{-
(c) The University of Glasgow 2006
(c) The GRASP/AQUA Project, Glasgow University, 1998
\section[PatSyn]{@PatSyn@: Pattern synonyms}
-}{-# LANGUAGE CPP #-}modulePatSyn(-- * Main data typesPatSyn ,mkPatSyn ,-- ** Type deconstructionpatSynName ,patSynArity ,patSynIsInfix ,patSynArgs ,patSynMatcher ,patSynBuilder ,patSynUnivTyVarBinders ,patSynExTyVars ,patSynExTyVarBinders ,patSynSig ,patSynInstArgTys ,patSynInstResTy ,patSynFieldLabels ,patSynFieldType ,tidyPatSynIds ,pprPatSynType )where#include "HsVersions.h"
importGhcPrelude importType importName importOutputable importUnique importUtil importBasicTypes importVar importFieldLabel importqualifiedData.DataasDataimportData.FunctionimportData.List{-
************************************************************************
* *
\subsection{Pattern synonyms}
* *
************************************************************************
-}-- | Pattern Synonym---- See Note [Pattern synonym representation]-- See Note [Pattern synonym signature contexts]dataPatSyn =MkPatSyn {psName ::Name ,psUnique ::Unique ,-- Cached from NamepsArgs ::[Type ],psArity ::Arity ,-- == length psArgspsInfix ::Bool,-- True <=> declared infixpsFieldLabels ::[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 variablespsUnivTyVars ::[TyVarBinder ],-- Required dictionaries (may mention psUnivTyVars)psReqTheta ::ThetaType ,-- Existentially-quantified type varspsExTyVars ::[TyVarBinder ],-- Provided dictionaries (may mention psUnivTyVars or psExTyVars)psProvTheta ::ThetaType ,-- Result typepsResultTy ::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)-- -> rpsBuilder ::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 #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}
* *
************************************************************************
-}instanceEqPatSyn where(== )=(==)`on`getUnique (/= )=(/=)`on`getUnique instanceUniquable PatSyn wheregetUnique =psUniqueinstanceNamedThing PatSyn wheregetName =patSynName instanceOutputable PatSyn whereppr =ppr .getName instanceOutputableBndr PatSyn wherepprInfixOcc =pprInfixName .getName pprPrefixOcc =pprPrefixName .getName instanceData.DataPatSyn where-- don't traverse?toConstr _=abstractConstr "PatSyn"gunfold __=error"gunfold"dataTypeOf _=mkNoRepType"PatSyn"{-
************************************************************************
* *
\subsection{Construction}
* *
************************************************************************
-}-- | Build a new pattern synonymmkPatSyn::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=lengthorig_args ,psResultTy=orig_res_ty ,psMatcher=matcher ,psBuilder=builder ,psFieldLabels=field_labels }-- | The 'Name' of the 'PatSyn', giving it a unique, rooted identificationpatSynName::PatSyn ->Name patSynName =psName-- | Should the 'PatSyn' be presented infix?patSynIsInfix::PatSyn ->BoolpatSynIsInfix =psInfix-- | Arity of the pattern synonympatSynArity::PatSyn ->Arity patSynArity =psAritypatSynArgs::PatSyn ->[Type ]patSynArgs =psArgspatSynFieldLabels::PatSyn ->[FieldLabel ]patSynFieldLabels =psFieldLabels-- | Extract the type for any given labelled field of the 'DataCon'patSynFieldType::PatSyn ->FieldLabelString ->Type patSynFieldType ps label=casefind((==label).flLabel.fst)(psFieldLabelsps `zip`psArgsps )ofJust(_,ty )->ty Nothing->pprPanic "dataConFieldType"(ppr ps <+> ppr label)patSynUnivTyVarBinders::PatSyn ->[TyVarBinder ]patSynUnivTyVarBinders =psUnivTyVarspatSynExTyVars::PatSyn ->[TyVar ]patSynExTyVars ps =binderVars (psExTyVarsps )patSynExTyVarBinders::PatSyn ->[TyVarBinder ]patSynExTyVarBinders =psExTyVarspatSynSig::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 =psMatcherpatSynBuilder::PatSyn ->Maybe(Id ,Bool)patSynBuilder =psBuildertidyPatSynIds::(Id ->Id )->PatSyn ->PatSyn tidyPatSynIds tidy_fn ps @(MkPatSyn {psMatcher=matcher ,psBuilder=builder })=ps {psMatcher=tidy_pr matcher ,psBuilder=fmaptidy_pr builder }wheretidy_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 existentialpatSynInstArgTys (MkPatSyn {psName=name ,psUnivTyVars=univ_tvs ,psExTyVars=ex_tvs ,psArgs=arg_tys })inst_tys =ASSERT2(tyvars`equalLength`inst_tys ,text"patSynInstArgTys"<+>pprname$$pprtyvars$$pprinst_tys)map(substTyWith tyvars inst_tys )arg_tys wheretyvars =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* tyvarspatSynInstResTy (MkPatSyn {psName=name ,psUnivTyVars=univ_tvs ,psResultTy=res_ty })inst_tys =ASSERT2(univ_tvs`equalLength`inst_tys ,text"patSynInstResTy"<+>pprname$$ppruniv_tvs$$pprinst_tys)substTyWith (binderVars univ_tvs )inst_tys res_ty -- | Print the type of a pattern synonym. The foralls are printed explicitlypprPatSynType::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 ]wheresigma_ty =mkForAllTys ex_tvs $mkInvisFunTys prov_theta $mkVisFunTys orig_args orig_res_ty insert_empty_ctxt =nullreq_theta &&not(nullprov_theta &&nullex_tvs )