{-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE MultiWayIf #-} {-# LANGUAGE TypeFamilies #-} {- (c) The University of Glasgow 2006 (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 Pattern-matching bindings (HsBinds and MonoBinds) Handles @HsBinds@; those at the top level require different handling, in that the @Rec@/@NonRec@/etc structure is thrown away (whereas at lower levels it is preserved with @let@/@letrec@s). -} module GHC.HsToCore.Binds ( dsTopLHsBinds, dsLHsBinds , dsImpSpecs, decomposeRuleLhs , dsHsWrapper, dsHsWrappers , dsEvTerm, dsTcEvBinds, dsTcEvBinds_s, dsEvBinds , dsWarnOrphanRule ) where import GHC.Prelude import GHC.Driver.DynFlags import GHC.Driver.Config import qualified GHC.LanguageExtensions as LangExt import GHC.Unit.Module import {-# SOURCE #-} GHC.HsToCore.Expr ( dsLExpr ) import {-# SOURCE #-} GHC.HsToCore.Match ( matchWrapper ) import GHC.HsToCore.Pmc.Utils( tracePm ) import GHC.HsToCore.Monad import GHC.HsToCore.Errors.Types import GHC.HsToCore.GuardedRHSs import GHC.HsToCore.Utils import GHC.HsToCore.Pmc ( addTyCs, pmcGRHSs ) import GHC.Hs -- lots of things import GHC.Core -- lots of things import GHC.Core.SimpleOpt import GHC.Core.Opt.OccurAnal ( occurAnalyseExpr ) import GHC.Core.InstEnv ( CanonicalEvidence(..) ) import GHC.Core.Make import GHC.Core.Utils import GHC.Core.Opt.Arity ( etaExpand ) import GHC.Core.Unfold.Make import GHC.Core.FVs import GHC.Core.Predicate import GHC.Core.TyCon import GHC.Core.Type import GHC.Core.Coercion import GHC.Core.Rules import GHC.Core.Ppr( pprCoreBinders ) import GHC.Core.TyCo.Compare( eqType ) import GHC.Builtin.Names import GHC.Builtin.Types ( naturalTy, typeSymbolKind, charTy ) import GHC.Tc.Types.Evidence import GHC.Types.Id import GHC.Types.Id.Info import GHC.Types.Name import GHC.Types.Var.Set import GHC.Types.Var.Env import GHC.Types.Var( EvVar, mkLocalVar ) import GHC.Types.SrcLoc import GHC.Types.Basic import GHC.Types.Unique.Set( nonDetEltsUniqSet ) import GHC.Data.Maybe import GHC.Data.OrdList import GHC.Data.Graph.Directed import GHC.Data.Bag import GHC.Utils.Constants (debugIsOn) import GHC.Utils.Misc import GHC.Utils.Monad import GHC.Utils.Outputable import GHC.Utils.Panic import Control.Monad {-********************************************************************** * * Desugaring a MonoBinds * * **********************************************************************-} -- | Desugar top level binds, strict binds are treated like normal -- binds since there is no good time to force before first usage. dsTopLHsBinds :: LHsBinds GhcTc -> DsM (OrdList (Id,CoreExpr)) dsTopLHsBinds binds -- see Note [Strict binds checks] | not (null unlifted_binds) || not (null bang_binds) = do { mapM_ (top_level_err UnliftedTypeBinds) unlifted_binds ; mapM_ (top_level_err StrictBinds) bang_binds ; return nilOL } | otherwise = do { (force_vars, prs) <- dsLHsBinds binds ; when debugIsOn $ do { xstrict <- xoptM LangExt.Strict ; massertPpr (null force_vars || xstrict) (ppr binds $$ ppr force_vars) } -- with -XStrict, even top-level vars are listed as force vars. ; return (toOL prs) } where unlifted_binds = filter (isUnliftedHsBind . unLoc) binds bang_binds = filter (isBangedHsBind . unLoc) binds top_level_err bindsType (L loc bind) = putSrcSpanDs (locA loc) $ diagnosticDs (DsTopLevelBindsNotAllowed bindsType bind) {- Note [Return non-recursive bindings in dependency order] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ For recursive bindings, the desugarer has no choice: it returns a single big Rec{...} group. But for /non-recursive/ bindings, the desugarer guarantees to desugar them to a sequence of non-recurive Core bindings, in dependency order. Why is this important? Partly it saves a bit of work in the first run of the occurrence analyser. But more importantly, for linear types, non-recursive lets can be linear whereas recursive-let can't. Since we check the output of the desugarer for linearity (see also Note [Linting linearity]), desugaring non-recursive lets to recursive lets would break linearity checks. An alternative is to refine the typing rule for recursive lets so that we don't have to care (see in particular #23218 and #18694), but the outcome of this line of work is still unclear. In the meantime, being a little precise in the desugarer is cheap. (paragraph written on 2023-06-09) In dsLHSBinds (and dependencies), a single binding can be desugared to multiple bindings. For instance because the source binding has the {-# SPECIALIZE #-} pragma. In: f _ = … where {-# SPECIALIZE g :: F Int -> F Int #-} g :: C a => F a -> F a g _ = … The g binding desugars to let { $sg = … } in g [RULES: "SPEC g" g @Int $dC = $sg] g = … In order to avoid generating a letrec that will immediately be reordered, we make sure to return the binding in dependency order [$sg, g]. -} -- | Desugar all other kind of bindings, Ids of strict binds are returned to -- later be forced in the binding group body, see Note [Desugar Strict binds] -- -- Invariant: the desugared bindings are returned in dependency order, -- see Note [Return non-recursive bindings in dependency order] dsLHsBinds :: LHsBinds GhcTc -> DsM ([Id], [(Id,CoreExpr)]) dsLHsBinds binds = do { ds_bs <- mapM dsLHsBind binds ; return (foldr (\(a, a') (b, b') -> (a ++ b, a' ++ b')) ([], []) ds_bs) } ------------------------ dsLHsBind :: LHsBind GhcTc -> DsM ([Id], [(Id,CoreExpr)]) dsLHsBind (L loc bind) = do dflags <- getDynFlags putSrcSpanDs (locA loc) $ dsHsBind dflags bind -- | Desugar a single binding (or group of recursive binds). -- -- Invariant: the desugared bindings are returned in dependency order, -- see Note [Return non-recursive bindings in dependency order] dsHsBind :: DynFlags -> HsBind GhcTc -> DsM ([Id], [(Id,CoreExpr)]) -- ^ The Ids of strict binds, to be forced in the body of the -- binding group see Note [Desugar Strict binds] and all -- bindings and their desugared right hand sides. dsHsBind dflags (VarBind { var_id = var , var_rhs = expr }) = do { core_expr <- dsLExpr expr -- Dictionary bindings are always VarBinds, -- so we only need do this here ; let core_bind@(id,_) = makeCorePair dflags var False 0 core_expr force_var = if xopt LangExt.Strict dflags then [id] else [] ; return (force_var, [core_bind]) } dsHsBind dflags b@(FunBind { fun_id = L loc fun , fun_matches = matches , fun_ext = (co_fn, tick) }) = dsHsWrapper co_fn $ \core_wrap -> do { (args, body) <- matchWrapper (mkPrefixFunRhs (L loc (idName fun)) noAnn) Nothing matches ; let body' = mkOptTickBox tick body rhs = core_wrap (mkLams args body') core_binds@(id,_) = makeCorePair dflags fun False 0 rhs force_var -- Bindings are strict when -XStrict is enabled | xopt LangExt.Strict dflags , matchGroupArity matches == 0 -- no need to force lambdas = [id] | isBangedHsBind b = [id] | otherwise = [] ; --pprTrace "dsHsBind" (vcat [ ppr fun <+> ppr (idInlinePragma fun) -- , ppr (mg_alts matches) -- , ppr args, ppr core_binds, ppr body']) $ return (force_var, [core_binds]) } dsHsBind dflags (PatBind { pat_lhs = pat, pat_rhs = grhss , pat_ext = (ty, (rhs_tick, var_ticks)) }) = do { rhss_nablas <- pmcGRHSs PatBindGuards grhss ; body_expr <- dsGuarded grhss ty rhss_nablas ; let body' = mkOptTickBox rhs_tick body_expr pat' = decideBangHood dflags pat ; (force_var,sel_binds) <- mkSelectorBinds var_ticks pat PatBindRhs body' -- We silently ignore inline pragmas; no makeCorePair -- Not so cool, but really doesn't matter ; let force_var' = if isBangedLPat pat' then [force_var] else [] ; return (force_var', sel_binds) } dsHsBind dflags (XHsBindsLR (AbsBinds { abs_tvs = tyvars, abs_ev_vars = dicts , abs_exports = exports , abs_ev_binds = ev_binds , abs_binds = binds, abs_sig = has_sig })) = addTyCs FromSource (listToBag dicts) $ -- addTyCs: push type constraints deeper -- for inner pattern match check -- See Check, Note [Long-distance information] dsTcEvBinds_s ev_binds $ \ds_ev_binds -> do do { ds_binds <- dsLHsBinds