Remove strange extra print (a temporary debug?)

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

diff --git a/compiler/typecheck/TcPat.lhs b/compiler/typecheck/TcPat.lhs
index 933adb8..8dd5a7a 100644 (file)
--- 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