From: simonpj@microsoft.com <unknown>
Date: Tue, 30 Dec 2008 16:59:06 +0000 (+0000)
Subject: Improve error message in deriving (fix Trac #2851)
X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=commitdiff_plain;h=1fab5eeaf45798ee7832497d6518883be451bfca

Improve error message in deriving (fix Trac #2851)
---

diff --git a/compiler/typecheck/TcDeriv.lhs b/compiler/typecheck/TcDeriv.lhs
index 419ec94..5053a06 100644
--- 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