{- Data/Singletons/Single.hs (c) Richard Eisenberg 2013 rae@cs.brynmawr.edu This file contains functions to refine constructs to work with singleton types. It is an internal module to the singletons package. -} {-# LANGUAGE TemplateHaskell, TupleSections, ParallelListComp, CPP #-} module Data.Singletons.Single where import Prelude hiding ( exp ) import Language.Haskell.TH hiding ( cxt ) import Language.Haskell.TH.Syntax (NameSpace(..), Quasi(..)) import Data.Singletons.Deriving.Ord import Data.Singletons.Deriving.Bounded import Data.Singletons.Deriving.Enum import Data.Singletons.Deriving.Show import Data.Singletons.Deriving.Util import Data.Singletons.Util import Data.Singletons.Promote import Data.Singletons.Promote.Defun import Data.Singletons.Promote.Monad ( promoteM ) import Data.Singletons.Promote.Type import Data.Singletons.Names import Data.Singletons.Single.Monad import Data.Singletons.Single.Type import Data.Singletons.Single.Data import Data.Singletons.Single.Defun import Data.Singletons.Single.Fixity import Data.Singletons.Single.Eq import Data.Singletons.Syntax import Data.Singletons.Partition import Language.Haskell.TH.Desugar import qualified Language.Haskell.TH.Desugar.OMap.Strict as OMap import Language.Haskell.TH.Desugar.OMap.Strict (OMap) import qualified Language.Haskell.TH.Desugar.OSet as OSet import Language.Haskell.TH.Desugar.OSet (OSet) import qualified Data.Map.Strict as Map import Data.Map.Strict ( Map ) import Data.Maybe import qualified Data.Set as Set import Control.Monad import Data.List import qualified GHC.LanguageExtensions.Type as LangExt {- How singletons works ~~~~~~~~~~~~~~~~~~~~ Singling, on the surface, doesn't seem all that complicated. Promote the type, and singletonize all the terms. That's essentially what was done singletons < 1.0. But, now we want to deal with higher-order singletons. So, things are a little more complicated. The way to understand all of this is that *every* variable maps to something of type (Sing t), for an appropriately-kinded t. This includes functions, which use the "SLambda" instance of Sing. To apply singleton functions, we use the applySing function. That, in and of itself, wouldn't be too hard, but it's really annoying from the user standpoint. After dutifully singling `map`, a user doesn't want to have to use two `applySing`s to actually use it. So, any let-bound identifier is eta-expanded so that the singled type has the same number of arrows as the original type. (If there is no original type signature, then it has as many arrows as the original had patterns.) Then, we store a use of one of the singFunX functions in the SgM environment so that every use of a let-bound identifier has a proper type (Sing t). It would be consistent to avoid this eta-expansion for local lets (as opposed to top-level lets), but that seemed like more bother than it was worth. It may also be possible to be cleverer about nested eta-expansions and contractions, but that also seemed not to be worth it. Though I haven't tested it, my hope is that the eta-expansions and contractions have no runtime effect, especially because SLambda is a *newtype* instance, not a *data* instance. Note that to maintain the desired invariant, we must also be careful to eta- contract constructors. This is the point of buildDataLets. -} -- | Generate singleton definitions from a type that is already defined. -- For example, the singletons package itself uses -- -- > $(genSingletons [''Bool, ''Maybe, ''Either, ''[]]) -- -- to generate singletons for Prelude types. genSingletons :: DsMonad q => [Name] -> q [Dec] genSingletons names = do checkForRep names ddecs <- concatMapM (singInfo <=< dsInfo <=< reifyWithLocals) names return $ decsToTH ddecs -- | Make promoted and singleton versions of all declarations given, retaining -- the original declarations. -- See for -- further explanation. singletons :: DsMonad q => q [Dec] -> q [Dec] singletons qdecs = do decs <- qdecs ddecs <- withLocalDeclarations decs $ dsDecs decs singDecs <- singTopLevelDecs decs ddecs return (decs ++ decsToTH singDecs) -- | Make promoted and singleton versions of all declarations given, discarding -- the original declarations. Note that a singleton based on a datatype needs -- the original datatype, so this will fail if it sees any datatype declarations. -- Classes, instances, and functions are all fine. singletonsOnly :: DsMonad q => q [Dec] -> q [Dec] singletonsOnly = (>>= wrapDesugar singTopLevelDecs) -- | Create instances of 'SEq' and type-level @(==)@ for each type in the list singEqInstances :: DsMonad q => [Name] -> q [Dec] singEqInstances = concatMapM singEqInstance -- | Create instance of 'SEq' and type-level @(==)@ for the given type singEqInstance :: DsMonad q => Name -> q [Dec] singEqInstance name = do promotion <- promoteEqInstance name dec <- singEqualityInstance sEqClassDesc name return $ dec ++ promotion -- | Create instances of 'SEq' (only -- no instance for @(==)@, which 'SEq' generally -- relies on) for each type in the list singEqInstancesOnly :: DsMonad q => [Name] -> q [Dec] singEqInstancesOnly = concatMapM singEqInstanceOnly -- | Create instances of 'SEq' (only -- no instance for @(==)@, which 'SEq' generally -- relies on) for the given type singEqInstanceOnly :: DsMonad q => Name -> q [Dec] singEqInstanceOnly name = singEqualityInstance sEqClassDesc name -- | Create instances of 'SDecide', 'TestEquality', and 'TestCoercion' for each -- type in the list. singDecideInstances :: DsMonad q => [Name] -> q [Dec] singDecideInstances = concatMapM singDecideInstance -- | Create instance of 'SDecide', 'TestEquality', and 'TestCoercion' for the -- given type. singDecideInstance :: DsMonad q => Name -> q [Dec] singDecideInstance name = singEqualityInstance sDecideClassDesc name -- generalized function for creating equality instances singEqualityInstance :: DsMonad q => EqualityClassDesc q -> Name -> q [Dec] singEqualityInstance desc@(_, _, className, _) name = do (tvbs, cons) <- getDataD ("I cannot make an instance of " ++ show className ++ " for it.") name dtvbs <- mapM dsTvb tvbs let data_ty = foldTypeTvbs (DConT name) dtvbs dcons <- concatMapM (dsCon dtvbs data_ty) cons let tyvars = map (DVarT . extractTvbName) dtvbs kind = foldType (DConT name) tyvars (scons, _) <- singM [] $ mapM (singCtor name) dcons eqInstance <- mkEqualityInstance Nothing kind dcons scons desc testInstances <- if className == sDecideClassName then traverse (mkTestInstance Nothing kind name dcons) [TestEquality, TestCoercion] else pure [] return $ decsToTH (eqInstance:testInstances) -- | Create instances of 'SOrd' for the given types singOrdInstances :: DsMonad q => [Name] -> q [Dec] singOrdInstances = concatMapM singOrdInstance -- | Create instance of 'SOrd' for the given type singOrdInstance :: DsMonad q => Name -> q [Dec] singOrdInstance = singInstance mkOrdInstance "Ord" -- | Create instances of 'SBounded' for the given types singBoundedInstances :: DsMonad q => [Name] -> q [Dec] singBoundedInstances = concatMapM singBoundedInstance -- | Create instance of 'SBounded' for the given type singBoundedInstance :: DsMonad q => Name -> q [Dec] singBoundedInstance = singInstance mkBoundedInstance "Bounded" -- | Create instances of 'SEnum' for the given types singEnumInstances :: DsMonad q => [Name] -> q [Dec] singEnumInstances = concatMapM singEnumInstance -- | Create instance of 'SEnum' for the given type singEnumInstance :: DsMonad q => Name -> q [Dec] singEnumInstance = singInstance mkEnumInstance "Enum" -- | Create instance of 'SShow' for the given type -- -- (Not to be confused with 'showShowInstance'.) singShowInstance :: DsMonad q => Name -> q [Dec] singShowInstance = singInstance (mkShowInstance ForPromotion) "Show" -- | Create instances of 'SShow' for the given types -- -- (Not to be confused with 'showSingInstances'.) singShowInstances :: DsMonad q => [Name] -> q [Dec] singShowInstances = concatMapM singShowInstance -- | Create instance of 'Show' for the given singleton type -- -- (Not to be confused with 'singShowInstance'.) showSingInstance :: DsMonad q => Name -> q [Dec] showSingInstance name = do (tvbs, cons) <- getDataD ("I cannot make an instance of Show for it.") name dtvbs <- mapM dsTvb tvbs let data_ty = foldTypeTvbs (DConT name) dtvbs dcons <- concatMapM (dsCon dtvbs data_ty) cons let tyvars = map (DVarT . extractTvbName) dtvbs kind = foldType (DConT name) tyvars data_decl = DataDecl name dtvbs dcons deriv_show_decl = DerivedDecl { ded_mb_cxt = Nothing , ded_type = kind , ded_type_tycon = name , ded_decl = data_decl } (show_insts, _) <- singM [] $ singDerivedShowDecs deriv_show_decl pure $ decsToTH show_insts -- | Create instances of 'Show' for the given singleton types -- -- (Not to be confused with 'singShowInstances'.) showSingInstances :: DsMonad q => [Name] -> q [Dec] showSingInstances = concatMapM showSingInstance -- | Create an instance for @'SingI' TyCon{N}@, where @N@ is the positive -- number provided as an argument. -- -- Note that the generated code requires the use of the @QuantifiedConstraints@ -- language extension. singITyConInstances :: DsMonad q => [Int] -> q [Dec] singITyConInstances = concatMapM singITyConInstance -- | Create an instance for @'SingI' TyCon{N}@, where @N@ is the positive -- number provided as an argument. -- -- Note that the generated code requires the use of the @QuantifiedConstraints@ -- language extension. singITyConInstance :: DsMonad q => Int -> q [Dec] singITyConInstance n | n <= 0 = fail $ "Argument must be a positive number (given " ++ show n ++ ")" | otherwise = do as <- replicateM n (qNewName "a") ks <- replicateM n (qNewName "k") k_last <- qNewName "k_last" f <- qNewName "f" x <- qNewName "x" let k_penult = last ks k_fun = ravel (map DVarT ks) (DVarT k_last) f_ty = DVarT f a_tys = map DVarT as mk_fun arrow t1 t2 = arrow `DAppT` t1 `DAppT` t2 matchable_apply_fun = mk_fun DArrowT (DVarT k_penult) (DVarT k_last) unmatchable_apply_fun = mk_fun (DConT tyFunArrowName) (DVarT k_penult) (DVarT k_last) ctxt = [ DForallT (map DPlainTV as) (map (DAppT (DConT singIName)) a_tys) (DConT singIName `DAppT` foldType f_ty a_tys) , DConT equalityName `DAppT` (DConT applyTyConName `DSigT` mk_fun DArrowT matchable_apply_fun unmatchable_apply_fun) `DAppT` DConT