Improve error message in deriving (fix Trac #2851)

diff --git a/compiler/typecheck/TcDeriv.lhs b/compiler/typecheck/TcDeriv.lhs
index 419ec94..5053a06 100644 (file)
--- a/compiler/typecheck/TcDeriv.lhs
+++ b/compiler/typecheck/TcDeriv.lhs
@@ -1314,7 +1314,9 @@ standaloneCtxt ty = hang (ptext (sLit "In the stand-alone deriving instance for"
 
 derivInstCtxt :: Class -> [Type] -> Message
 derivInstCtxt clas inst_tys
-  = ptext (sLit "When deriving the instance for") <+> parens (pprClassPred clas inst_tys)
+  = vcat [ptext (sLit "Alternative fix: use a standalone 'deriving instance' declaration"),
+          nest 2 (ptext (sLit "instead, so you can specify the instance context yourself")),
+         ptext (sLit "When deriving the instance for") <+> parens (pprClassPred clas inst_tys)]
 
 badDerivedPred :: PredType -> Message
 badDerivedPred pred