Suggest missing 'valueOf' in error message

src/comp/Error.hs

@@ -690,7 +690,7 @@ data ErrMsg =
         | EBadInstanceOverlap String String Position
 
         | EUndefinedTask String
-        | EUnboundCon String
+        | EUnboundCon String (Maybe String)
         | EUnboundVar String
         | EUnboundField String
         | EUnboundClCon String
@@ -1879,8 +1879,13 @@ getErrorText (EDeriveCtx s) =
 
 getErrorText (EUndefinedTask c) =
     (Type 2, empty, s2par ("Unsupported system task " ++ ishow c))
-getErrorText (EUnboundCon c) =
-    (Type 3, empty, s2par ("Unbound constructor " ++ ishow c))
+getErrorText (EUnboundCon c maybeSuggest) =
+    (Type 3, empty,
+     let intro_msg = "Unbound constructor " ++ ishow c
+         msg = case maybeSuggest of
+           Nothing -> intro_msg
+           Just fname -> intro_msg ++ ".  Perhaps " ++ quote fname ++ " is missing?"
+     in s2par msg)
 getErrorText (EUnboundVar c) =
     (Type 4, empty, s2par ("Unbound variable " ++ ishow c))
 getErrorText (EUnboundField c) =

src/comp/TIMonad.hs

@@ -468,9 +468,17 @@ findCons ct i = do
          Nothing -> errorAtId (EConstrAmb (pfpString ct')) i
          Just di -> case [ a | ConInfo {ci_id = i', ci_assump = a} <- cs, qualEq di i'] of
                    [a] -> return (updAssumpPos i a, di)
-                   []  -> errorAtId EUnboundCon i
+                   []  -> errSuggest r i
                    _   -> internalError "findCons ambig"
-     Nothing -> errorAtId EUnboundCon i
+     Nothing -> errSuggest r i
+  where
+    errSuggest :: SymTab -> Id -> TI (Assump, Id)
+    errSuggest r i =
+      let mSuggest = case findType r i of
+            Just (TypeInfo _ KNum _ _) -> Just "valueOf"
+            Just (TypeInfo _ KStr _ _) -> Just "stringOf"
+            _ -> Nothing
+      in err (getIdPosition i, EUnboundCon (pfpString i) mSuggest)
 
 findTyCon :: Id -> TI TyCon
 findTyCon i = do