applyTyConAux1Name ] pure $ decToTH $ DInstanceD Nothing Nothing ctxt (DConT singIName `DAppT` (DConT (mkTyConName n) `DAppT` (f_ty `DSigT` k_fun))) [DLetDec $ DFunD singMethName [DClause [] $ wrapSingFun 1 DWildCardT $ DLamE [x] $ DVarE withSingIName `DAppE` DVarE x `DAppE` DVarE singMethName]] singInstance :: DsMonad q => DerivDesc q -> String -> Name -> q [Dec] singInstance mk_inst inst_name name = do (tvbs, cons) <- getDataD ("I cannot make an instance of " ++ inst_name ++ " for it.") name dtvbs <- mapM dsTvb tvbs let data_ty = foldTypeTvbs (DConT name) dtvbs dcons <- concatMapM (dsCon dtvbs data_ty) cons let data_decl = DataDecl name dtvbs dcons raw_inst <- mk_inst Nothing data_ty data_decl (a_inst, decs) <- promoteM [] $ promoteInstanceDec OMap.empty raw_inst decs' <- singDecsM [] $ (:[]) <$> singInstD a_inst return $ decsToTH (decs ++ decs') singInfo :: DsMonad q => DInfo -> q [DDec] singInfo (DTyConI dec _) = singTopLevelDecs [] [dec] singInfo (DPrimTyConI _name _numArgs _unlifted) = fail "Singling of primitive type constructors not supported" singInfo (DVarI _name _ty _mdec) = fail "Singling of value info not supported" singInfo (DTyVarI _name _ty) = fail "Singling of type variable info not supported" singInfo (DPatSynI {}) = fail "Singling of pattern synonym info not supported" singTopLevelDecs :: DsMonad q => [Dec] -> [DDec] -> q [DDec] singTopLevelDecs locals raw_decls = withLocalDeclarations locals $ do decls <- expand raw_decls -- expand type synonyms PDecs { pd_let_decs = letDecls , pd_class_decs = classes , pd_instance_decs = insts , pd_data_decs = datas , pd_ty_syn_decs = ty_syns , pd_open_type_family_decs = o_tyfams , pd_closed_type_family_decs = c_tyfams , pd_derived_eq_decs = derivedEqDecs , pd_derived_show_decs = derivedShowDecs } <- partitionDecs decls ((letDecEnv, classes', insts'), promDecls) <- promoteM locals $ do defunTypeDecls ty_syns c_tyfams o_tyfams promoteDataDecs datas (_, letDecEnv) <- promoteLetDecs noPrefix letDecls classes' <- mapM promoteClassDec classes let meth_sigs = foldMap (lde_types . cd_lde) classes insts' <- mapM (promoteInstanceDec meth_sigs) insts mapM_ promoteDerivedEqDec derivedEqDecs return (letDecEnv, classes', insts') singDecsM locals $ do let letBinds = concatMap buildDataLets datas ++ concatMap buildMethLets classes (newLetDecls, singIDefunDecls, newDecls) <- bindLets letBinds $ singLetDecEnv letDecEnv $ do newDataDecls <- concatMapM singDataD datas newClassDecls <- mapM singClassD classes' newInstDecls <- mapM singInstD insts' newDerivedEqDecs <- concatMapM singDerivedEqDecs derivedEqDecs newDerivedShowDecs <- concatMapM singDerivedShowDecs derivedShowDecs return $ newDataDecls ++ newClassDecls ++ newInstDecls ++ newDerivedEqDecs ++ newDerivedShowDecs return $ promDecls ++ (map DLetDec newLetDecls) ++ singIDefunDecls ++ newDecls -- see comment at top of file buildDataLets :: DataDecl -> [(Name, DExp)] buildDataLets (DataDecl _name _tvbs cons) = concatMap con_num_args cons where con_num_args :: DCon -> [(Name, DExp)] con_num_args (DCon _tvbs _cxt name fields _rty) = (name, wrapSingFun (length (tysOfConFields fields)) (promoteValRhs name) (DConE $ singDataConName name)) : rec_selectors fields rec_selectors :: DConFields -> [(Name, DExp)] rec_selectors (DNormalC {}) = [] rec_selectors (DRecC fields) = let names = map fstOf3 fields in [ (name, wrapSingFun 1 (promoteValRhs name) (DVarE $ singValName name)) | name <- names ] -- see comment at top of file buildMethLets :: UClassDecl -> [(Name, DExp)] buildMethLets (ClassDecl { cd_lde = LetDecEnv { lde_types = meth_sigs } }) = map mk_bind (OMap.assocs meth_sigs) where mk_bind (meth_name, meth_ty) = ( meth_name , wrapSingFun (countArgs meth_ty) (promoteValRhs meth_name) (DVarE $ singValName meth_name) ) singClassD :: AClassDecl -> SgM DDec singClassD (ClassDecl { cd_cxt = cls_cxt , cd_name = cls_name , cd_tvbs = cls_tvbs , cd_fds = cls_fundeps , cd_lde = LetDecEnv { lde_defns = default_defns , lde_types = meth_sigs , lde_infix = fixities , lde_proms = promoted_defaults , lde_bound_kvs = meth_bound_kvs } }) = bindContext [foldTypeTvbs (DConT cls_name) cls_tvbs] $ do (sing_sigs, _, tyvar_names, cxts, res_kis, singIDefunss) <- unzip6 <$> zipWithM (singTySig no_meth_defns meth_sigs meth_bound_kvs) meth_names (map promoteValRhs meth_names) emitDecs $ concat singIDefunss let default_sigs = catMaybes $ zipWith4 mk_default_sig meth_names sing_sigs tyvar_names res_kis res_ki_map = Map.fromList (zip meth_names (map (fromMaybe always_sig) res_kis)) sing_meths <- mapM (uncurry (singLetDecRHS (Map.fromList tyvar_names) (Map.fromList cxts) res_ki_map)) (OMap.assocs default_defns) fixities' <- traverse (uncurry singInfixDecl) $ OMap.assocs fixities cls_cxt' <- mapM singPred cls_cxt return $ DClassD cls_cxt' (singClassName cls_name) cls_tvbs cls_fundeps -- they are fine without modification (map DLetDec (sing_sigs ++ sing_meths ++ fixities') ++ default_sigs) where no_meth_defns = error "Internal error: can't find declared method type" always_sig = error "Internal error: no signature for default method" meth_names = map fst $ OMap.assocs meth_sigs