binds -- dsAbsBinds does the hard work ; dsAbsBinds dflags tyvars dicts exports ds_ev_binds ds_binds (isSingleton binds) has_sig } dsHsBind _ (PatSynBind{}) = panic "dsHsBind: PatSynBind" ----------------------- dsAbsBinds :: DynFlags -> [TyVar] -> [EvVar] -> [ABExport] -> [CoreBind] -- Desugared evidence bindings -> ([Id], [(Id,CoreExpr)]) -- Desugared value bindings -> Bool -- Single source binding -> Bool -- Single binding with signature -> DsM ([Id], [(Id,CoreExpr)]) dsAbsBinds dflags tyvars dicts exports ds_ev_binds (force_vars, bind_prs) is_singleton has_sig -- A very important common case: one exported variable -- Non-recursive bindings come through this way -- So do self-recursive bindings -- gbl_id = wrap (/\tvs \dicts. let ev_binds -- letrec bind_prs -- in lcl_id) | [export] <- exports , ABE { abe_poly = global_id, abe_mono = local_id , abe_wrap = wrap, abe_prags = prags } <- export , Just force_vars' <- case force_vars of [] -> Just [] [v] | v == local_id -> Just [global_id] _ -> Nothing -- If there is a variable to force, it's just the -- single variable we are binding here = do { dsHsWrapper wrap $ \core_wrap -> do -- Usually the identity { let rhs = core_wrap $ mkLams tyvars $ mkLams dicts $ mkCoreLets ds_ev_binds $ body body | has_sig , [(_, lrhs)] <- bind_prs = lrhs | otherwise = mkLetRec bind_prs (Var local_id) ; (spec_binds, rules) <- dsSpecs rhs prags ; let global_id' = addIdSpecialisations global_id rules main_bind = makeCorePair dflags global_id' (isDefaultMethod prags) (dictArity dicts) rhs ; return (force_vars', fromOL spec_binds ++ [main_bind]) } } -- Another common case: no tyvars, no dicts -- In this case we can have a much simpler desugaring -- lcl_id{inl-prag} = rhs -- Auxiliary binds -- gbl_id = lcl_id |> co -- Main binds -- -- See Note [The no-tyvar no-dict case] | null tyvars, null dicts = do { let wrap_first_bind f ((main, main_rhs):other_binds) = ((main, f main_rhs):other_binds) wrap_first_bind _ [] = panic "dsAbsBinds received an empty binding list" mk_main :: ABExport -> DsM (Id, CoreExpr) mk_main (ABE { abe_poly = gbl_id, abe_mono = lcl_id , abe_wrap = wrap }) -- No SpecPrags (no dicts) -- Can't be a default method (default methods are singletons) = do { dsHsWrapper wrap $ \core_wrap -> do { return ( gbl_id `setInlinePragma` defaultInlinePragma , core_wrap (Var lcl_id)) } } ; main_prs <- mapM mk_main exports ; let bind_prs' = map mk_aux_bind bind_prs -- When there's a single source binding, we wrap the evidence binding in a -- separate let-rec (DSB1) inside the first desugared binding (DSB2). -- See Note [The no-tyvar no-dict case]. final_prs | is_singleton = wrap_first_bind (mkCoreLets ds_ev_binds) bind_prs' | otherwise = flattenBinds ds_ev_binds ++ bind_prs' ; return (force_vars, final_prs ++ main_prs ) } -- The general case -- See Note [Desugaring AbsBinds] | otherwise = do { let aux_binds = Rec (map mk_aux_bind bind_prs) -- Monomorphic recursion possible, hence Rec new_force_vars = get_new_force_vars force_vars locals = map abe_mono exports all_locals = locals ++ new_force_vars tup_expr = mkBigCoreVarTup all_locals tup_ty = exprType tup_expr ; let poly_tup_rhs = mkLams tyvars $ mkLams dicts $ mkCoreLets ds_ev_binds $ mkLet aux_binds $ tup_expr ; poly_tup_id <- newSysLocalMDs (exprType poly_tup_rhs) -- Find corresponding global or make up a new one: sometimes -- we need to make new export to desugar strict binds, see -- Note [Desugar Strict binds] ; (exported_force_vars, extra_exports) <- get_exports force_vars ; let mk_bind (ABE { abe_wrap = wrap , abe_poly = global , abe_mono = local, abe_prags = spec_prags }) -- See Note [ABExport wrapper] in "GHC.Hs.Binds" = do { tup_id <- newSysLocalMDs tup_ty ; dsHsWrapper wrap $ \core_wrap -> do { let rhs = core_wrap $ mkLams tyvars $ mkLams dicts $ mkBigTupleSelector all_locals local tup_id $ mkVarApps (Var poly_tup_id) (tyvars ++ dicts) rhs_for_spec = Let (NonRec poly_tup_id poly_tup_rhs) rhs ; (spec_binds, rules) <- dsSpecs rhs_for_spec spec_prags ; let global' = (global `setInlinePragma` defaultInlinePragma) `addIdSpecialisations` rules -- Kill the INLINE pragma because it applies to -- the user written (local) function. The global -- Id is just the selector. Hmm. ; return (fromOL spec_binds ++ [(global', rhs)]) } } ; export_binds_s <- mapM mk_bind (exports ++ extra_exports) ; return ( exported_force_vars , (poly_tup_id, poly_tup_rhs) : concat export_binds_s) } where mk_aux_bind :: (Id,CoreExpr) -> (Id,CoreExpr) mk_aux_bind (lcl_id, rhs) = let lcl_w_inline = lookupVarEnv inline_env lcl_id `orElse` lcl_id in makeCorePair dflags lcl_w_inline False 0 rhs inline_env :: IdEnv Id -- Maps a monomorphic local Id to one with -- the inline pragma from the source -- The type checker put the inline pragma -- on the *global* Id, so we need to transfer it inline_env = mkVarEnv [ (lcl_id, setInlinePragma lcl_id prag) | ABE { abe_mono = lcl_id, abe_poly = gbl_id } <- exports , let prag = idInlinePragma gbl_id ] global_env :: IdEnv Id -- Maps local Id to its global exported Id global_env = mkVarEnv [ (local, global) | ABE { abe_mono = local, abe_poly = global } <- exports ] -- find variables that are not exported get_new_force_vars lcls = foldr (\lcl acc -> case lookupVarEnv global_env lcl of Just _ -> acc Nothing -> lcl:acc) [] lcls -- find exports or make up new exports for force variables get_exports :: [Id] -> DsM ([Id], [ABExport]) get_exports lcls = foldM (\(glbls, exports) lcl -> case lookupVarEnv global_env lcl of Just glbl -> return (glbl:glbls, exports) Nothing -> do export <- mk_export lcl let glbl = abe_poly export return (glbl:glbls, export:exports)) ([],[]) lcls mk_export local = do global <- newSysLocalMDs (exprType (mkLams tyvars (mkLams dicts (Var local)))) return (ABE { abe_poly = global , abe_mono = local , abe_wrap = WpHole , abe_prags = SpecPrags [] }) -- | This is where we apply INLINE and INLINABLE pragmas. All we need to -- do is to attach the unfolding information to the Id. -- -- Other decisions about whether to inline are made in -- `calcUnfoldingGuidance` but the decision about whether to then expose -- the unfolding in the interface file is made in `GHC.Iface.Tidy.addExternal` -- using this information. ------------------------ makeCorePair :: DynFlags -> Id -> Bool -> Arity -> CoreExpr -> (Id, CoreExpr) makeCorePair dflags gbl_id is_default_method dict_arity rhs | is_default_method -- Default methods are *always* inlined -- See Note [INLINE and default methods] in GHC.Tc.TyCl.Instance = (gbl_id `setIdUnfolding` mkCompulsoryUnfolding' simpl_opts rhs, rhs) | otherwise = case inlinePragmaSpec inline_prag of NoUserInlinePrag -> (gbl_id, rhs) NoInline {} -> (gbl_id, rhs) Opaque {} -> (gbl_id, rhs) Inlinable {} -> (gbl_id `setIdUnfolding` inlinable_unf, rhs) Inline {} -> inline_pair where simpl_opts = initSimpleOpts dflags inline_prag = idInlinePragma gbl_id inlinable_unf = mkInlinableUnfolding simpl_opts StableUserSrc rhs inline_pair | Just arity <- inlinePragmaSat inline_prag -- Add an Unfolding for an INLINE (but not for NOINLINE) -- And eta-expand the RHS; see Note [Eta-expanding INLINE things] , let real_arity = dict_arity + arity -- NB: The arity passed to mkInlineUnfoldingWithArity -- must take account of the dictionaries = ( gbl_id `setIdUnfolding` mkInlineUnfoldingWithArity simpl_opts StableUserSrc real_arity rhs , etaExpand real_arity rhs) | otherwise = pprTrace "makeCorePair: arity missing" (ppr gbl_id) $ (gbl_id `setIdUnfolding` mkInlineUnfoldingNoArity simpl_opts StableUserSrc rhs, rhs) dictArity :: [Var] -> Arity -- Don't count coercion variables in arity dictArity dicts = count isId dicts {- Note [Desugaring AbsBinds] ~~~~~~~~~~~~~~~~~~~~~~~~~~ In the general AbsBinds case we desugar the binding to this: tup a (d:Num a) = let fm = ...gm... gm = ...fm... in (fm,gm) f a d = case tup a d of { (fm,gm) -> fm } g a d = case tup a d of { (fm,gm) -> fm } Note [Rules and inlining] ~~~~~~~~~~~~~~~~~~~~~~~~~ Common special case: no type or dictionary abstraction This is a bit less trivial than you might suppose The naive way would be to desugar to something like f_lcl = ...f_lcl... -- The "binds" from AbsBinds M.f = f_lcl -- Generated from "exports" But we don't want that, because if M.f isn't exported, it'll be inlined unconditionally at every call site (its rhs is trivial). That would be ok unless it has RULES, which would thereby be completely lost. Bad, bad, bad. Instead we want to generate M.f = ...f_lcl... f_lcl = M.f Now all is cool. The RULES are attached to M.f (by SimplCore), and f_lcl is rapidly inlined away. This does not happen in the same way to polymorphic binds, because they desugar to M.f = /\a. let f_lcl = ...f_lcl... in f_lcl Although I'm a bit worried about whether full laziness might float the f_lcl binding out and then inline M.f at its call site Note [Specialising in no-dict case] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Even if there are no tyvars or dicts, we may have specialisation pragmas. Class methods can generate AbsBinds [] [] [( ... spec-prag] { AbsBinds [tvs] [dicts] ...blah } So the overloading is in the nested AbsBinds. A good example is in GHC.Float: class (Real a, Fractional a) => RealFrac a where round :: (Integral b) => a -> b instance RealFrac Float where {-# SPECIALIZE round :: Float -> Int #-} The top-level AbsBinds for $cround has no tyvars or dicts (because the instance does not). But the method is locally overloaded! Note [The no-tyvar no-dict case] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Suppose we are desugaring AbsBinds { tyvars = [] , dicts = [] , exports = [ ABE f fm, ABE g gm ] , binds = B , ev_binds = EB } That is: no type variables or dictionary abstractions. Here, `f` and `fm` are the polymorphic and monomorphic versions of `f`; in this special case they will both have the same type. Specialising Note [Desugaring AbsBinds] for this case gives the desugaring tup = letrec EB' in letrec B' in (fm,gm) f = case tup of { (fm,gm) -> fm } g = case tup of { (fm,gm) -> fm } where B' is the result of desugaring B. This desugaring is a little silly: we don't need the intermediate tuple (contrast with the general case where fm and f have different types). So instead, in this case, we desugar to EB'; B'; f=fm; g=gm This is done in the `null tyvars, null dicts` case of `dsAbsBinds`. But there is a wrinkle (DSB1). If the original binding group was /non-recursive/, we want to return a bunch of non-recursive bindings in dependency order: see Note [Return non-recursive bindings in dependency order]. But there is no guarantee that EB', the desugared evidence bindings, will be non-recursive. Happily, in the non-recursive case, B will have just a single binding (f = rhs), so we can wrap EB' around its RHS, thus: fm = letrec EB' in rhs; f = fm There is a sub-wrinkle (DSB2). If B is a /pattern/ bindings, it will desugar to a "main" binding followed by a bunch of selectors. The main binding always comes first, so we can pick it out and wrap EB' around its RHS. For example AbsBinds { tyvars = [] , dicts = [] , exports = [ ABE p pm, ABE q qm ] , binds = PatBind (pm, Just qm) rhs , ev_binds = EB } can desguar to pt = let EB' in case rhs of (pm,Just qm) -> (pm,qm) pm = case pt of (pm,qm) -> pm qm = case pt of (pm,qm) -> qm p = pm q = qm The first three bindings come from desugaring the PatBind, and subsequently wrapping the RHS of the main binding in EB'. Why got to this trouble? It's a common case, and it removes the quadratic-sized tuple desugaring. Less clutter, hopefully faster compilation, especially in a case where there are a *lot* of bindings. Note [Eta-expanding INLINE things] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider foo :: Eq a => a -> a {-# INLINE foo #-} foo x = ... If (foo d) ever gets floated out as a common sub-expression (which can happen as a result of method sharing), there's a danger that we never get to do the inlining, which is a Terribly Bad thing given that the user said "inline"! To avoid this we preemptively eta-expand the definition, so that foo has the arity with which it is declared in the source code. In this example it has arity 2 (one for the Eq and one for x). Doing this should mean that (foo d) is a PAP and we don't share it. Note [Nested arities] ~~~~~~~~~~~~~~~~~~~~~ For reasons that are not entirely clear, method bindings come out looking like this: AbsBinds [] [] [$cfromT <= [] fromT] $cfromT [InlPrag=INLINE] :: T Bool -> Bool { AbsBinds [] [] [fromT <= [] fromT_1] fromT :: T Bool -> Bool { fromT_1 ((TBool b)) = not b } } } Note the nested AbsBind. The arity for the unfolding on $cfromT should be gotten from the binding for fromT_1. It might be better to have just one level of AbsBinds, but that requires more thought! Note [Desugar Strict binds] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ See https://gitlab.haskell.org/ghc/ghc/wikis/strict-pragma Desugaring strict variable bindings looks as follows (core below ==>) let !x = rhs in body ==> let x = rhs in x `seq` body -- seq the variable and if it is a pattern binding the desugaring looks like let !pat = rhs in body ==> let x = rhs -- bind the rhs to a new variable pat = x in x `seq` body -- seq the new variable if there is no variable in the pattern desugaring looks like let False = rhs in body ==> let x = case rhs of {False -> (); _ -> error "Match failed"} in x `seq` body In order to force the Ids in the binding group they are passed around in the dsHsBind family of functions, and later seq'ed in GHC.HsToCore.Expr.ds_val_bind. Consider a recursive group like this letrec f : g = rhs[f,g] in Without `Strict`, we get a translation like this: let t = /\a. letrec tm = rhs[fm,gm] fm = case t of fm:_ -> fm gm = case t of _:gm -> gm in (fm,gm) in let f = /\a. case t a of (fm,_) -> fm in let g = /\a. case t a of (_,gm) -> gm in Here `tm` is the monomorphic binding for `rhs`. With `Strict`, we want to force `tm`, but NOT `fm` or `gm`. Alas, `tm` isn't in scope in the `in ` part. The simplest thing is to return it in the polymorphic tuple `t`, thus: let t = /\a. letrec tm = rhs[fm,gm] fm = case t of fm:_ -> fm gm = case t of _:gm -> gm in (tm, fm, gm) in let f = /\a. case t a of (_,fm,_) -> fm in let g = /\a. case t a of (_,_,gm) -> gm in let tm = /\a. case t a of (tm,_,_) -> tm in tm `seq` See https://gitlab.haskell.org/ghc/ghc/wikis/strict-pragma for a more detailed explanation of the desugaring of strict bindings. Wrinkle 1: forcing linear variables Consider let %1 !x = rhs in ==> let x = rhs in x `seq` In the desugared version x is used in both arguments of seq. This isn't recognised a linear. So we can't strictly speaking use seq. Instead, the code is really desugared as let x = rhs in case x of x { _ -> } The shadowing with the case-binder is crucial. The linear linter (see Note [Linting linearity] in GHC.Core.Lint) understands this as linear. This is what the seqVar function does. To be more precise, suppose x has multiplicity p, the fully annotated seqVar (in Core, p is really stored inside x) is case x of %p x { _ -> } In linear Core, case u of %p y { _ -> v } consumes u with multiplicity p, and makes y available with multiplicity p in v. Which is exactly what we want. Wrinkle 2: linear patterns Consider the following linear binding (linear lets are always non-recursive): let %1 f : g = rhs in The general case would desugar it to let t = let tm = rhs fm = case tm of fm:_ -> fm gm = case tm of _:gm -> gm in (tm, fm, gm) in let f = case t a of (_,fm,_) -> fm in let g = case t a of (_,_,gm) -> gm in let tm = case t a of (tm,_,_) -> tm in tm `seq` But all the case expression drop variables, which is prohibited by linearity. But because this is a non-recursive let (in particular we're desugaring a single binding), we can (and do) desugar the binding as a simple case-expression instead: case rhs of { (f:g) -> } This is handled by the special case: a non-recursive PatBind in GHC.HsToCore.Expr.ds_val_bind. Note [Strict binds checks] ~~~~~~~~~~~~~~~~~~~~~~~~~~ There are several checks around properly formed strict bindings. They all link to this Note. These checks must be here in the desugarer because we cannot know whether or not a type is unlifted until after zonking, due to representation polymorphism. These checks all used to be handled in the typechecker in checkStrictBinds (before Jan '17). We define an "unlifted bind" to be any bind that binds an unlifted id. Note that x :: Char (# True, x #) = blah is *not* an unlifted bind. Unlifted binds are detected by GHC.Hs.Utils.isUnliftedHsBind. Define a "banged bind" to have a top-level bang. Detected by GHC.Hs.Pat.isBangedHsBind. Define a "strict bind" to be either an unlifted bind or a banged bind. The restrictions are: 1. Strict binds may not be top-level. Checked in dsTopLHsBinds. 