basicTypes/PatSyn.hs

    {-
    (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 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

            -- Universially-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
            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]
      }

    {- 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 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 [Pattern 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 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,
                    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)

    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, psOrigResTy = 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( length tyvars == length 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: 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 (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, psOrigResTy = 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)