From 1fab5eeaf45798ee7832497d6518883be451bfca Mon Sep 17 00:00:00 2001 From: "simonpj@microsoft.com" Date: Tue, 30 Dec 2008 16:59:06 +0000 Subject: [PATCH] Improve error message in deriving (fix Trac #2851) --- compiler/typecheck/TcDeriv.lhs | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) 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 -- 1.7.10.4