basicTypes/PatSyn.hs

    {-
    (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, patSynTyDetails, patSynType,
            patSynMatcher, patSynBuilder,
            patSynExTyVars, patSynSig,
            patSynInstArgTys, patSynInstResTy,
            tidyPatSynIds
        ) where

    #include "HsVersions.h"

    import Type
    import TcType( mkSigmaTy )
    import Name
    import Outputable
    import Unique
    import Util
    import BasicTypes
    import FastString
    import Var
    import HsBinds( HsPatSynDetails(..) )

    import qualified Data.Data as Data
    import qualified Data.Typeable
    import Data.Function

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Pattern synonyms}
    *                                                                      *
    ************************************************************************
    -}

    -- | A pattern synonym
    -- See Note [Pattern synonym representation]
    data PatSyn
      = MkPatSyn {
            psName        :: Name,
            psUnique      :: Unique,      -- Cached from Name

            psArgs        :: [Type],
            psArity       :: Arity,       -- == length psArgs
            psInfix       :: Bool,        -- True <=> declared infix

            psUnivTyVars  :: [TyVar],     -- Universially-quantified type variables
            psReqTheta    :: ThetaType,   -- Required dictionaries
            psExTyVars    :: [TyVar],     -- Existentially-quantified type vars
            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 (r :: ?) univ_tvs. req_theta
                 --                       => res_ty
                 --                       -> (forall ex_tvs. Void# -> r)
                 --                       -> (Void# -> r)
                 --                       -> r
                 --
                 -- Otherwise type is
                 --   forall (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. (prov_theta, req_theta)
                 --                       =>  arg_tys -> res_ty
                 -- See Note [Builder for pattern synonyms with unboxed type]
      }
      deriving Data.Typeable.Typeable

    {-
    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:

            provides: (Show (Maybe t), Ord b)
            requires: (Eq t, Num t)

    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. (Show (Maybe t), Ord b, Eq t, Num t)
                => 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.

    ************************************************************************
    *                                                                      *
    \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], ThetaType) -- ^ Universially-quantified type variables
                                     --   and required dicts
             -> ([TyVar], 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
             -> PatSyn
    mkPatSyn name declared_infix
             (univ_tvs, req_theta)
             (ex_tvs, prov_theta)
             orig_args
             orig_res_ty
             matcher builder
        = 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,
                    psOrigResTy = orig_res_ty,
                    psMatcher = matcher,
                    psBuilder = builder }

    -- | The 'Name' of the 'PatSyn', giving it a unique, rooted identification
    patSynName :: PatSyn -> Name
    patSynName = psName

    patSynType :: PatSyn -> Type
    -- The full pattern type, used only in error messages
    patSynType (MkPatSyn { psUnivTyVars = univ_tvs, psReqTheta = req_theta
                         , psExTyVars   = ex_tvs,   psProvTheta = prov_theta
                         , psArgs = orig_args, psOrigResTy = orig_res_ty })
      = mkSigmaTy univ_tvs req_theta $
        mkSigmaTy ex_tvs prov_theta $
        mkFunTys orig_args orig_res_ty

    -- | 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

    patSynTyDetails :: PatSyn -> HsPatSynDetails Type
    patSynTyDetails (MkPatSyn { psInfix = is_infix, psArgs = arg_tys })
      | is_infix, [left,right] <- arg_tys
      = InfixPatSyn left right
      | otherwise
      = PrefixPatSyn arg_tys

    patSynExTyVars :: PatSyn -> [TyVar]
    patSynExTyVars = psExTyVars

    patSynSig :: PatSyn -> ([TyVar], [TyVar], ThetaType, ThetaType, [Type], Type)
    patSynSig (MkPatSyn { psUnivTyVars = univ_tvs, psExTyVars = ex_tvs
                        , psProvTheta = prov, psReqTheta = req
                        , psArgs = arg_tys, psOrigResTy = res_ty })
      = (univ_tvs, ex_tvs, prov, req, 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
              , ptext (sLit "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
               , ptext (sLit "patSynInstResTy") <+> ppr name $$ ppr univ_tvs $$ ppr inst_tys )
        substTyWith univ_tvs inst_tys res_ty