{- (c) The University of Glasgow 2006 (c) The GRASP/AQUA Project, Glasgow University, 1998 \section[PatSyn]{@PatSyn@: Pattern synonyms} -} {-# LANGUAGE CPP #-} module PatSyn ( -- * Main data types PatSyn, mkPatSyn, -- ** Type deconstruction patSynName, patSynArity, patSynIsInfix, patSynArgs, patSynMatcher, patSynBuilder, patSynUnivTyVarBinders, patSynExTyVars, patSynExTyVarBinders, patSynSig, patSynInstArgTys, patSynInstResTy, patSynFieldLabels, patSynFieldType, tidyPatSynIds, pprPatSynType ) where #include "HsVersions.h" import GhcPrelude import Type import Name import Outputable import Unique import Util import BasicTypes import Var import FieldLabel import qualified Data.Data as Data import Data.Function import Data.List {- ************************************************************************ * * \subsection{Pattern synonyms} * * ************************************************************************ -} -- | Pattern Synonym -- -- See Note [Pattern synonym representation] -- See Note [Pattern synonym signature contexts] data PatSyn = MkPatSyn { psName :: Name, psUnique :: Unique, -- Cached from Name psArgs :: [Type], psArity :: Arity, -- == length psArgs psInfix :: Bool, -- True <=> declared infix psFieldLabels :: [FieldLabel], -- List of fields for a -- record pattern synonym -- INVARIANT: either empty if no -- record pat syn or same length as -- psArgs -- Universally-quantified type variables psUnivTyVars :: [TyVarBinder], -- Required dictionaries (may mention psUnivTyVars) psReqTheta :: ThetaType, -- Existentially-quantified type vars psExTyVars :: [TyVarBinder], -- Provided dictionaries (may mention psUnivTyVars or psExTyVars) psProvTheta :: ThetaType, -- Result type psResultTy :: Type, -- Mentions only psUnivTyVars -- See Note [Pattern synonym result type] -- See Note [Matchers and builders for pattern synonyms] psMatcher :: (Id, Bool), -- Matcher function. -- If Bool is True then prov_theta and arg_tys are empty -- and type is -- forall (p :: RuntimeRep) (r :: TYPE p) univ_tvs. -- req_theta -- => res_ty -- -> (forall ex_tvs. Void# -> r) -- -> (Void# -> r) -- -> r -- -- Otherwise type is -- forall (p :: RuntimeRep) (r :: TYPE r) univ_tvs. -- req_theta -- => res_ty -- -> (forall ex_tvs. prov_theta => arg_tys -> r) -- -> (Void# -> r) -- -> r psBuilder :: Maybe (Id, Bool) -- Nothing => uni-directional pattern synonym -- Just (builder, is_unlifted) => bi-directional -- Builder function, of type -- forall univ_tvs, ex_tvs. (req_theta, prov_theta) -- => arg_tys -> res_ty -- See Note [Builder for pattern synonyms with unboxed type] } {- Note [Pattern synonym signature contexts] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In a pattern synonym signature we write pattern P :: req => prov => t1 -> ... tn -> res_ty Note that the "required" context comes first, then the "provided" context. Moreover, the "required" context must not mention existentially-bound type variables; that is, ones not mentioned in res_ty. See lots of discussion in Trac #10928. If there is no "provided" context, you can omit it; but you can't omit the "required" part (unless you omit both). Example 1: pattern P1 :: (Num a, Eq a) => b -> Maybe (a,b) pattern P1 x = Just (3,x) We require (Num a, Eq a) to match the 3; there is no provided context. Example 2: data T2 where MkT2 :: (Num a, Eq a) => a -> a -> T2 pattern P2 :: () => (Num a, Eq a) => a -> T2 pattern P2 x = MkT2 3 x When we match against P2 we get a Num dictionary provided. We can use that to check the match against 3. Example 3: pattern P3 :: Eq a => a -> b -> T3 b This signature is illegal because the (Eq a) is a required constraint, but it mentions the existentially-bound variable 'a'. You can see it's existential because it doesn't appear in the result type (T3 b). Note [Pattern synonym result type] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider data T a b = MkT b a pattern P :: a -> T [a] Bool pattern P x = MkT True [x] P's psResultTy is (T a Bool), and it really only matches values of type (T [a] Bool). For example, this is ill-typed f :: T p q -> String f (P x) = "urk" This is different to the situation with GADTs: data S a where MkS :: Int -> S Bool Now MkS (and pattern synonyms coming from MkS) can match a value of type (S a), not just (S Bool); we get type refinement. That in turn means that if you have a pattern P x :: T [ty] Bool it's not entirely straightforward to work out the instantiation of P's universal tyvars. You have to /match/ the type of the pattern, (T [ty] Bool) against the psResultTy for the pattern synonym, T [a] Bool to get the instantiation a := ty. This is very unlike DataCons, where univ tyvars match 1-1 the arguments of the TyCon. Note [Pattern synonym representation] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider the following pattern synonym declaration pattern P x = MkT [x] (Just 42) where data T a where MkT :: (Show a, Ord b) => [b] -> a -> T a so pattern P has type b -> T (Maybe t) with the following typeclass constraints: requires: (Eq t, Num t) provides: (Show (Maybe t), Ord b) In this case, the fields of MkPatSyn will be set as follows: psArgs = [b] psArity = 1 psInfix = False psUnivTyVars = [t] psExTyVars = [b] psProvTheta = (Show (Maybe t), Ord b) psReqTheta = (Eq t, Num t) psResultTy = T (Maybe t) Note [Matchers and builders for pattern synonyms] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ For each pattern synonym P, we generate * a "matcher" function, used to desugar uses of P in patterns, which implements pattern matching * A "builder" function (for bidirectional pattern synonyms only), used to desugar uses of P in expressions, which constructs P-values. For the above example, the matcher function has type: $mP :: forall (r :: ?) t. (Eq t, Num t) => T (Maybe t) -> (forall b. (Show (Maybe t), Ord b) => b -> r) -> (Void# -> r) -> r with the following implementation: $mP @r @t $dEq $dNum scrut cont fail = case scrut of MkT @b $dShow $dOrd [x] (Just 42) -> cont @b $dShow $dOrd x _ -> fail Void# Notice that the return type 'r' has an open kind, so that it can be instantiated by an unboxed type; for example where we see f (P x) = 3# The extra Void# argument for the failure continuation is needed so that it is lazy even when the result type is unboxed. For the same reason, if the pattern has no arguments, an extra Void# argument is added to the success continuation as well. For *bidirectional* pattern synonyms, we also generate a "builder" function which implements the pattern synonym in an expression context. For our running example, it will be: $bP :: forall t b. (Eq t, Num t, Show (Maybe t), Ord b) => b -> T (Maybe t) $bP x = MkT [x] (Just 42) NB: the existential/universal and required/provided split does not apply to the builder since you are only putting stuff in, not getting stuff out. Injectivity of bidirectional pattern synonyms is checked in tcPatToExpr which walks the pattern and returns its corresponding expression when available. Note [Builder for pattern synonyms with unboxed type] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ For bidirectional pattern synonyms that have no arguments and have an unboxed type, we add an extra Void# argument to the builder, else it would be a top-level declaration with an unboxed type. pattern P = 0# $bP :: Void# -> Int# $bP _ = 0# This means that when typechecking an occurrence of P in an expression, we must remember that the builder has this void argument. This is done by TcPatSyn.patSynBuilderOcc. Note [Pattern synonyms and the data type Type] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The type of a pattern synonym is of the form (See Note [Pattern synonym signatures] in TcSigs): forall univ_tvs. req => forall ex_tvs. prov => ... We cannot in general represent this by a value of type Type: - if ex_tvs is empty, then req and prov cannot be distinguished from each other - if req is empty, then univ_tvs and ex_tvs cannot be distinguished from each other, and moreover, prov is seen as the "required" context (as it is the only context) ************************************************************************ * * \subsection{Instances} * * ************************************************************************ -} instance Eq PatSyn where (==) = (==) `on` getUnique (/=) = (/=) `on` getUnique instance Uniquable PatSyn where getUnique = psUnique instance NamedThing PatSyn where getName = patSynName instance Outputable PatSyn where ppr = ppr . getName instance OutputableBndr PatSyn where pprInfixOcc = pprInfixName . getName pprPrefixOcc = pprPrefixName . getName instance Data.Data PatSyn where -- don't traverse? toConstr _ = abstractConstr "PatSyn" gunfold _ _ = error "gunfold" dataTypeOf _ = mkNoRepType "PatSyn" {- ************************************************************************ * * \subsection{Construction} * * ************************************************************************ -} -- | Build a new pattern synonym mkPatSyn :: Name -> Bool -- ^ Is the pattern synonym declared