X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcPat.lhs;h=8dd5a7aec909783202606efcb40cda13322da996;hb=d2359864dfb6d8cc419b8ee55f1948c93f796d0b;hp=933adb8888b135764c243f2fd3a0b1bdff96e2e0;hpb=f4510d27c5883fe7e8570f4dd49d45a8b0122f2c;p=ghc-hetmet.git

diff --git a/compiler/typecheck/TcPat.lhs b/compiler/typecheck/TcPat.lhs
index 933adb8..8dd5a7a 100644
--- a/compiler/typecheck/TcPat.lhs
+++ b/compiler/typecheck/TcPat.lhs
@@ -132,7 +132,7 @@ tcCheckExistentialPat pats [] pat_tys body_ty
   = return ()	-- Short cut for case when there are no existentials
 
 tcCheckExistentialPat pats ex_tvs pat_tys body_ty
-  = addErrCtxtM (sigPatCtxt (collectPatsBinders pats) ex_tvs pat_tys body_ty)	$
+  = addErrCtxtM (sigPatCtxt pats ex_tvs pat_tys body_ty)	$
     checkSigTyVarsWrt (tcTyVarsOfTypes (body_ty:pat_tys)) ex_tvs
 
 data PatState = PS {
@@ -332,11 +332,26 @@ tc_pat pstate (BangPat pat) pat_ty thing_inside
 --
 -- Nor should a lazy pattern bind any existential type variables
 -- because they won't be in scope when we do the desugaring
+--
+-- Note [Hopping the LIE in lazy patterns]
+-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+-- In a lazy pattern, we must *not* discharge constraints from the RHS
+-- from dictionaries bound in the pattern.  E.g.
+--	f ~(C x) = 3
+-- We can't discharge the Num constraint from dictionaries bound by
+-- the pattern C!  
+--
+-- So we have to make the constraints from thing_inside "hop around" 
+-- the pattern.  Hence the getLLE and extendLIEs later.
+
 tc_pat pstate lpat@(LazyPat pat) pat_ty thing_inside
-  = do	{ (pat', pat_tvs, res) <- tc_lpat pat pat_ty pstate $ \ _ ->
-				  thing_inside pstate
-					-- Ignore refined pstate',
-					-- revert to pstate
+  = do	{ (pat', pat_tvs, (res,lie)) 
+		<- tc_lpat pat pat_ty pstate $ \ _ ->
+		   getLIE (thing_inside pstate)
+		-- Ignore refined pstate', revert to pstate
+	; extendLIEs lie
+	-- getLIE/extendLIEs: see Note [Hopping the LIE in lazy patterns]
+
 	-- Check no existentials
 	; if (null pat_tvs) then return ()
 	  else lazyPatErr lpat pat_tvs
@@ -894,7 +909,7 @@ existentialExplode pat
 		text "In the binding group for"])
 	4 (ppr pat)
 
-sigPatCtxt bound_ids bound_tvs pat_tys body_ty tidy_env 
+sigPatCtxt pats bound_tvs pat_tys body_ty tidy_env 
   = do	{ pat_tys' <- mapM zonkTcType pat_tys
 	; body_ty' <- zonkTcType body_ty
 	; let (env1,  tidy_tys)    = tidyOpenTypes tidy_env (map idType show_ids)
@@ -907,6 +922,7 @@ sigPatCtxt bound_ids bound_tvs pat_tys body_ty tidy_env
 		      ptext SLIT("The body has type:") <+> ppr tidy_body_ty
 		]) }
   where
+    bound_ids = collectPatsBinders pats
     show_ids = filter is_interesting bound_ids
     is_interesting id = any (`elemVarSet` idFreeTyVars id) bound_tvs