mk_default_sig meth_name (DSigD s_name sty) bound_kvs (Just res_ki) = DDefaultSigD s_name <$> add_constraints meth_name sty bound_kvs res_ki mk_default_sig _ _ _ _ = error "Internal error: a singled signature isn't a signature." add_constraints meth_name sty (_, bound_kvs) res_ki = do -- Maybe monad prom_dflt <- OMap.lookup meth_name promoted_defaults let default_pred = foldType (DConT equalityName) -- NB: Need the res_ki here to prevent ambiguous -- kinds in result-inferred default methods. -- See #175 [ foldApply (promoteValRhs meth_name) tvs `DSigT` res_ki , foldApply prom_dflt tvs ] return $ DForallT tvbs (default_pred : cxt) (ravel args res) where (tvbs, cxt, args, res) = unravel sty bound_kv_set = Set.fromList bound_kvs -- Filter out explicitly bound kind variables. Otherwise, if you had -- the following class (#312): -- -- class Foo a where -- bar :: a -> b -> b -- bar _ x = x -- -- Then it would be singled to: -- -- class SFoo a where -- sBar :: forall b (x :: a) (y :: b). Sing x -> Sing y -> Sing (sBar x y) -- default :: forall b (x :: a) (y :: b). -- (Bar b x y) ~ (BarDefault b x y) => ... -- -- Which applies Bar/BarDefault to b, which shouldn't happen. tvs = map tvbToType $ filter (\tvb -> extractTvbName tvb `Set.member` bound_kv_set) tvbs singInstD :: AInstDecl -> SgM DDec singInstD (InstDecl { id_cxt = cxt, id_name = inst_name, id_arg_tys = inst_tys , id_sigs = inst_sigs, id_meths = ann_meths }) = do bindContext cxt $ do cxt' <- mapM singPred cxt inst_kis <- mapM promoteType inst_tys meths <- concatMapM (uncurry sing_meth) ann_meths return (DInstanceD Nothing Nothing cxt' (foldl DAppT (DConT s_inst_name) inst_kis) meths) where s_inst_name = singClassName inst_name sing_meth :: Name -> ALetDecRHS -> SgM [DDec] sing_meth name rhs = do mb_s_info <- dsReify (singValName name) inst_kis <- mapM promoteType inst_tys let mk_subst cls_tvbs = Map.fromList $ zip (map extractTvbName vis_cls_tvbs) inst_kis where -- This is a half-hearted attempt to address the underlying problem -- in #358, where we can sometimes have more class type variables -- (due to implicit kind arguments) than class arguments. This just -- ensures that the explicit type variables are properly mapped -- to the class arguments, leaving the implicit kind variables -- unmapped. That could potentially cause *other* problems, but -- those are perhaps best avoided by using InstanceSigs. At the -- very least, this workaround will make error messages slightly -- less confusing. vis_cls_tvbs = drop (length cls_tvbs - length inst_kis) cls_tvbs sing_meth_ty :: OSet Name -> DType -> SgM (DType, [Name], DCxt, DKind) sing_meth_ty bound_kvs inner_ty = do -- Make sure to expand through type synonyms here! Not doing so -- resulted in #167. raw_ty <- expand inner_ty (s_ty, _num_args, tyvar_names, ctxt, _arg_kis, res_ki) <- singType bound_kvs (promoteValRhs name) raw_ty pure (s_ty, tyvar_names, ctxt, res_ki) (s_ty, tyvar_names, ctxt, m_res_ki) <- case OMap.lookup name inst_sigs of Just inst_sig -> do -- We have an InstanceSig, so just single that type. Take care to -- avoid binding the variables bound by the instance head as well. let inst_bound = foldMap fvDType (cxt ++ inst_kis) (s_ty, tyvar_names, ctxt, res_ki) <- sing_meth_ty inst_bound inst_sig pure (s_ty, tyvar_names, ctxt, Just res_ki) Nothing -> case mb_s_info of -- We don't have an InstanceSig, so we must compute the type to use -- in the singled instance ourselves through reification. Just (DVarI _ (DForallT cls_tvbs _cls_pred s_ty) _) -> do let subst = mk_subst cls_tvbs (sing_tvbs, ctxt, _args, res_ty) = unravel s_ty m_res_ki = case res_ty of _sing `DAppT` (_prom_func `DSigT` res_ki) -> Just (substKind subst res_ki) _ -> Nothing pure ( substType subst s_ty , map extractTvbName sing_tvbs , map (substType subst) ctxt , m_res_ki ) _ -> do mb_info <- dsReify name case mb_info of Just (DVarI _ (DForallT cls_tvbs _cls_pred inner_ty) _) -> do let subst = mk_subst cls_tvbs cls_kvb_names = foldMap (foldMap fvDType . extractTvbKind) cls_tvbs cls_tvb_names = OSet.fromList $ map extractTvbName cls_tvbs cls_bound = cls_kvb_names `OSet.union` cls_tvb_names (s_ty, tyvar_names, ctxt, res_ki) <- sing_meth_ty cls_bound inner_ty pure ( substType subst s_ty , tyvar_names , ctxt , Just (substKind subst res_ki) ) _ -> fail $ "Cannot find type of method " ++ show name let kind_map = maybe Map.empty (Map.singleton name) m_res_ki meth' <- singLetDecRHS (Map.singleton name tyvar_names) (Map.singleton name ctxt) kind_map name rhs return $ map DLetDec [DSigD (singValName name) s_ty, meth'] singLetDecEnv :: ALetDecEnv -> SgM a -> SgM ([DLetDec], [DDec], a) -- Return: -- -- 1. The singled let-decs -- 2. SingI instances for any defunctionalization symbols -- (see Data.Singletons.Single.Defun) -- 3. The result of running the `SgM a` action singLetDecEnv (LetDecEnv { lde_defns = defns , lde_types = types , lde_infix = infix_decls , lde_proms = proms , lde_bound_kvs = bound_kvs }) thing_inside = do let prom_list = OMap.assocs proms (typeSigs, letBinds, tyvarNames, cxts, res_kis, singIDefunss) <- unzip6 <$> mapM (uncurry (singTySig defns types bound_kvs)) prom_list