\section[TcPat]{Typechecking patterns}
\begin{code}
-module TcPat ( tcPat, tcPats, PatCtxt(..), badFieldCon, polyPatSig ) where
+module TcPat ( tcPat, tcPats, PatCtxt(..), badFieldCon, polyPatSig, refineTyVars ) where
#include "HsVersions.h"
instToId, tcInstStupidTheta, tcSyntaxName
)
import Id ( Id, idType, mkLocalId )
+import Var ( tyVarName )
import Name ( Name )
import TcSimplify ( tcSimplifyCheck, bindInstsOfLocalFuns )
-import TcEnv ( newLocalName, tcExtendIdEnv1, tcExtendTyVarEnv,
+import TcEnv ( newLocalName, tcExtendIdEnv1, tcExtendTyVarEnv2,
tcLookupClass, tcLookupDataCon, tcLookupId )
-import TcMType ( newTyFlexiVarTy, arityErr, tcSkolTyVars, isRigidType )
+import TcMType ( newTyFlexiVarTy, arityErr, tcSkolTyVars, readMetaTyVar )
import TcType ( TcType, TcTyVar, TcSigmaType, TcTauType, zipTopTvSubst,
- SkolemInfo(PatSkol), isSkolemTyVar, pprSkolemTyVar,
+ SkolemInfo(PatSkol), isSkolemTyVar, isMetaTyVar, pprTcTyVar,
+ TvSubst, mkOpenTvSubst, substTyVar, substTy, MetaDetails(..),
mkTyVarTys, mkClassPred, mkTyConApp, isOverloadedTy )
+import VarEnv ( mkVarEnv ) -- ugly
import Kind ( argTypeKind, liftedTypeKind )
import TcUnify ( tcSubPat, Expected(..), zapExpectedType,
zapExpectedTo, zapToListTy, zapToTyConApp )
import TcHsType ( UserTypeCtxt(..), TcSigInfo( sig_tau ), TcSigFun, tcHsPatSigType )
import TysWiredIn ( stringTy, parrTyCon, tupleTyCon )
-import Unify ( MaybeErr(..), tcRefineTys, tcMatchTys )
+import Unify ( MaybeErr(..), gadtRefineTys, BindFlag(..) )
import Type ( substTys, substTheta )
-import CmdLineOpts ( opt_IrrefutableTuples )
+import StaticFlags ( opt_IrrefutableTuples )
import TyCon ( TyCon )
import DataCon ( DataCon, dataConTyCon, isVanillaDataCon, dataConInstOrigArgTys,
dataConFieldLabels, dataConSourceArity, dataConSig )
import PrelNames ( eqStringName, eqName, geName, negateName, minusName,
integralClassName )
import BasicTypes ( isBoxed )
-import SrcLoc ( Located(..), noLoc, unLoc )
+import SrcLoc ( Located(..), SrcSpan, noLoc, unLoc )
+import Maybes ( catMaybes )
import ErrUtils ( Message )
import Outputable
import FastString
%************************************************************************
\begin{code}
-data PatCtxt = LamPat Bool | LetPat TcSigFun
- -- True <=> we are checking the case expression,
- -- so can do full-blown refinement
- -- False <=> inferring, do no refinement
+data PatCtxt = LamPat -- Used for lambda, case, do-notation etc
+ | LetPat TcSigFun -- Used for let(rec) bindings
-------------------
tcPatBndr :: PatCtxt -> Name -> Expected TcSigmaType -> TcM TcId
-tcPatBndr (LamPat _) bndr_name pat_ty
+tcPatBndr LamPat bndr_name pat_ty
= do { pat_ty' <- zapExpectedType pat_ty argTypeKind
-- If pat_ty is Expected, this returns the appropriate
-- SigmaType. In Infer mode, we create a fresh type variable.
= do { -- See Note [Pattern coercions] below
(sig_tvs, sig_ty) <- tcHsPatSigType PatSigCtxt sig
; tcSubPat sig_ty pat_ty
- ; (pat', tvs, res) <- tcExtendTyVarEnv sig_tvs $
- tc_lpat ctxt pat (Check sig_ty) thing_inside
+ ; subst <- refineTyVars sig_tvs -- See note [Type matching]
+ ; let tv_binds = [(tyVarName tv, substTyVar subst tv) | tv <- sig_tvs]
+ sig_ty' = substTy subst sig_ty
+ ; (pat', tvs, res)
+ <- tcExtendTyVarEnv2 tv_binds $
+ tc_lpat ctxt pat (Check sig_ty') thing_inside
+
; return (SigPatOut pat' sig_ty, tvs, res) }
tc_pat ctxt pat@(TypePat ty) pat_ty thing_inside
= do { data_con <- tcLookupDataCon con_name
; let tycon = dataConTyCon data_con
; ty_args <- zapToTyConApp tycon pat_ty
- ; (pat', tvs, res) <- tcConPat ctxt data_con tycon ty_args arg_pats thing_inside
+ ; (pat', tvs, res) <- tcConPat ctxt con_span data_con tycon ty_args arg_pats thing_inside
; return (pat', tvs, res) }
%************************************************************************
\begin{code}
-tcConPat :: PatCtxt -> DataCon -> TyCon -> [TcTauType]
+tcConPat :: PatCtxt -> SrcSpan -> DataCon -> TyCon -> [TcTauType]
-> HsConDetails Name (LPat Name) -> TcM a
-> TcM (Pat TcId, [TcTyVar], a)
-tcConPat ctxt data_con tycon ty_args arg_pats thing_inside
+tcConPat ctxt span data_con tycon ty_args arg_pats thing_inside
| isVanillaDataCon data_con
= do { let arg_tys = dataConInstOrigArgTys data_con ty_args
; tcInstStupidTheta data_con ty_args
; traceTc (text "tcConPat" <+> vcat [ppr data_con, ppr ty_args, ppr arg_tys])
; (arg_pats', tvs, res) <- tcConArgs ctxt data_con arg_pats arg_tys thing_inside
- ; return (ConPatOut data_con [] [] emptyLHsBinds
+ ; return (ConPatOut (L span data_con) [] [] emptyLHsBinds
arg_pats' (mkTyConApp tycon ty_args),
tvs, res) }
arg_tys' = substTys tenv arg_tys
res_tys' = substTys tenv res_tys
; dicts <- newDicts (SigOrigin rigid_info) theta'
- ; tcInstStupidTheta data_con tv_tys'
-- Do type refinement!