2. Unlifted binds must also be banged. (There is no trouble to compile an unbanged unlifted bind, but an unbanged bind looks lazy, and we don't want users to be surprised by the strictness of an unlifted bind.) Checked in first clause of GHC.HsToCore.Expr.ds_val_bind. 3. Unlifted binds may not have polymorphism (#6078). (That is, no quantified type variables or constraints.) Checked in first clause of GHC.HsToCore.Expr.ds_val_bind. 4. Unlifted binds may not be recursive. Checked in second clause of ds_val_bind. Note [Desugaring new-form SPECIALISE pragmas] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ "New-form" SPECIALISE pragmas generate a SpecPragE record in the typechecker, which is desugared in this module by `dsSpec`. For the context see Note [Handling new-form SPECIALISE pragmas] in GHC.Tc.Gen.Sig Suppose we have f :: forall a b c d. (Ord a, Ord b, Eq c, Ix d) => ... f = rhs {-# SPECIALISE f @p @[p] @[Int] @(q,r) #-} The type-checker generates `the_call` which looks like spe_bndrs = (dx1 :: Ord p) (dx2::Ix q) (dx3::Ix r) the_call = let d6 = dx1 d2 = $fOrdList d6 d3 = $fEqList $fEqInt d7 = dx1 -- Solver may introduce d1 = d7 -- these indirections d4 = $fIxPair dx2 dx3 in f @p @p @[Int] @(q,r) (d1::Ord p) (d2::Ord [p]) (d3::Eq [Int]) (d4::Ix (q,r) We /could/ generate RULE f d1 d2 d3 d4 e1..en = $sf d1 d2 d3 d4 $sf d1 d2 d3 d4 = d1 d2 d3 d4 But that would do no specialisation at all! What we want is this: RULE f d1 _d2 _d3 d4 e1..en = $sf d1 d4 $sf d1 d4 = let d7 = d1 -- Renaming dx1 = d1 -- Renaming d6 = d1 -- Renaming d2 = $fOrdList d6 d3 = $fEqList $fEqInt in rhs d1 d2 d3 d4 Notice that: * We pass some, but not all, of the matched dictionaries to $sf * We get specialisations for d2 and d3, but not for d1, nor d4. * We had to introduce some renaming bindings at the top to line things up The transformation goes in these steps (S1) decomposeCall: decompose `the_call` into - `binds`: the enclosing let-bindings - `rule_lhs_args`: the arguments of the call itself If the expression in the SPECIALISE pragma had a type signature, such as SPECIALISE g :: Eq b => Int -> b -> b then the desugared expression may have type abstractions and applications "in the way", like this: (/\b. (\d:Eq b). let d1 = $dfOrdInt in f @Int @b d1 d) @b (d2:Eq b) The lambdas come from the type signature, which is then re-instantiated, hence the applications of those lambdas. To handle these, `decomposeCall` uses the simple optimiser to simplify this to let { d = d2; d1 = $dfOrdInt } in f @Int @b d1 d Wrinkle (S1a): do no inlining in this "simple optimiser" phase: use `simpleOptExprNoInline`. E.g. we don't want to turn it into f @Int @b $dfOrdInt d2 because the latter is harder to match. Similarly if we have let { d1=d; d2=d } in f d1 d2 we don't want to inline d1/d2 to get this f d d TL;DR: as a result the dictionary arguments of the actual call, `rule_lhs_args` are all distinct dictionary variables, not expressions. (S2) Compute `rule_bndrs`: the free vars of `rule_lhs_args`, which will be the forall'd template variables of the RULE. In the example, rule_bndrs = d1,d2,d3,d4 These variables will get values from a successful RULE match. (S3) `getRenamings`: starting from the rule_bndrs, make bindings for all other variables that are equal to them. In the example, we make renaming-bindings for d7, dx1, d6. Our goal is to bind as many variables as possible, by deducing them from `rule_bndrs`. If we have the binding d1 = d7 (where `d1` is a rule_bndr) we want to generate the renaming d7 = d1 which instead deduces d7 from d1 NB1: we don't actually have to remove the original bindings; it's harmless to leave them NB2: We also reverse bindings like d1 = d2 |> co, to get d2 = d1 |> sym co It's easy and slightly improves our ability to get bindings for local variables from `rule_bndrs`. I'm not sure if it matters in practice, but it's easy to do. NB3: `getRenamings` works innermost first, so that we get transitivity. E.g in our example we have d7 = dx1 d1 = d7 where d1 is rule_bndr `getRenamings` generate the renamings dx1 = d1 d7 = d2 (S4) `pickSpecBinds`: pick the bindings we want to keep in the specialised function. We start from `known_vars`, the variables we know, namely the `rule_bndrs` and the binders from (S3), which are all equal to one of the `rule_bndrs`. Then we keep a binding if the free vars of its RHS are all known. In our example, `d2` and `d3` are both picked, but `d4` is not. The non-picked ones won't end up being specialised. (S5) Finally, work out which of the `rule_bndrs` we must pass on to specialised function. We just filter out ones bound by a renaming or a picked binding. -} ------------------------ dsSpecs :: CoreExpr -- Its rhs -> TcSpecPrags -> DsM ( OrdList (Id,CoreExpr) -- Binding for specialised Ids , [CoreRule] ) -- Rules for the Global Ids -- See Note [Overview of SPECIALISE pragmas] in GHC.Tc.Gen.Sig dsSpecs _ IsDefaultMethod = return (nilOL, []) dsSpecs poly_rhs (SpecPrags sps) = do { pairs <- mapMaybeM (dsLSpec poly_rhs) sps ; let (spec_binds_s, rules) = unzip pairs ; return (concatOL spec_binds_s, rules) } dsImpSpecs :: [LTcSpecPrag] -> DsM (OrdList (Id,CoreExpr), [CoreRule]) dsImpSpecs imp_specs = do { spec_prs <- mapMaybeM spec_one imp_specs ; let (spec_binds, spec_rules) = unzip spec_prs ; return (concatOL spec_binds, spec_rules) } where spec_one (L loc prag) = putSrcSpanDs loc $ dsSpec (get_rhs prag) prag get_rhs (SpecPrag poly_id _ _) = get_rhs1 poly_id get_rhs (SpecPragE { spe_fn_id = poly_id }) = get_rhs1 poly_id get_rhs1 poly_id | Just unfolding <- maybeUnfoldingTemplate (realIdUnfolding poly_id) = unfolding -- Imported Id; this is its unfolding -- Use realIdUnfolding so we get the unfolding -- even when it is a loop breaker. -- We want to specialise recursive functions! | otherwise = pprPanic "dsImpSpecs" (ppr poly_id) -- The type checker has checked that it *has* an unfolding dsLSpec :: CoreExpr -> Located TcSpecPrag -> DsM (Maybe (OrdList (Id,CoreExpr), CoreRule)) dsLSpec poly_rhs (L loc prag) = putSrcSpanDs loc $ dsSpec poly_rhs prag dsSpec :: CoreExpr -- RHS to be specialised -> TcSpecPrag -> DsM (Maybe (OrdList (Id,CoreExpr), CoreRule)) dsSpec poly_rhs (SpecPrag poly_id spec_co spec_inl) -- SpecPrag case: See Note [Handling old-form SPECIALISE pragmas] in GHC.Tc.Gen.Sig | (spec_bndrs, spec_app) <- collectHsWrapBinders spec_co -- spec_co looks like -- \spec_bndrs. [] spec_args -- perhaps with the body of the lambda wrapped in some WpLets -- E.g. /\a \(d:Eq a). let d2 = $df d in [] (Maybe a) d2 = dsHsWrapper spec_app $ \core_app -> dsSpec_help (idName poly_id) poly_id poly_rhs spec_inl spec_bndrs (core_app (Var poly_id)) dsSpec poly_rhs (SpecPragE { spe_fn_nm = poly_nm , spe_fn_id = poly_id , spe_inl = spec_inl , spe_bndrs = bndrs , spe_call = the_call }) -- SpecPragE case: See Note [Handling new-form SPECIALISE pragmas] in GHC.Tc.Gen.Sig = do { ds_call <- unsetGOptM Opt_EnableRewriteRules $ -- Note [Desugaring RULE left hand sides] unsetWOptM Opt_WarnIdentities $ zapUnspecables $ dsLExpr the_call ; dsSpec_help poly_nm poly_id poly_rhs spec_inl bndrs ds_call } dsSpec_help :: Name -> Id -> CoreExpr -- Function to specialise -> InlinePragma -> [Var] -> CoreExpr -> DsM (Maybe (OrdList (Id,CoreExpr), CoreRule)) dsSpec_help poly_nm poly_id poly_rhs spec_inl orig_bndrs ds_call = do { -- Decompose the call -- Step (S1) of Note [Desugaring new-form SPECIALISE pragmas] mb_call_info <- decomposeCall poly_id ds_call ; case mb_call_info of { Nothing -> return Nothing ; Just (binds, rule_lhs_args) -> do { dflags <- getDynFlags ; this_mod <- getModule ; uniq <- newUnique ; let locals = mkVarSet orig_bndrs `extendVarSetList` bindersOfBinds binds is_local :: Var -> Bool is_local v = v `elemVarSet` locals -- Find `rule_bndrs`: (S2) of Note [Desugaring new-form SPECIALISE pragmas] rule_bndrs = scopedSort (exprsSomeFreeVarsList is_local rule_lhs_args) -- getRenamings: (S3) of Note [Desugaring new-form SPECIALISE pragmas] rn_binds = getRenamings orig_bndrs binds rule_bndrs -- pickSpecBinds: (S4) of Note [Desugaring new-form SPECIALISE pragmas] known_vars = mkVarSet rule_bndrs `extendVarSetList` bindersOfBinds rn_binds picked_binds = pickSpecBinds is_local known_vars binds -- Find `spec_bndrs`: (S5) of Note [Desugaring new-form SPECIALISE pragmas] -- Make spec_bndrs, the variables to pass to the specialised -- function, by filtering out the rule_bndrs that aren't needed spec_binds_bndr_set = mkVarSet (bindersOfBinds picked_binds) `minusVarSet` exprsFreeVars (rhssOfBinds rn_binds) spec_bndrs = filterOut (`elemVarSet` spec_binds_bndr_set) rule_bndrs mk_spec_body fn_body = mkLets (rn_binds ++ picked_binds) $ mkApps fn_body rule_lhs_args -- NB: not mkCoreApps! That uses exprType on fun -- which fails in specUnfolding, sigh poly_name = idName poly_id spec_occ = mkSpecOcc (getOccName poly_name) spec_name = mkInternalName uniq spec_occ (getSrcSpan poly_name) id_inl = idInlinePragma poly_id simpl_opts = initSimpleOpts dflags fn_unf = realIdUnfolding poly_id spec_unf = specUnfolding simpl_opts spec_bndrs mk_spec_body rule_lhs_args fn_unf spec_info = vanillaIdInfo `setInlinePragInfo` specFunInlinePrag poly_id id_inl spec_inl `setUnfoldingInfo` spec_unf spec_id = mkLocalVar (idDetails poly_id) spec_name ManyTy spec_ty spec_info -- Specialised binding is toplevel, hence Many. -- The RULE looks like -- RULE "USPEC" forall rule_bndrs. f rule_lhs_args = $sf spec_bndrs -- The specialised function looks like -- $sf spec_bndrs = mk_spec_body -- We also use mk_spec_body to specialise the methods in f's stable unfolding -- NB: spec_bindrs is a subset of rule_bndrs rule = mkSpecRule dflags this_mod False rule_act (text "USPEC") poly_id rule_bndrs rule_lhs_args (mkVarApps (Var spec_id) spec_bndrs) rule_ty = exprType (mkApps (Var poly_id) rule_lhs_args) spec_ty = mkLamTypes spec_bndrs rule_ty spec_rhs = mkLams spec_bndrs (mk_spec_body poly_rhs) result = (unitOL (spec_id, spec_rhs), rule) -- NB: do *not* use makeCorePair on (spec_id,spec_rhs), because -- makeCorePair overwrites the unfolding, which we have -- just created using specUnfolding ; tracePm "dsSpec(new route)" $ vcat [ text "poly_id" <+> ppr poly_id , text "unfolding" <+> ppr (realIdUnfolding poly_id) , text "orig_bndrs" <+> pprCoreBinders orig_bndrs , text "locals" <+> ppr locals , text "fvs" <+> ppr (exprsSomeFreeVarsList is_local rule_lhs_args) , text "ds_call" <+> ppr ds_call , text "binds" <+> ppr binds , text "rule_lhs_args" <+> ppr rule_lhs_args , text "rule_bndrs" <+> ppr rule_bndrs , text "spec_bndrs" <+> ppr spec_bndrs , text "rn_binds" <+> ppr rn_binds , text "picked_binds" <+> ppr picked_binds ] ; dsWarnOrphanRule rule ; case checkUselessSpecPrag poly_id rule_lhs_args spec_bndrs no_act_spec spec_inl rule_act of Nothing -> return (Just result) Just reason -> do { diagnosticDs $ DsUselessSpecialisePragma poly_nm is_dfun reason ; if uselessSpecialisePragmaKeepAnyway reason then return (Just result) else return Nothing } } } } where -- See Note [Activation pragmas for SPECIALISE] -- no_act_spec is True if the user didn't write an explicit -- phase specification in the SPECIALISE pragma id_inl = idInlinePragma poly_id inl_prag_act = inlinePragmaActivation id_inl spec_prag_act = inlinePragmaActivation spec_inl no_act_spec = case inlinePragmaSpec spec_inl of NoInline _ -> isNeverActive spec_prag_act Opaque _ -> isNeverActive spec_prag_act _ -> isAlwaysActive spec_prag_act rule_act | no_act_spec = inl_prag_act -- Inherit | otherwise = spec_prag_act -- Specified by user is_dfun = case idDetails poly_id of DFunId {} -> True _ -> False decomposeCall :: Id -> CoreExpr -> DsM (Maybe ([CoreBind], [CoreExpr] )) -- Decompose the call into (let in f ) -- See (S1) in Note [Desugaring new-form SPECIALISE pragmas] decomposeCall poly_id ds_call = do { dflags <- getDynFlags ; let simpl_opts = initSimpleOpts dflags core_call = simpleOptExprNoInline simpl_opts ds_call -- simpleOptExprNoInline: see Wrinkle (S1a)! ; case go [] core_call of { Nothing -> do { diagnosticDs (DsRuleLhsTooComplicated ds_call core_call) ; return Nothing } ; Just result -> return (Just result) } } where go :: [CoreBind] -> CoreExpr -> Maybe ([CoreBind],[CoreExpr]) go acc (Let bind body) = go (bind:acc) body go acc (Cast e _) = go acc e -- Discard outer casts -- ToDo: document this go acc e | (Var fun, args) <- collectArgs e = assertPpr (fun == poly_id) (ppr fun $$ ppr poly_id) $ Just (reverse acc, args) | otherwise = Nothing -- Is this SPECIALISE pragma useless? checkUselessSpecPrag :: Id -> [CoreExpr] -> [Var] -> Bool -> InlinePragma -> Activation -> Maybe UselessSpecialisePragmaReason checkUselessSpecPrag poly_id rule_lhs_args spec_bndrs no_act_spec spec_inl rule_act | isJust (isClassOpId_maybe poly_id) -- There is no point in trying to specialise a class op -- Moreover, classops don't (currently) have an inl_sat arity set -- (it would be Just 0) and that in turn makes makeCorePair bleat = Just UselessSpecialiseForClassMethodSelector | no_act_spec, isNeverActive rule_act -- Function is NOINLINE, and the specialisation inherits that -- See Note [Activation pragmas for SPECIALISE] = Just UselessSpecialiseForNoInlineFunction | all is_nop_arg rule_lhs_args, not (isInlinePragma spec_inl) -- The specialisation does nothing. -- But don't complain if it is SPECIALISE INLINE (#4444) = Just UselessSpecialiseNoSpecialisation | otherwise = Nothing where is_nop_arg (Type {}) = True is_nop_arg (Coercion {}) = True is_nop_arg (Cast e _) = is_nop_arg e is_nop_arg (Tick _ e) = is_nop_arg e is_nop_arg (Var x) = x `elem` spec_bndrs is_nop_arg _ = False getRenamings :: [Var] -> [CoreBind] -- orig_bndrs and bindings -> [Var] -- rule_bndrs -> [CoreBind] -- Binds some of the orig_bndrs to a rule_bndr getRenamings orig_bndrs binds rule_bndrs = [ NonRec b e | b <- orig_bndrs , not (b `elem` rule_bndrs) , Just e <- [lookupVarEnv final_renamings b] ] where init_renamings, final_renamings :: IdEnv CoreExpr -- In this function, IdEnv maps a local variable to (v |> co), -- where `v` is a rule_bndr init_renamings = mkVarEnv [ (v, Var v) | v <- rule_bndrs, isId v ] final_renamings = go binds go :: [CoreBind] -> IdEnv CoreExpr go [] = init_renamings go (bind : binds) | NonRec b rhs <- bind , Just (v, mco) <- getCastedVar rhs , Just e <- lookupVarEnv renamings b = extendVarEnv renamings v (mkCastMCo e (mkSymMCo mco)) | otherwise = renamings where renamings = go binds pickSpecBinds :: (Var -> Bool) -> VarSet -> [CoreBind] -> [CoreBind] pickSpecBinds _ _ [] = [] pickSpecBinds is_local known_bndrs (bind:binds) | all keep_me (rhssOfBind bind) , let known_bndrs' = known_bndrs `extendVarSetList` bindersOf bind = bind : pickSpecBinds is_local known_bndrs' binds | otherwise = pickSpecBinds is_local known_bndrs binds where keep_me rhs = isEmptyVarSet (exprSomeFreeVars bad_var rhs) bad_var v = is_local v && not (v `elemVarSet` known_bndrs) getCastedVar :: CoreExpr -> Maybe (Var, MCoercionR) getCastedVar (Var v) = Just (v, MRefl) getCastedVar (Cast (Var v) co) = Just (v, MCo co) getCastedVar _ = Nothing specFunInlinePrag :: Id -> InlinePragma -> InlinePragma -> InlinePragma -- See Note [Activation pragmas for SPECIALISE] specFunInlinePrag poly_id id_inl spec_inl | not (isDefaultInlinePragma spec_inl) = spec_inl | isGlobalId poly_id -- See Note [Specialising imported functions] -- in OccurAnal , isStrongLoopBreaker (idOccInfo poly_id) = neverInlinePragma | otherwise = id_inl -- Get the INLINE pragma from SPECIALISE declaration, or, -- failing that, from the original Id dsWarnOrphanRule :: CoreRule -> DsM () dsWarnOrphanRule rule = when (ruleIsOrphan rule) $ diagnosticDs (DsOrphanRule rule) {- Note [SPECIALISE on INLINE functions] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We used to warn that using SPECIALISE for a function marked INLINE would be a no-op; but it isn't! Especially with worker/wrapper split we might have {-# INLINE f #-} f :: Ord a => Int -> a -> ... f d x y = case x of I# x' -> $wf d x' y We might want to specialise 'f' so that we in turn specialise '$wf'. We can't even /name/ '$wf' in the source code, so we can't specialise it even if we wanted to. #10721 is a case in point. Note [Activation pragmas for SPECIALISE] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ From a user SPECIALISE pragma for f, we generate a) A top-level binding spec_fn = rhs b) A RULE f dOrd = spec_fn We need two pragma-like things: * spec_fn's inline pragma: inherited from f's inline pragma (ignoring activation on SPEC), unless overridden by SPEC INLINE * Activation of RULE: from SPECIALISE pragma (if activation given) otherwise from f's inline pragma This is not obvious (see #5237)! Examples Rule activation Inline prag on spec'd fn --------------------------------------------------------------------- SPEC [n] f :: ty [n] Always, or NOINLINE [n] copy f's prag NOINLINE f SPEC [n] f :: ty [n] NOINLINE copy f's prag NOINLINE [k] f SPEC [n] f :: ty [n] NOINLINE [k] copy f's prag INLINE [k] f SPEC [n] f :: ty [n] INLINE [k] copy f's prag SPEC INLINE [n] f :: ty [n] INLINE [n] (ignore INLINE prag on f, same activation for rule and spec'd fn) NOINLINE [k] f SPEC f :: ty [n] INLINE [k] ************************************************************************ * * \subsection{Adding inline pragmas} * * ************************************************************************ -} decomposeRuleLhs :: DynFlags -> [Var] -> CoreExpr -> VarSet -- Free vars of the RHS -> Either DsMessage ([Var], Id, [CoreExpr]) -- (decomposeRuleLhs bndrs lhs) takes apart the LHS of a RULE, -- The 'bndrs' are the quantified binders of the rules, but decomposeRuleLhs -- may add some extra dictionary binders (see Note [Free dictionaries on rule LHS]) -- -- Returns an error message if the LHS isn't of the expected shape -- Note [Decomposing the left-hand side of a RULE] decomposeRuleLhs dflags orig_bndrs orig_lhs rhs_fvs | Var funId <- fun2 , Just con <- isDataConId_maybe funId = Left (DsRuleIgnoredDueToConstructor con) -- See Note [No RULES on datacons] | otherwise = case decompose fun2 args2 of Nothing -> -- pprTrace "decomposeRuleLhs 3" (vcat [ text "orig_bndrs:" <+> ppr orig_bndrs -- , text "orig_lhs:" <+> ppr orig_lhs -- , text "rhs_fvs:" <+> ppr rhs_fvs -- , text "lhs1:" <+> ppr lhs1 -- , text "lhs2:" <+> ppr lhs2 -- , text "fun2:" <+> ppr fun2 -- , text "args2:" <+> ppr args2 -- ]) $ Left (DsRuleLhsTooComplicated orig_lhs lhs2) Just (fn_id, args) | not (null unbound) -> -- Check for things unbound on LHS -- See Note [Unused spec binders] -- pprTrace "decomposeRuleLhs 1" (vcat [ text "orig_bndrs:" <+> ppr orig_bndrs -- , text "orig_lhs:" <+> ppr orig_lhs -- , text "lhs_fvs:" <+> ppr lhs_fvs -- , text "rhs_fvs:" <+> ppr rhs_fvs -- , text "unbound:" <+> ppr unbound -- ]) $ Left (DsRuleBindersNotBound unbound orig_bndrs orig_lhs lhs2) | otherwise -> -- pprTrace "decomposeRuleLhs 2" (vcat [ text "orig_bndrs:" <+> ppr orig_bndrs -- , text "orig_lhs:" <+> ppr orig_lhs -- , text "lhs1:" <+> ppr lhs1 -- , text "trimmed_bndrs:" <+> ppr trimmed_bndrs -- , text "extra_dicts:" <+> ppr extra_dicts -- , text "fn_id:" <+> ppr fn_id -- , text "args:" <+> ppr args -- , text "args fvs:" <+> ppr (exprsFreeVarsList args) -- ]) $ Right (trimmed_bndrs ++ extra_dicts, fn_id, args) where -- See Note [Variables unbound on the LHS] lhs_fvs = exprsFreeVars args all_fvs = lhs_fvs `unionVarSet` rhs_fvs trimmed_bndrs = filter (`elemVarSet` all_fvs) orig_bndrs unbound = filterOut (`elemVarSet` lhs_fvs) trimmed_bndrs -- Needed on RHS but not bound on LHS -- extra_dicts: see Note [Free dictionaries on rule LHS] -- ToDo: extra_dicts is needed. E.g. the SPECIALISE rules for `ap` in GHC.Base extra_dicts = [ mkLocalIdOrCoVar (localiseName (idName d)) ManyTy (idType d) | d <- exprsSomeFreeVarsList is_extra args ] is_extra v = isSimplePredTy (idType v) -- isSimplePredTy: includes coercions and dictionaries -- But NOT dictionary functions; and in particular not -- superclass-selector fuctions && not (v `elemVarSet` orig_bndr_set) && not (v == fn_id) -- fn_id: do not quantify over the function itself, which may -- itself be a dictionary (in pathological cases, #10251) where simpl_opts = initSimpleOpts dflags orig_bndr_set = mkVarSet orig_bndrs lhs1 = drop_dicts orig_lhs lhs2 = simpleOptExpr simpl_opts lhs1 -- See Note [Simplify rule LHS] (fun2,args2) = collectArgs lhs2 decompose (Var fn_id) args | not (fn_id `elemVarSet` orig_bndr_set) = Just (fn_id, args) decompose _ _ = Nothing drop_dicts :: CoreExpr -> CoreExpr drop_dicts e = wrap_lets needed bnds body where needed = orig_bndr_set `minusVarSet` exprFreeVars body (bnds, body) = split_lets (occurAnalyseExpr e) -- The occurAnalyseExpr drops dead bindings which is -- crucial to ensure that every binding is used later; -- which in turn makes wrap_lets work right split_lets :: CoreExpr -> ([(DictId,CoreExpr)], CoreExpr) split_lets (Let (NonRec d r) body) | isPredTy (idType d) -- isPredTy: catches dictionaries, yes, but also catches -- dictionary /functions/ arising from solving a -- quantified contraint (see #24370) = ((d,r):bs, body') where (bs, body') = split_lets body -- handle "unlifted lets" too, needed for "map/coerce" split_lets (Case r d _ [Alt DEFAULT _ body]) | isCoVar d = ((d,r):bs, body') where (bs, body') = split_lets body split_lets e = ([], e) wrap_lets :: VarSet -> [(DictId,CoreExpr)] -> CoreExpr -> CoreExpr wrap_lets _ [] body = body wrap_lets needed ((d, r) : bs) body | rhs_fvs `intersectsVarSet` needed = mkCoreLet (NonRec d r) (wrap_lets needed' bs body) | otherwise = wrap_lets needed bs body where rhs_fvs = exprFreeVars r needed' = (needed `minusVarSet` rhs_fvs) `extendVarSet` d {- Note [Variables unbound on the LHS] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We obviously want to complain about RULE forall x. f True = not x because the forall'd variable `x` is not bound on the LHS. It can be a bit delicate when dictionaries are involved. Consider #22471 {-# RULES "foo" forall (f :: forall a. [a] -> Int). foo (\xs. 1 + f xs) = 2 + foo f #-} We get two dicts on the LHS, one from `1` and one from `+`. For reasons described in Note [The SimplifyRule Plan] in GHC.Tc.Gen.Sig, we quantify separately over those dictionaries: forall f (d1::Num Int) (d2 :: Num Int). foo (\xs. (+) d1 (fromInteger d2 1) xs) = ... Now the desugarer shortcircuits (fromInteger d2 1) to (I# 1); so d2 is not mentioned at all (on LHS or RHS)! We don't want to complain about and unbound d2. Hence the trimmed_bndrs. Note [Decomposing the left-hand side of a RULE] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ There are several things going on here. * drop_dicts: see Note [Drop dictionary bindings on rule LHS] * simpleOptExpr: see Note [Simplify rule LHS] * extra_dict_bndrs: see Note [Free dictionaries on rule LHS] Note [Free dictionaries on rule LHS] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When the LHS of a specialisation rule, (/\as\ds. f es) has a free dict, which is presumably in scope at the function definition site, we can quantify over it too. *Any* dict with that type will do. So for example when you have f :: Eq a => a -> a f = ... SPECIALISE f :: Int -> Int ... Then we get the SpecPrag SpecPrag (f Int dInt) And from that we want the rule RULE forall dInt. f Int dInt = f_spec f_spec = let f = in f Int dInt But be careful! That dInt might be GHC.Base.