{- (c) The University of Glasgow 2006 (c) The GRASP/AQUA Project, Glasgow University, 1998 \section[PatSyn]{@PatSyn@: Pattern synonyms} -} {-# LANGUAGE CPP, DeriveDataTypeable #-} module PatSyn ( -- * Main data types PatSyn, mkPatSyn, -- ** Type deconstruction patSynName, patSynArity, patSynIsInfix, patSynArgs, patSynMatcher, patSynBuilder, patSynUnivTyBinders, patSynExTyVars, patSynExTyBinders, patSynSig, patSynInstArgTys, patSynInstResTy, patSynFieldLabels, patSynFieldType, tidyPatSynIds, pprPatSynType ) where #include "HsVersions.h" import Type import TcType( mkSpecSigmaTy ) import Name import Outputable import Unique import Util import BasicTypes import Var import FieldLabel import qualified Data.Data as Data import qualified Data.Typeable import Data.Function import Data.List {- ************************************************************************ * * \subsection{Pattern synonyms} * * ************************************************************************ -} -- | A pattern synonym -- See Note [Pattern synonym representation] -- See Note [Pattern synonym signatures] 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 psUnivTyVars :: [TyVar], -- Universially-quantified type variables psUnivTyBinders :: [TyBinder], -- same, with visibility info psReqTheta :: ThetaType, -- Required dictionaries -- these constraints are very much like -- stupid thetas (which is a useful -- guideline when implementing) -- but are actually needed. psExTyVars :: [TyVar], -- Existentially-quantified type vars psExTyBinders :: [TyBinder], -- same, with visibility info psProvTheta :: ThetaType, -- Provided dictionaries psOrigResTy :: Type, -- Mentions only psUnivTyVars -- 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] } deriving Data.Typeable.Typeable {- Note [Pattern synonym signatures] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 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 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) psOrigResTy = 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 [Patterns synonyms and the data type Type] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The type of a pattern synonym is of the form (See Note [Pattern synonym signatures]): 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 Ord PatSyn where (<=) = (<=) `on` getUnique (<) = (<) `on` getUnique (>=) = (>=) `on` getUnique (>) = (>) `on` getUnique compare = compare `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? -> ([TyVar], [TyBinder], ThetaType) -- ^ Universially-quantified type variables -- and required dicts -> ([TyVar], [TyBinder], 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, univ_bndrs, req_theta) (ex_tvs, ex_bndrs, prov_theta) orig_args orig_res_ty matcher builder field_labels = MkPatSyn {psName = name, psUnique = getUnique name, psUnivTyVars = univ_tvs, psUnivTyBinders = univ_bndrs, psExTyVars = ex_tvs, psExTyBinders = ex_bndrs, psProvTheta = prov_theta, psReqTheta = req_theta, psInfix = declared_infix, psArgs = orig_args, psArity = length orig_args, psOrigResTy = 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) patSynUnivTyBinders :: PatSyn -> [TyBinder] patSynUnivTyBinders = psUnivTyBinders patSynExTyVars :: PatSyn -> [TyVar] patSynExTyVars = psExTyVars patSynExTyBinders :: PatSyn -> [TyBinder] patSynExTyBinders = psExTyBinders patSynSig :: PatSyn -> ([TyVar], ThetaType, [TyVar], ThetaType, [Type], Type) patSynSig (MkPatSyn { psUnivTyVars = univ_tvs, psExTyVars = ex_tvs , psProvTheta = prov, psReqTheta = req , psArgs = arg_tys, psOrigResTy = res_ty }) = (univ_tvs, req, 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( length tyvars == length inst_tys , text "patSynInstArgTys" <+> ppr name $$ ppr tyvars $$ ppr inst_tys ) map (substTyWith tyvars inst_tys) arg_tys where tyvars = 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: unlikepatSynInstArgTys, the inst_tys should be just the *universal* tyvars patSynInstResTy (MkPatSyn { psName = name, psUnivTyVars = univ_tvs , psOrigResTy = res_ty }) inst_tys = ASSERT2( length univ_tvs == length inst_tys , text "patSynInstResTy" <+> ppr name $$ ppr univ_tvs $$ ppr inst_tys ) substTyWith 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, psOrigResTy = orig_res_ty }) = sep [ pprForAllImplicit univ_tvs , pprThetaArrowTy req_theta , ppWhen insert_empty_ctxt $ parens empty <+> darrow , pprType sigma_ty ] where sigma_ty = mkSpecSigmaTy ex_tvs prov_theta $ mkFunTys orig_args orig_res_ty insert_empty_ctxt = null req_theta && not (null prov_theta && null ex_tvs)