; traceTc (text "tcGadtPat" <+> vcat [ppr data_con, ppr tvs', ppr arg_tys', ppr res_tys',
text "ty-args:" <+> ppr ty_args ])
; refineAlt ctxt data_con tvs' ty_args res_tys' $ do
- { ((arg_pats', inner_tvs, res), lie_req)
- <- getLIE (tcConArgs ctxt data_con arg_pats arg_tys' thing_inside)
+ { ((arg_pats', inner_tvs, res), lie_req) <- getLIE $
+ do { tcInstStupidTheta data_con tv_tys'
+ -- The stupid-theta mentions the newly-bound tyvars, so
+ -- it must live inside the getLIE, so that the
+ -- tcSimplifyCheck will apply the type refinement to it
+ ; tcConArgs ctxt data_con arg_pats arg_tys' thing_inside }
; dict_binds <- tcSimplifyCheck doc tvs' dicts lie_req
- ; return (ConPatOut data_con
+ ; return (ConPatOut (L span data_con)
tvs' (map instToId dicts) dict_binds
arg_pats' (mkTyConApp tycon ty_args),
tvs' ++ inner_tvs, res) } }
-> TcM a -> TcM a
refineAlt ctxt con ex_tvs ctxt_tys pat_tys thing_inside
= do { old_subst <- getTypeRefinement
- ; let refiner | can_i_refine ctxt = tcRefineTys
- | otherwise = tcMatchTys
- ; case refiner ex_tvs old_subst pat_tys ctxt_tys of
+ ; case gadtRefineTys bind_fn old_subst pat_tys ctxt_tys of
Failed msg -> failWithTc (inaccessibleAlt msg)
Succeeded new_subst -> do {
traceTc (text "refineTypes:match" <+> ppr con <+> ppr new_subst)
; setTypeRefinement new_subst thing_inside } }
where
- can_i_refine (LamPat can_refine) = can_refine
- can_i_refine other_ctxt = False
+ bind_fn tv | isMetaTyVar tv = WildCard -- Wobbly types behave as wild cards
+ | otherwise = BindMe
+\end{code}
+
+Note [Type matching]
+~~~~~~~~~~~~~~~~~~~~
+This little function @refineTyVars@ is a little tricky. Suppose we have a pattern type
+signature
+ f = \(x :: Term a) -> body
+Then 'a' should be bound to a wobbly type. But if we have
+ f :: Term b -> some-type
+ f = \(x :: Term a) -> body
+then 'a' should be bound to the rigid type 'b'. So we want to
+ * instantiate the type sig with fresh meta tyvars (e.g. \alpha)
+ * unify with the type coming from the context
+ * read out whatever information has been gleaned
+ from that unificaiton (e.g. unifying \alpha with 'b')
+ * and replace \alpha by 'b' in the type, when typechecking the
+ pattern inside the type sig (x in this case)
+It amounts to combining rigid info from the context and from the sig.
+
+Exactly the same thing happens for 'smart function application'.
+
+\begin{code}
+refineTyVars :: [TcTyVar] -- Newly instantiated meta-tyvars of the function
+ -> TcM TvSubst -- Substitution mapping any of the meta-tyvars that
+ -- has been unifies to what it was instantiated to
+-- Just one level of de-wobblification though. What a hack!
+refineTyVars tvs
+ = do { mb_prs <- mapM mk_pr tvs
+ ; return (mkOpenTvSubst (mkVarEnv (catMaybes mb_prs))) }
+ where
+ mk_pr tv = do { details <- readMetaTyVar tv
+ ; case details of
+ Indirect ty -> return (Just (tv,ty))
+ other -> return Nothing
+ }
\end{code}
%************************************************************************
lazyPatErr pat tvs
= failWithTc $
hang (ptext SLIT("A lazy (~) pattern connot bind existential type variables"))
- 2 (vcat (map get tvs))
- where
- get tv = ASSERT( isSkolemTyVar tv ) pprSkolemTyVar tv
+ 2 (vcat (map pprTcTyVar tvs))
inaccessibleAlt msg
= hang (ptext SLIT("Inaccessible case alternative:")) 2 msg
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