$fOrdInt, which is an External Name, and you can't bind them in a lambda or forall without getting things confused. Likewise it might have a stable unfolding or something, which would be utterly bogus. So we really make a fresh Id, with the same unique and type as the old one, but with an Internal name and no IdInfo. Note [Drop dictionary bindings on rule LHS] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ drop_dicts drops dictionary bindings on the LHS where possible. E.g. let d:Eq [Int] = $fEqList $fEqInt in f d --> f d Reasoning here is that there is only one d:Eq [Int], and so we can quantify over it. That makes 'd' free in the LHS, but that is later picked up by extra_dict_bndrs (see Note [Unused spec binders]). NB 1: We can only drop the binding if the RHS of the binding doesn't mention one of the orig_bndrs, which we assume occur on RHS of the rule. Example f :: (Eq a) => b -> a -> a {-# SPECIALISE f :: Eq a => b -> [a] -> [a] #-} Here we want to end up with RULE forall d:Eq a. f ($dfEqList d) = f_spec d Of course, the ($dfEqlist d) in the pattern makes it less likely to match, but there is no other way to get d:Eq a NB 2: We do drop_dicts *before* simplOptExpr, so that we expect all the evidence bindings to be wrapped around the outside of the LHS. (After simplOptExpr they'll usually have been inlined.) dsHsWrapper does dependency analysis, so that civilised ones will be simple NonRec bindings. We don't handle recursive dictionaries! NB3: In the common case of a non-overloaded, but perhaps-polymorphic specialisation, we don't need to bind *any* dictionaries for use in the RHS. For example (#8331) {-# SPECIALIZE INLINE useAbstractMonad :: ReaderST s Int #-} useAbstractMonad :: MonadAbstractIOST m => m Int Here, deriving (MonadAbstractIOST (ReaderST s)) is a lot of code but the RHS uses no dictionaries, so we want to end up with RULE forall s (d :: MonadAbstractIOST (ReaderT s)). useAbstractMonad (ReaderT s) d = $suseAbstractMonad s #8848 is a good example of where there are some interesting dictionary bindings to discard. The drop_dicts algorithm is based on these observations: * Given (let d = rhs in e) where d is a DictId, matching 'e' will bind e's free variables. * So we want to keep the binding if one of the needed variables (for which we need a binding) is in fv(rhs) but not already in fv(e). * The "needed variables" are simply the orig_bndrs. Consider f :: (Eq a, Show b) => a -> b -> String ... SPECIALISE f :: (Show b) => Int -> b -> String ... Then orig_bndrs includes the *quantified* dictionaries of the type namely (dsb::Show b), but not the one for Eq Int So we work inside out, applying the above criterion at each step. Note [Simplify rule LHS] ~~~~~~~~~~~~~~~~~~~~~~~~ simplOptExpr occurrence-analyses and simplifies the LHS: (a) Inline any remaining dictionary bindings (which hopefully occur just once) (b) Substitute trivial lets, so that they don't get in the way. Note that we substitute the function too; we might have this as a LHS: let f71 = M.f Int in f71 (c) Do eta reduction. To see why, consider the fold/build rule, which without simplification looked like: fold k z (build (/\a. g a)) ==> ... This doesn't match unless you do eta reduction on the build argument. Similarly for a LHS like augment g (build h) we do not want to get augment (\a. g a) (build h) otherwise we don't match when given an argument like augment (\a. h a a) (build h) Note [Unused spec binders] ~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider f :: a -> a ... SPECIALISE f :: Eq a => a -> a ... It's true that this *is* a more specialised type, but the rule we get is something like this: f_spec d = f RULE: f = f_spec d Note that the rule is bogus, because it mentions a 'd' that is not bound on the LHS! But it's a silly specialisation anyway, because the constraint is unused. We could bind 'd' to (error "unused") but it seems better to reject the program because it's almost certainly a mistake. That's what the isDeadBinder call detects. Note [No RULES on datacons] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ Previously, `RULES` like "JustNothing" forall x . Just x = Nothing were allowed. Simon Peyton Jones says this seems to have been a mistake, that such rules have never been supported intentionally, and that he doesn't know if they can break in horrible ways. Furthermore, Ben Gamari and Reid Barton are considering trying to detect the presence of "static data" that the simplifier doesn't need to traverse at all. Such rules do not play well with that. So for now, we ban them altogether as requested by #13290. See also #7398. ************************************************************************ * * Desugaring evidence * * ************************************************************************ Note [Desugaring WpFun] ~~~~~~~~~~~~~~~~~~~~~~~ See comments on WpFun in GHC.Tc.Types.Evidence for what WpFun means. Roughly: (WpFun w_arg w_res)[ e ] = \x. w_res[ e w_arg[x] ] This eta-expansion risk duplicating work, if `e` is not in HNF. At one stage I thought we could avoid that by desugaring to let f = e in \x. w_res[ f w_arg[x] ] But that /fundamentally/ doesn't work, because `w_res` may bind evidence that is used in `e`. This question arose when thinking about deep subsumption; see https://github.com/ghc-proposals/ghc-proposals/pull/287#issuecomment-1125419649). -} dsHsWrappers :: [HsWrapper] -> ([CoreExpr -> CoreExpr] -> DsM a) -> DsM a dsHsWrappers (wp:wps) k = dsHsWrapper wp $ \wrap -> dsHsWrappers wps $ \wraps -> k (wrap:wraps) dsHsWrappers [] k = k [] dsHsWrapper :: HsWrapper -> ((CoreExpr -> CoreExpr) -> DsM a) -> DsM a dsHsWrapper hs_wrap thing_inside = ds_hs_wrapper hs_wrap $ \ core_wrap -> addTyCs FromSource (hsWrapDictBinders hs_wrap) $ -- addTyCs: Add type evidence to the refinement type -- predicate of the coverage checker -- See Note [Long-distance information] in "GHC.HsToCore.Pmc" -- FromSource might not be accurate (we don't have any -- origin annotations for things in this module), but at -- worst we do superfluous calls to the pattern match -- oracle. thing_inside core_wrap ds_hs_wrapper :: HsWrapper -> ((CoreExpr -> CoreExpr) -> DsM a) -> DsM a ds_hs_wrapper wrap = go wrap where go WpHole k = k $ \e -> e go (WpTyApp ty) k = k $ \e -> App e (Type ty) go (WpEvLam ev) k = k $ Lam ev go (WpTyLam tv) k = k $ Lam tv go (WpCast co) k = assert (coercionRole co == Representational) $ k $ \e -> mkCastDs e co go (WpEvApp tm) k = do { core_tm <- dsEvTerm tm ; k $ \e -> e `App` core_tm } go (WpLet ev_binds) k = dsTcEvBinds ev_binds $ \bs -> k (mkCoreLets bs) go (WpCompose c1 c2) k = go c1 $ \w1 -> go c2 $ \w2 -> k (w1 . w2) go (WpFun c1 c2 st) k = -- See Note [Desugaring WpFun] do { x <- newSysLocalDs st ; go c1 $ \w1 -> go c2 $ \w2 -> let app f a = mkCoreApp (text "dsHsWrapper") f a arg = w1 (Var x) in k (\e -> (Lam x (w2 (app e arg)))) } -------------------------------------- dsTcEvBinds_s :: [TcEvBinds] -> ([CoreBind] -> DsM a) -> DsM a dsTcEvBinds_s [] k = k [] dsTcEvBinds_s (b:rest) k = assert (null rest) $ -- Zonker ensures null dsTcEvBinds b k dsTcEvBinds :: TcEvBinds -> ([CoreBind] -> DsM a) -> DsM a dsTcEvBinds (TcEvBinds {}) = panic "dsEvBinds" -- Zonker has got rid of this dsTcEvBinds (EvBinds bs) = dsEvBinds bs -- * Desugars the ev_binds, sorts them into dependency order, and -- passes the resulting [CoreBind] to thing_inside -- * Extends the DsM (dsl_unspecable field) with specialisability information -- for each binder in ev_binds, before