infix_decls' <- traverse (uncurry singInfixDecl) $ OMap.assocs infix_decls let res_ki_map = Map.fromList [ (name, res_ki) | ((name, _), Just res_ki) <- zip prom_list res_kis ] bindLets letBinds $ do let_decs <- mapM (uncurry (singLetDecRHS (Map.fromList tyvarNames) (Map.fromList cxts) res_ki_map)) (OMap.assocs defns) thing <- thing_inside return (infix_decls' ++ typeSigs ++ let_decs, concat singIDefunss, thing) singTySig :: OMap Name ALetDecRHS -- definitions -> OMap Name DType -- type signatures -> OMap Name (OSet Name) -- bound kind variables -> Name -> DType -- the type is the promoted type, not the type sig! -> SgM ( DLetDec -- the new type signature , (Name, DExp) -- the let-bind entry , (Name, [Name]) -- the scoped tyvar names in the tysig , (Name, DCxt) -- the context of the type signature , Maybe DKind -- the result kind in the tysig , [DDec] -- SingI instances for defun symbols ) singTySig defns types bound_kvs name prom_ty = let sName = singValName name in case OMap.lookup name types of Nothing -> do num_args <- guess_num_args (sty, tyvar_names) <- mk_sing_ty num_args singIDefuns <- singDefuns name VarName [] (map (const Nothing) tyvar_names) Nothing return ( DSigD sName sty , (name, wrapSingFun num_args prom_ty (DVarE sName)) , (name, tyvar_names) , (name, []) , Nothing , singIDefuns ) Just ty -> do all_bound_kvs <- lookup_bound_kvs (sty, num_args, tyvar_names, ctxt, arg_kis, res_ki) <- singType all_bound_kvs prom_ty ty bound_cxt <- askContext singIDefuns <- singDefuns name VarName (bound_cxt ++ ctxt) (map Just arg_kis) (Just res_ki) return ( DSigD sName sty , (name, wrapSingFun num_args prom_ty (DVarE sName)) , (name, tyvar_names) , (name, ctxt) , Just res_ki , singIDefuns ) where guess_num_args :: SgM Int guess_num_args = case OMap.lookup name defns of Nothing -> fail "Internal error: promotion known for something not let-bound." Just (AValue _ n _) -> return n Just (AFunction _ n _) -> return n lookup_bound_kvs :: SgM (OSet Name) lookup_bound_kvs = case OMap.lookup name bound_kvs of Nothing -> fail $ "Internal error: " ++ nameBase name ++ " has no type variable " ++ "bindings, despite having a type signature" Just kvs -> pure kvs -- create a Sing t1 -> Sing t2 -> ... type of a given arity and result type mk_sing_ty :: Int -> SgM (DType, [Name]) mk_sing_ty n = do arg_names <- replicateM n (qNewName "arg") return ( DForallT (map DPlainTV arg_names) [] (ravel (map (\nm -> singFamily `DAppT` DVarT nm) arg_names) (singFamily `DAppT` (foldl apply prom_ty (map DVarT arg_names)))) , arg_names ) singLetDecRHS :: Map Name [Name] -> Map Name DCxt -- the context of the type signature -- (might not be known) -> Map Name DKind -- result kind (might not be known) -> Name -> ALetDecRHS -> SgM DLetDec singLetDecRHS bound_names cxts res_kis name ld_rhs = bindContext (Map.findWithDefault [] name cxts) $ case ld_rhs of AValue prom num_arrows exp -> DValD (DVarP (singValName name)) <$> (wrapUnSingFun num_arrows prom <$> singExp exp (Map.lookup name res_kis)) AFunction prom_fun num_arrows clauses -> let tyvar_names = case Map.lookup name bound_names of Nothing -> [] Just ns -> ns res_ki = Map.lookup name res_kis in DFunD (singValName name) <$> mapM (singClause prom_fun num_arrows tyvar_names res_ki) clauses singClause :: DType -- the promoted function -> Int -- the number of arrows in the type. If this is more -- than the number of patterns, we need to eta-expand -- with unSingFun. -> [Name] -- the names of the forall'd vars in the type sig of this -- function. This list should have at least the length as the -- number of patterns in the clause -> Maybe DKind -- result kind, if known -> ADClause -> SgM DClause singClause prom_fun num_arrows bound_names res_ki (ADClause var_proms pats exp) = do -- Fix #166: when (num_arrows - length pats < 0) $ fail $ "Function being promoted to " ++ (pprint (typeToTH prom_fun)) ++ " has too many arguments." (sPats, sigPaExpsSigs) <- evalForPair $ mapM (singPat (Map.fromList var_proms)) pats sBody <- singExp exp res_ki -- when calling unSingFun, the promoted pats aren't in scope, so we use the -- bound_names instead let pattern_bound_names = zipWith const bound_names pats -- this does eta-expansion. See comment at top of file. sBody' = wrapUnSingFun (num_arrows - length pats) (foldl apply prom_fun (map DVarT pattern_bound_names)) sBody return $ DClause sPats $ mkSigPaCaseE sigPaExpsSigs sBody' singPat :: Map Name Name -- from term-level names to type-level names -> ADPat -> QWithAux SingDSigPaInfos SgM DPat singPat var_proms = go where go :: ADPat -> QWithAux SingDSigPaInfos SgM DPat go (ADLitP _lit) = fail "Singling of literal patterns not yet supported" go (ADVarP name) = do tyname <- case Map.lookup name var_proms of Nothing -> fail "Internal error: unknown variable when singling pattern" Just tyname -> return tyname pure $ DVarP (singValName name) `DSigP` (singFamily `DAppT` DVarT tyname) go (ADConP name pats) = DConP (singDataConName name) <$> mapM go pats go (ADTildeP pat) = do qReportWarning "Lazy pattern converted into regular pattern during singleton generation." go pat go (ADBangP pat) = DBangP <$> go