X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcPat.lhs;h=5fda4f4bf82e7ad985ebb08b16331cb11c3e14d3;hb=8b08c15b8ace5a76e341939081fbb6ad2736ddd1;hp=a5d420930dd7adf7d8b1aa4816a2a849b0db18f3;hpb=3bcc65b82524d1b37e81492885a9eab7b48920ca;p=ghc-hetmet.git

diff --git a/compiler/typecheck/TcPat.lhs b/compiler/typecheck/TcPat.lhs
index a5d4209..5fda4f4 100644
--- a/compiler/typecheck/TcPat.lhs
+++ b/compiler/typecheck/TcPat.lhs
@@ -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
@@ -540,7 +555,7 @@ tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside
 	; ex_tvs' <- tcInstSkolTyVars skol_info ex_tvs	-- Get location from monad,
 							-- not from ex_tvs
 	; let tenv     = zipTopTvSubst (univ_tvs ++ ex_tvs)
-				      (ctxt_res_tys ++ mkTyVarTys ex_tvs')
+				       (ctxt_res_tys ++ mkTyVarTys ex_tvs')
 	      eq_spec' = substEqSpec tenv eq_spec
 	      theta'   = substTheta  tenv theta
 	      arg_tys' = substTys    tenv arg_tys
@@ -904,8 +919,7 @@ sigPatCtxt pats bound_tvs pat_tys body_ty tidy_env
 		 sep [ptext SLIT("When checking an existential match that binds"),
 		      nest 4 (vcat (zipWith ppr_id show_ids tidy_tys)),
 		      ptext SLIT("The pattern(s) have type(s):") <+> vcat (map ppr tidy_pat_tys),
-		      ptext SLIT("The body has type:") <+> ppr tidy_body_ty,
-		      ppr pats
+		      ptext SLIT("The body has type:") <+> ppr tidy_body_ty
 		]) }
   where
     bound_ids = collectPatsBinders pats