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[ghc-hetmet.git] / compiler / typecheck / TcPat.lhs

diff --git a/compiler/typecheck/TcPat.lhs b/compiler/typecheck/TcPat.lhs
index a5d4209..5fda4f4 100644 (file)
--- 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