[project @ 2002-05-23 15:51:26 by simonpj]

Don't report ambiguity errors
	if other type errors have happened

This saves a gratuitous error cascade when the type checker
recovers from one error by giving f type (forall a.a), and
then find an ambiguity problem as a direct result.

diff --git a/ghc/compiler/typecheck/TcModule.lhs b/ghc/compiler/typecheck/TcModule.lhs
index 9251283..6b76101 100644 (file)
--- a/ghc/compiler/typecheck/TcModule.lhs
+++ b/ghc/compiler/typecheck/TcModule.lhs
@@ -445,8 +445,9 @@ tcModule pcs hst (RnResult { rr_decls = decls, rr_mod = this_mod,
                           lie_rules     `plusLIE`
                           lie_main
        in
-       tcSimplifyTop lie_alldecls      `thenTc` \ const_inst_binds ->
-        traceTc (text "endsimpltop")   `thenTc_`
+       tcSimplifyTop lie_alldecls              `thenTc` \ const_inst_binds ->
+        traceTc (text "endsimpltop")           `thenTc_`
+       
        
            -- Backsubstitution.    This must be done last.
            -- Even tcSimplifyTop may do some unification.

diff --git a/ghc/compiler/typecheck/TcMonad.lhs b/ghc/compiler/typecheck/TcMonad.lhs
index de83f05..7b06460 100644 (file)
--- a/ghc/compiler/typecheck/TcMonad.lhs
+++ b/ghc/compiler/typecheck/TcMonad.lhs
@@ -17,7 +17,7 @@ module TcMonad(
 
        checkTc, checkTcM, checkMaybeTc, checkMaybeTcM, 
        failTc, failWithTc, addErrTc, addErrsTc, warnTc, 
-       recoverTc, checkNoErrsTc, recoverNF_Tc, discardErrsTc,
+       recoverTc, checkNoErrsTc, ifErrsTc, recoverNF_Tc, discardErrsTc,
        addErrTcM, addInstErrTcM, failWithTcM,
 
        tcGetEnv, tcSetEnv,
@@ -408,6 +408,19 @@ checkNoErrsTc main
          errs_var = getTcErrs down
 
 
+ifErrsTc :: TcM r -> TcM r -> TcM r
+--     ifErrsTc bale_out main
+-- does 'bale_out' if there are errors in errors collection
+-- and does 'main' otherwise
+-- Useful to avoid error cascades
+
+ifErrsTc bale_out main
+  = getErrsTc  `thenNF_Tc` \ (warns, errs) -> 
+    if isEmptyBag errs then
+          main
+    else       
+          bale_out
+
 -- (tryTc_ r m) tries m; if it succeeds it returns it,
 -- otherwise it returns r.  Any error messages added by m are discarded,
 -- whether or not m succeeds.

diff --git a/ghc/compiler/typecheck/TcSimplify.lhs b/ghc/compiler/typecheck/TcSimplify.lhs
index c7280a3..c28105a 100644 (file)
--- a/ghc/compiler/typecheck/TcSimplify.lhs
+++ b/ghc/compiler/typecheck/TcSimplify.lhs
@@ -1609,15 +1609,25 @@ tcSimplifyTop wanted_lie
                -- Collect together all the bad guys
        bad_guys = non_stds ++ concat std_bads
     in
-       -- Disambiguate the ones that look feasible
-    mapTc disambigGroup std_oks                `thenTc` \ binds_ambig ->
 
-       -- And complain about the ones that don't
+    ifErrsTc (returnTc []) (
+       -- Don't check for ambiguous things
+       -- if there has been an error; errors often
+       -- give rise to spurious ambiguous Insts
+       
+    
+       -- And complain about the ones that don't fall under
+       -- the Haskell rules for disambiguation
        -- This group includes both non-existent instances
        --      e.g. Num (IO a) and Eq (Int -> Int)
        -- and ambiguous dictionaries
        --      e.g. Num a
-    addTopAmbigErrs bad_guys           `thenNF_Tc_`
+        addTopAmbigErrs bad_guys       `thenNF_Tc_`
+
+       -- Disambiguate the ones that look feasible
+        mapTc disambigGroup std_oks
+    )                                  `thenTc` \ binds_ambig ->
+
 
     returnTc (binds `andMonoBinds` andMonoBindList binds_ambig)
   where