (a :: j) = Compose (f (g a)) deriving Generic1 This ultimately generates the following instance: instance forall (f :: Type -> Type) (g :: j -> Type). Functor f => Generic1 (Compose f g) where ... Note that because of the inferred `Functor f` constraint, `k` was instantiated to be `Type`. GHC's deriving machinery doesn't realize this until it performs constraint inference (in GHC.Tc.Deriv.Infer.inferConstraintsStock), however, which is *after* the initial DerivInstTys has been created. As a result, the `dit_dc_inst_arg_env` field might need to be updated after constraint inference, as the inferred constraints might instantiate the field types further. This is accomplished by way of `substDerivInstTys`, which substitutes all of the fields in a `DerivInstTys`, including the `dit_dc_inst_arg_env`. It is important to do this in inferConstraintsStock, as the deriving/should_compile/T20387 test case will not compile otherwise. -}