infix? -> ([TyVarBinder], ThetaType) -- ^ Universially-quantified type -- variables and required dicts -> ([TyVarBinder], ThetaType) -- ^ Existentially-quantified type -- variables and provided dicts -> [Type] -- ^ Original arguments -> Type -- ^ Original result type -> (Id, Bool) -- ^ Name of matcher -> Maybe (Id, Bool) -- ^ Name of builder -> [FieldLabel] -- ^ Names of fields for -- a record pattern synonym -> PatSyn -- NB: The univ and ex vars are both in TyBinder form and TyVar form for -- convenience. All the TyBinders should be Named! mkPatSyn name declared_infix (univ_tvs, req_theta) (ex_tvs, prov_theta) orig_args orig_res_ty matcher builder field_labels = MkPatSyn {psName = name, psUnique = getUnique name, psUnivTyVars = univ_tvs, psExTyVars = ex_tvs, psProvTheta = prov_theta, psReqTheta = req_theta, psInfix = declared_infix, psArgs = orig_args, psArity = length orig_args, psResultTy = orig_res_ty, psMatcher = matcher, psBuilder = builder, psFieldLabels = field_labels } -- | The 'Name' of the 'PatSyn', giving it a unique, rooted identification patSynName :: PatSyn -> Name patSynName = psName -- | Should the 'PatSyn' be presented infix? patSynIsInfix :: PatSyn -> Bool patSynIsInfix = psInfix -- | Arity of the pattern synonym patSynArity :: PatSyn -> Arity patSynArity = psArity patSynArgs :: PatSyn -> [Type] patSynArgs = psArgs patSynFieldLabels :: PatSyn -> [FieldLabel] patSynFieldLabels = psFieldLabels -- | Extract the type for any given labelled field of the 'DataCon' patSynFieldType :: PatSyn -> FieldLabelString -> Type patSynFieldType ps label = case find ((== label) . flLabel . fst) (psFieldLabels ps `zip` psArgs ps) of Just (_, ty) -> ty Nothing -> pprPanic "dataConFieldType" (ppr ps <+> ppr label) patSynUnivTyVarBinders :: PatSyn -> [TyVarBinder] patSynUnivTyVarBinders = psUnivTyVars patSynExTyVars :: PatSyn -> [TyVar] patSynExTyVars ps = binderVars (psExTyVars ps) patSynExTyVarBinders :: PatSyn -> [TyVarBinder] patSynExTyVarBinders = psExTyVars patSynSig :: PatSyn -> ([TyVar], ThetaType, [TyVar], ThetaType, [Type], Type) patSynSig (MkPatSyn { psUnivTyVars = univ_tvs, psExTyVars = ex_tvs , psProvTheta = prov, psReqTheta = req , psArgs = arg_tys, psResultTy = res_ty }) = (binderVars univ_tvs, req, binderVars ex_tvs, prov, arg_tys, res_ty) patSynMatcher :: PatSyn -> (Id,Bool) patSynMatcher = psMatcher patSynBuilder :: PatSyn -> Maybe (Id, Bool) patSynBuilder = psBuilder tidyPatSynIds :: (Id -> Id) -> PatSyn -> PatSyn tidyPatSynIds tidy_fn ps@(MkPatSyn { psMatcher = matcher, psBuilder = builder }) = ps { psMatcher = tidy_pr matcher, psBuilder = fmap tidy_pr builder } where tidy_pr (id, dummy) = (tidy_fn id, dummy) patSynInstArgTys :: PatSyn -> [Type] -> [Type] -- Return the types of the argument patterns -- e.g. data D a = forall b. MkD a b (b->a) -- pattern P f x y = MkD (x,True) y f -- D :: forall a. forall b. a -> b -> (b->a) -> D a -- P :: forall c. forall b. (b->(c,Bool)) -> c -> b -> P c -- patSynInstArgTys P [Int,bb] = [bb->(Int,Bool), Int, bb] -- NB: the inst_tys should be both universal and existential patSynInstArgTys (MkPatSyn { psName = name, psUnivTyVars = univ_tvs , psExTyVars = ex_tvs, psArgs = arg_tys }) inst_tys = ASSERT2( tyvars `equalLength` inst_tys , text "patSynInstArgTys" <+> ppr name $$ ppr tyvars $$ ppr inst_tys ) map (substTyWith tyvars inst_tys) arg_tys where tyvars = binderVars (univ_tvs ++ ex_tvs) patSynInstResTy :: PatSyn -> [Type] -> Type -- Return the type of whole pattern -- E.g. pattern P x y = Just (x,x,y) -- P :: a -> b -> Just (a,a,b) -- (patSynInstResTy P [Int,Bool] = Maybe (Int,Int,Bool) -- NB: unlike patSynInstArgTys, the inst_tys should be just the *universal* tyvars patSynInstResTy (MkPatSyn { psName = name, psUnivTyVars = univ_tvs , psResultTy = res_ty }) inst_tys = ASSERT2( univ_tvs `equalLength` inst_tys , text "patSynInstResTy" <+> ppr name $$ ppr univ_tvs $$ ppr inst_tys ) substTyWith (binderVars univ_tvs) inst_tys res_ty -- | Print the type of a pattern synonym. The foralls are printed explicitly pprPatSynType :: PatSyn -> SDoc pprPatSynType (MkPatSyn { psUnivTyVars = univ_tvs, psReqTheta = req_theta , psExTyVars = ex_tvs, psProvTheta = prov_theta , psArgs = orig_args, psResultTy = orig_res_ty }) = sep [ pprForAll univ_tvs , pprThetaArrowTy req_theta , ppWhen insert_empty_ctxt $ parens empty <+> darrow , pprType sigma_ty ] where sigma_ty = mkForAllTys ex_tvs $ mkFunTys prov_theta $ mkFunTys orig_args orig_res_ty insert_empty_ctxt = null req_theta && not (null prov_theta && null ex_tvs)