invoking thing_inside dsEvBinds :: Bag EvBind -> ([CoreBind] -> DsM a) -> DsM a dsEvBinds ev_binds thing_inside = do { ds_binds <- mapBagM dsEvBind ev_binds ; let comps = sort_ev_binds ds_binds ; go comps thing_inside } where go ::[SCC (Node EvVar (CanonicalEvidence, CoreExpr))] -> ([CoreBind] -> DsM a) -> DsM a go (comp:comps) thing_inside = do { unspecables <- getUnspecables ; let (core_bind, new_unspecables) = ds_component unspecables comp ; addUnspecables new_unspecables $ go comps $ \ core_binds -> thing_inside (core_bind:core_binds) } go [] thing_inside = thing_inside [] ds_component mb_unspecables (AcyclicSCC node) = (NonRec v rhs, new_unspecables) where ((v, rhs), (this_canonical, deps)) = unpack_node node new_unspecables = case mb_unspecables of Nothing -> [] Just unspecs | transitively_unspecable -> [v] | otherwise -> [] where transitively_unspecable = is_unspecable this_canonical || any (`elemVarSet` unspecs) deps ds_component mb_unspecables (CyclicSCC nodes) = (Rec pairs, new_unspecables) where (pairs, direct_canonicity) = unzip $ map unpack_node nodes new_unspecables = case mb_unspecables of Nothing -> [] Just unspecs | transitively_unspecable -> map fst pairs | otherwise -> [] where transitively_unspecable = or [ is_unspecable this_canonical || any (`elemVarSet` unspecs) deps | (this_canonical, deps) <- direct_canonicity ] -- Bindings from a given SCC are transitively specialisable if -- all are specialisable and all their remote dependencies are -- also specialisable; see Note [Desugaring non-canonical evidence] unpack_node DigraphNode { node_key = v, node_payload = (canonical, rhs), node_dependencies = deps } = ((v, rhs), (canonical, deps)) is_unspecable :: CanonicalEvidence -> Bool is_unspecable EvNonCanonical = True is_unspecable EvCanonical = False sort_ev_binds :: Bag (Id, CanonicalEvidence, CoreExpr) -> [SCC (Node EvVar (CanonicalEvidence, CoreExpr))] -- We do SCC analysis of the evidence bindings, /after/ desugaring -- them. This is convenient: it means we can use the GHC.Core -- free-variable functions rather than having to do accurate free vars -- for EvTerm. sort_ev_binds ds_binds = stronglyConnCompFromEdgedVerticesUniqR edges where edges :: [ Node EvVar (CanonicalEvidence, CoreExpr) ] edges = foldr ((:) . mk_node) [] ds_binds mk_node :: (Id, CanonicalEvidence, CoreExpr) -> Node EvVar (CanonicalEvidence, CoreExpr) mk_node (var, canonical, rhs) = DigraphNode { node_payload = (canonical, rhs) , node_key = var , node_dependencies = nonDetEltsUniqSet $ exprFreeVars rhs `unionVarSet` coVarsOfType (varType var) } -- It's OK to use nonDetEltsUniqSet here as graphFromEdgedVerticesUniq -- is still deterministic even if the edges are in nondeterministic order -- as explained in Note [Deterministic SCC] in GHC.Data.Graph.Directed. dsEvBind :: EvBind -> DsM (Id, CanonicalEvidence, CoreExpr) dsEvBind (EvBind { eb_lhs = v, eb_rhs = r, eb_info = info }) = do e <- dsEvTerm r let canonical = case info of EvBindGiven{} -> EvCanonical EvBindWanted{ ebi_canonical = canonical } -> canonical return (v, canonical, e) {-********************************************************************** * * Desugaring EvTerms * * **********************************************************************-} dsEvTerm :: EvTerm -> DsM CoreExpr dsEvTerm (EvExpr e) = return e dsEvTerm (EvTypeable ty ev) = dsEvTypeable ty ev dsEvTerm (EvFun { et_tvs = tvs, et_given = given , et_binds = ev_binds, et_body = wanted_id }) = do { dsTcEvBinds ev_binds $ \ds_ev_binds -> return $ (mkLams (tvs ++ given) $ mkCoreLets ds_ev_binds $ Var wanted_id) } {-********************************************************************** * * Desugaring Typeable dictionaries * * **********************************************************************-} dsEvTypeable :: Type -> EvTypeable -> DsM CoreExpr -- Return a CoreExpr :: Typeable ty -- This code is tightly coupled to the representation -- of TypeRep, in base library Data.Typeable.Internal dsEvTypeable ty ev = do { tyCl <- dsLookupTyCon typeableClassName -- Typeable ; let kind = typeKind ty typeable_data_con = tyConSingleDataCon tyCl -- "Data constructor" -- for Typeable ; rep_expr <- ds_ev_typeable ty ev -- :: TypeRep a -- Package up the method as `Typeable` dictionary ; return $ mkConApp typeable_data_con [Type kind, Type ty, rep_expr] } type TypeRepExpr = CoreExpr -- | Returns a @CoreExpr :: TypeRep ty@ ds_ev_typeable :: Type -> EvTypeable -> DsM CoreExpr ds_ev_typeable ty (EvTypeableTyCon tc kind_ev) = do { mkTrCon <- dsLookupGlobalId mkTrConName -- mkTrCon :: forall k (a :: k). TyCon -> TypeRep k -> TypeRep a ; someTypeRepTyCon <- dsLookupTyCon someTypeRepTyConName ; someTypeRepDataCon <- dsLookupDataCon someTypeRepDataConName -- SomeTypeRep :: forall k (a :: k). TypeRep a -> SomeTypeRep ; tc_rep <- tyConRep tc -- :: TyCon ; let ks = tyConAppArgs ty -- Construct a SomeTypeRep toSomeTypeRep :: Type -> EvTerm -> DsM CoreExpr toSomeTypeRep t ev = do rep <- getRep ev t return $ mkCoreConApps someTypeRepDataCon [Type (typeKind t), Type t, rep] ; kind_arg_reps <- sequence $ zipWith toSomeTypeRep ks kind_ev -- :: TypeRep t ; let -- :: [SomeTypeRep] kind_args = mkListExpr (mkTyConTy someTypeRepTyCon) kind_arg_reps -- Note that we use the kind of the type, not the TyCon from which it -- is constructed since the latter may be kind polymorphic whereas the -- former we know is not (we checked in the solver). ; let expr = mkApps (Var mkTrCon) [ Type (typeKind ty) , Type ty , tc_rep , kind_args ] -- ; pprRuntimeTrace "Trace mkTrTyCon" (ppr expr) expr ; return expr } ds_ev_typeable ty (EvTypeableTyApp ev1 ev2) | Just (t1,t2) <- splitAppTy_maybe ty = do { e1 <- getRep ev1 t1 ; e2 <- getRep ev2 t2 ; mkTrAppChecked <- dsLookupGlobalId mkTrAppCheckedName -- mkTrAppChecked :: forall k1 k2 (a :: k1 -> k2) (b :: k1). -- TypeRep a -> TypeRep b -> TypeRep (a b) ; let (_, k1, k2) = splitFunTy (typeKind t1) -- drop the multiplicity, -- since it's a kind ; let expr = mkApps (mkTyApps (Var mkTrAppChecked) [ k1, k2, t1, t2 ]) [ e1, e2 ] -- ; pprRuntimeTrace "Trace mkTrAppChecked" (ppr expr) expr ; return expr } ds_ev_typeable ty (EvTypeableTrFun evm ev1 ev2) | Just (_af,m,t1,t2) <- splitFunTy_maybe ty = do { e1 <- getRep ev1 t1 ; e2 <- getRep ev2 t2 ; em <- getRep evm m ; mkTrFun <- dsLookupGlobalId mkTrFunName -- mkTrFun :: forall (m :: Multiplicity) r1 r2 (a :: TYPE r1) (b :: TYPE r2). -- TypeRep m -> TypeRep a -> TypeRep b -> TypeRep (a % m -> b) ; let r1 = getRuntimeRep t1 r2 = getRuntimeRep t2 ; return $ mkApps (mkTyApps (Var mkTrFun) [m, r1, r2, t1, t2]) [ em, e1, e2 ] } ds_ev_typeable ty (EvTypeableTyLit ev) = -- See Note [Typeable for Nat and Symbol] in GHC.Tc.Instance.Class do { fun <- dsLookupGlobalId tr_fun ; dict <- dsEvTerm ev -- Of type KnownNat/KnownSymbol ; return (mkApps (mkTyApps (Var fun) [ty]) [ dict ]) } where ty_kind = typeKind ty -- tr_fun is the Name of -- typeNatTypeRep :: KnownNat a => TypeRep a -- of typeSymbolTypeRep :: KnownSymbol a => TypeRep a tr_fun | ty_kind `eqType` naturalTy = typeNatTypeRepName | ty_kind `eqType` typeSymbolKind = typeSymbolTypeRepName | ty_kind `eqType` charTy = typeCharTypeRepName | otherwise = panic "dsEvTypeable: unknown type lit kind" ds_ev_typeable ty ev = pprPanic "dsEvTypeable" (ppr ty $$ ppr ev) getRep :: EvTerm -- ^ EvTerm for @Typeable ty@ -> Type -- ^ The type @ty@ -> DsM TypeRepExpr -- ^ Return @CoreExpr :: TypeRep ty@ -- namely @typeRep# dict@ -- Remember that -- typeRep# :: forall k (a::k). Typeable k a -> TypeRep a getRep ev ty = do { typeable_expr <- dsEvTerm ev ; typeRepId <- dsLookupGlobalId typeRepIdName ; let ty_args = [typeKind ty, ty] ; return (mkApps (mkTyApps (Var typeRepId) ty_args) [ typeable_expr ]) } tyConRep :: TyCon -> DsM CoreExpr -- Returns CoreExpr :: TyCon tyConRep tc | Just tc_rep_nm <- tyConRepName_maybe tc = do { tc_rep_id <- dsLookupGlobalId tc_rep_nm ; return (Var tc_rep_id) } | otherwise = pprPanic "tyConRep" (ppr tc)