pat go (ADSigP prom_pat pat ty) = do pat' <- go pat -- Normally, calling dPatToDExp would be dangerous, since it fails if the -- supplied pattern contains any wildcard patterns. However, promotePat -- (which produced the pattern we're passing into dPatToDExp) maintains -- an invariant that any promoted pattern signatures will be free of -- wildcard patterns in the underlying pattern. -- See Note [Singling pattern signatures]. addElement (dPatToDExp pat', DSigT prom_pat ty) pure pat' go ADWildP = pure DWildP -- | If given a non-empty list of 'SingDSigPaInfos', construct a case expression -- that brings singleton equality constraints into scope via pattern-matching. -- See @Note [Singling pattern signatures]@. mkSigPaCaseE :: SingDSigPaInfos -> DExp -> DExp mkSigPaCaseE exps_with_sigs exp | null exps_with_sigs = exp | otherwise = let (exps, sigs) = unzip exps_with_sigs scrutinee = mkTupleDExp exps pats = map (DSigP DWildP . DAppT (DConT singFamilyName)) sigs in DCaseE scrutinee [DMatch (mkTupleDPat pats) exp] -- Note [Annotate case return type] -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -- -- We're straining GHC's type inference here. One particular trouble area -- is determining the return type of a GADT pattern match. In general, GHC -- cannot infer return types of GADT pattern matches because the return type -- becomes "untouchable" in the case matches. See the OutsideIn paper. But, -- during singletonization, we *know* the return type. So, just add a type -- annotation. See #54. -- -- In particular, we add a type annotation in a somewhat unorthodox fashion. -- Instead of the usual `(x :: t)`, we use `id @t x`. See -- Note [The id hack; or, how singletons learned to stop worrying and avoid -- kind generalization] for an explanation of why we do this. -- Note [Why error is so special] -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -- Some of the transformations that happen before this point produce impossible -- case matches. We must be careful when processing these so as not to make -- an error GHC will complain about. When binding the case-match variables, we -- normally include an equality constraint saying that the scrutinee is equal -- to the matched pattern. But, we can't do this in inaccessible matches, because -- equality is bogus, and GHC (rightly) complains. However, we then have another -- problem, because GHC doesn't have enough information when type-checking the -- RHS of the inaccessible match to deem it type-safe. The solution: treat error -- as super-special, so that GHC doesn't look too hard at singletonized error -- calls. Specifically, DON'T do the applySing stuff. Just use sError, which -- has a custom type (Sing x -> a) anyway. -- Note [Singling pattern signatures] -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -- We want to single a pattern signature, like so: -- -- f :: Maybe a -> a -- f (Just x :: Maybe a) = x -- -- Naïvely, one might expect this to single straightfowardly as: -- -- sF :: forall (z :: Maybe a). Sing z -> Sing (F z) -- sF (SJust sX :: Sing (Just x :: Maybe a)) = sX -- -- But the way GHC typechecks patterns prevents this from working, as GHC won't -- know that the type `z` is actually `Just x` until /after/ the entirety of -- the `SJust sX` pattern has been typechecked. (See Trac #12018 for an -- extended discussion on this topic.) -- -- To work around this design, we resort to a somewhat unsightly trick: -- immediately after matching on all the patterns, we perform a case on every -- pattern with a pattern signature, like so: -- -- sF :: forall (z :: Maybe a). Sing z -> Sing (F z) -- sF (SJust sX :: Sing z) -- = case (SJust sX :: Sing z) of -- (_ :: Sing (Just x :: Maybe a)) -> sX -- -- Now GHC accepts the fact that `z` is `Just x`, and all is well. In order -- to support this construction, the type of singPat is augmented with some -- extra information in the form of SingDSigPaInfos: -- -- type SingDSigPaInfos = [(DExp, DType)] -- -- Where the DExps corresponds to the expressions we case on just after the -- patterns (`SJust sX :: Sing x`, in the example above), and the DTypes -- correspond to the singled pattern signatures to use in the case alternative -- (`Sing (Just x :: Maybe a)` in the example above). singPat appends to the -- list of SingDSigPaInfos whenever it processes a DSigPa (pattern signature), -- and call sites can pass these SingDSigPaInfos to mkSigPaCaseE to construct a -- case expression like the one featured above. -- -- Some interesting consequences of this design: -- -- 1. We must promote DPats to ADPats, a variation of DPat where the annotated -- DSigPa counterpart, ADSigPa, stores the type that the original DPat was -- promoted to. This is necessary since promoting the type might have -- generated fresh variable names, so we need to be able to use the same -- names when singling. -- -- 2. Also when promoting a DSigPa to an ADSigPa, we remove any wildcards from -- the underlying pattern. To see why this is necessary, consider singling -- this example: -- -- g (Just _ :: Maybe a) = "hi" -- -- This must single to something like this: -- -- sG (SJust _ :: Sing z) -- = case (SJust _ :: Sing z) of -- (_ :: Sing (Just _ :: Maybe a)) -> "hi" -- -- But `SJust _` is not a valid expression, and since the minimal th-desugar -- AST lacks as-patterns, we can't replace it with something like -- `sG x@(SJust _ :: Sing z) = case x of ...