Give a better error message when we try to print a value of unknown type

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

diff --git a/compiler/typecheck/TcType.lhs b/compiler/typecheck/TcType.lhs
index 728f58b..caea74f 100644 (file)
--- a/compiler/typecheck/TcType.lhs
+++ b/compiler/typecheck/TcType.lhs
@@ -323,6 +323,9 @@ data SkolemInfo
   | GenSkol [TcTyVar]  -- Bound when doing a subsumption check for 
            TcType      --      (forall tvs. ty)
 
+  | RuntimeUnkSkol      -- a type variable used to represent an unknown
+                        -- runtime type (used in the GHCi debugger)
+
   | UnkSkol            -- Unhelpful info (until I improve it)
 
 -------------------------------------
@@ -447,6 +450,7 @@ pprSkolTvBinding tv
     ppr_details (SkolemTv info)                = ppr_skol info
 
     ppr_skol UnkSkol = empty   -- Unhelpful; omit
+    ppr_skol RuntimeUnkSkol = quotes (ppr tv) <+> ptext SLIT("is an unknown runtime type")
     ppr_skol info    = quotes (ppr tv) <+> ptext SLIT("is bound by") 
                        <+> sep [pprSkolInfo info, nest 2 (ptext SLIT("at") <+> ppr (getSrcLoc tv))]
  
@@ -465,6 +469,7 @@ pprSkolInfo (GenSkol tvs ty) = sep [ptext SLIT("the polymorphic type"),
 -- For type variables the others are dealt with by pprSkolTvBinding.  
 -- For Insts, these cases should not happen
 pprSkolInfo UnkSkol = panic "UnkSkol"
+pprSkolInfo RuntimeUnkSkol = panic "RuntimeUnkSkol"
 
 instance Outputable MetaDetails where
   ppr Flexi        = ptext SLIT("Flexi")