`. But even if the th-desugar -- AST /did/ have as-patterns, we'd still be in trouble, as `Just _` isn't -- a valid type without the use of -XPartialTypeSignatures, which isn't a -- design we want to force upon others. -- -- We work around both issues by simply converting all wildcard patterns -- from the pattern that has a signature. That means our example becomes: -- -- sG (SJust sWild :: Sing z) -- = case (SJust sWild :: Sing z) of -- (_ :: Sing (Just wild :: Maybe a)) -> "hi" -- -- And now everything is hunky-dory. singExp :: ADExp -> Maybe DKind -- the kind of the expression, if known -> SgM DExp -- See Note [Why error is so special] singExp (ADVarE err `ADAppE` arg) _res_ki | err == errorName = DAppE (DVarE (singValName err)) <$> singExp arg (Just (DConT symbolName)) singExp (ADVarE name) _res_ki = lookupVarE name singExp (ADConE name) _res_ki = lookupConE name singExp (ADLitE lit) _res_ki = singLit lit singExp (ADAppE e1 e2) _res_ki = do e1' <- singExp e1 Nothing e2' <- singExp e2 Nothing -- `applySing undefined x` kills type inference, because GHC can't figure -- out the type of `undefined`. So we don't emit `applySing` there. if isException e1' then return $ e1' `DAppE` e2' else return $ (DVarE applySingName) `DAppE` e1' `DAppE` e2' singExp (ADLamE ty_names prom_lam names exp) _res_ki = do let sNames = map singValName names exp' <- singExp exp Nothing -- we need to bind the type variables... but DLamE doesn't allow SigT patterns. -- So: build a case let caseExp = DCaseE (mkTupleDExp (map DVarE sNames)) [DMatch (mkTupleDPat (map ((DWildP `DSigP`) . (singFamily `DAppT`) . DVarT) ty_names)) exp'] return $ wrapSingFun (length names) prom_lam $ DLamE sNames caseExp singExp (ADCaseE exp matches ret_ty) res_ki = -- See Note [Annotate case return type] and -- Note [The id hack; or, how singletons learned to stop worrying and -- avoid kind generalization] DAppE (DAppTypeE (DVarE 'id) (singFamily `DAppT` (ret_ty `maybeSigT` res_ki))) <$> (DCaseE <$> singExp exp Nothing <*> mapM (singMatch res_ki) matches) singExp (ADLetE env exp) res_ki = do -- We intentionally discard the SingI instances for exp's defunctionalization -- symbols, as we also do not generate the declarations for the -- defunctionalization symbols in the first place during promotion. (let_decs, _, exp') <- singLetDecEnv env $ singExp exp res_ki pure $ DLetE let_decs exp' singExp (ADSigE prom_exp exp ty) _ = do exp' <- singExp exp (Just ty) pure $ DSigE exp' $ DConT singFamilyName `DAppT` DSigT prom_exp ty -- See Note [DerivedDecl] in Data.Singletons.Syntax singDerivedEqDecs :: DerivedEqDecl -> SgM [DDec] singDerivedEqDecs (DerivedDecl { ded_mb_cxt = mb_ctxt , ded_type = ty , ded_type_tycon = ty_tycon , ded_decl = DataDecl _ _ cons }) = do (scons, _) <- singM [] $ mapM (singCtor ty_tycon) cons mb_sctxt <- mapM (mapM singPred) mb_ctxt kind <- promoteType ty sEqInst <- mkEqualityInstance mb_sctxt kind cons scons sEqClassDesc -- Beware! The user might have specified an instance context like this: -- -- deriving instance Eq a => Eq (T a Int) -- -- When we single the context, it will become (SEq a). But we do *not* want -- this for the SDecide instance! The simplest solution is to simply replace -- all occurrences of SEq with SDecide in the context. let mb_sctxtDecide = fmap (map sEqToSDecide) mb_sctxt sDecideInst <- mkEqualityInstance mb_sctxtDecide kind cons scons sDecideClassDesc testInsts <- traverse (mkTestInstance mb_sctxtDecide kind ty_tycon cons) [TestEquality, TestCoercion] return (sEqInst:sDecideInst:testInsts) -- Walk a DPred, replacing all occurrences of SEq with SDecide. sEqToSDecide :: DPred -> DPred sEqToSDecide = modifyConNameDType $ \n -> -- Why don't we directly compare n to sEqClassName? Because n is almost certainly -- produced from a call to singClassName, which uses unqualified Names. Ugh. if nameBase n == nameBase sEqClassName then sDecideClassName else n -- See Note [DerivedDecl] in Data.Singletons.Syntax singDerivedShowDecs :: DerivedShowDecl -> SgM [DDec] singDerivedShowDecs (DerivedDecl { ded_mb_cxt = mb_cxt , ded_type = ty , ded_type_tycon = ty_tycon , ded_decl = data_decl }) = do -- Generate a Show instance for a singleton type, like this: -- -- instance (ShowSing a, ShowSing b) => Show (SEither (z :: Either a b)) where -- showsPrec p (SLeft (sl :: Sing l)) = -- showParen (p > 10) $ showString "SLeft " . showsPrec 11 sl -- :: ShowSing' l => ShowS -- showsPrec p (SRight (sr :: Sing r)) = -- showParen (p > 10) $ showString "SRight " . showsPrec 11 sr -- :: ShowSing' r => ShowS -- -- Be careful: we want to generate an instance context that uses ShowSing, -- not SShow. show_sing_inst <- mkShowInstance (ForShowSing ty_tycon) mb_cxt ty data_decl pure [toInstanceD show_sing_inst] where toInstanceD :: UInstDecl -> DDec toInstanceD (InstDecl { id_cxt = cxt, id_name = inst_name , id_arg_tys = inst_tys, id_meths = ann_meths }) = DInstanceD Nothing Nothing cxt (foldType (DConT inst_name) inst_tys) (map (DLetDec . toFunD) ann_meths) toFunD :: (Name, ULetDecRHS) -> DLetDec toFunD (fun_name, UFunction clauses) = DFunD fun_name clauses toFunD (val_name, UValue rhs) = DValD (DVarP val_name) rhs isException :: DExp -> Bool isException (DVarE n) = nameBase n == "sUndefined" isException (DConE {}) = False isException (DLitE {}) = False isException (DAppE (DVarE fun) _) | nameBase fun == "sError" = True isException (DAppE fun _) = isException fun isException (DAppTypeE e _) = isException e isException (DLamE _ _) = False isException (DCaseE e _) = isException e isException (DLetE _ e) = isException e isException (DSigE e _) = isException e isException (DStaticE e) = isException e singMatch :: Maybe DKind -- ^ the result kind, if known -> ADMatch -> SgM DMatch singMatch res_ki (ADMatch var_proms pat exp) = do (sPat, sigPaExpsSigs) <- evalForPair $ singPat (Map.fromList var_proms) pat sExp <- singExp exp res_ki return $ DMatch sPat $ mkSigPaCaseE sigPaExpsSigs sExp singLit :: Lit -> SgM DExp singLit (IntegerL n) | n >= 0 = return $ DVarE sFromIntegerName `DAppE` (DVarE singMethName `DSigE` (singFamily `DAppT` DLitT (NumTyLit n))) | otherwise = do sLit <- singLit (IntegerL (-n)) return $ DVarE sNegateName `DAppE` sLit singLit (StringL str) = do let sing_str_lit = DVarE singMethName `DSigE` (singFamily `DAppT` DLitT (StrTyLit str)) os_enabled <- qIsExtEnabled LangExt.OverloadedStrings pure $ if os_enabled then DVarE sFromStringName `DAppE` sing_str_lit else sing_str_lit singLit lit = fail ("Only string and natural number literals can be singled: " ++ show lit) maybeSigT :: DType -> Maybe DKind -> DType maybeSigT ty Nothing = ty maybeSigT ty (Just ki) = ty `DSigT` ki {- Note [The id hack; or, how singletons learned to stop worrying and avoid kind generalization] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ GHC 8.8 was a time of great change. In particular, 8.8 debuted a fix for Trac #15141 (decideKindGeneralisationPlan is too complicated). To fix this, a wily GHC developer—who shall remain unnamed, but whose username rhymes with schmoldfire—decided to make decideKindGeneralisationPlan less complicated by, well, removing the whole thing. One consequence of this is that local definitions are now kind-generalized (whereas they would not have been previously). While schmoldfire had the noblest of intentions when authoring his fix, he unintentionally made life much harder for singletons. Why? Consider the following program: class Foo a where bar :: a -> (a -> b) -> b baz :: a quux :: Foo a => a -> a quux x = x `bar` \_ -> baz When singled, this program will turn into something like this: type family Quux (x :: a) :: a where Quux x = Bar x (LambdaSym1 x) sQuux :: forall a (x :: a). SFoo a => Sing x -> Sing (Quux x :: a) sQuux (sX :: Sing x) = sBar sX ((singFun1 @(LambdaSym1 x)) (\ sArg -> case sArg of { (_ :: Sing arg) -> (case sArg of { _ -> sBaz }) :: Sing (Case x arg arg) })) type family Case x arg t where Case x arg _ = Baz type family Lambda x t where Lambda x arg = Case x arg arg data LambdaSym1 x t type instance Apply (LambdaSym1 x) t = Lambda x t The high-level bit is the explicit `Sing (Case x arg arg)` signature. Question: what is the kind of `Case x arg arg`? The answer depends on whether local definitions are kind-generalized or not! 1. If local definitions are *not* kind-generalized (i.e., the status quo before GHC 8.8), then `Case x arg arg :: a`. 2. If local definitions *are* kind-generalized (i.e., the status quo in GHC 8.8 and later), then `Case x arg arg :: k` for some fresh kind variable `k`. Unfortunately, the kind of `Case x arg arg` *must* be `a` in order for `sQuux` to type-check. This means that the code above suddenly stopped working in GHC 8.8. What's more, we can't just remove these explicit signatures, as there is code elsewhere in `singletons` that crucially relies on them to guide type inference along (e.g., `sShowParen` in `Data.Singletons.Prelude.Show`). Luckily, there is an ingenious hack that lets us the benefits of explicit signatures without the pain of kind generalization: our old friend, the `id` function. The plan is as follows: instead of generating this code: (case sArg of ...) :: Sing (Case x arg arg) We instead generate this code: id @(Sing (Case x arg arg)) (case sArg of ...) That's it! This works because visible type arguments in terms do not get kind- generalized, unlike top-level or local signatures. Now `Case x arg arg`'s kind is not generalized, and all is well. We dub this: the `id` hack. One might wonder: will we need the `id` hack around forever? Perhaps not. While GHC 8.8 removed the decideKindGeneralisationPlan function, there have been rumblings that a future version of GHC may bring it back (in a limited form). If this happens, it is possibly that GHC's attitude towards kind-generalizing local definitons may change *again*, which could conceivably render the `id` hack unnecessary. This is all speculation, of course, so all we can do now is wait and revisit this design at a later date. -}