Re: [jhc] bug or feature: generated constraints of derived Show instance

On Sat, Nov 14, 2009 at 07:05:00PM +0100, Henning Thielemann wrote:

With the patch about unqualified methods in instance declarations I can compile most of my xml-basic package. However it stops at

data ProcessingInstruction name string = Known [Attr.T name string] | Unknown String deriving (Eq, Ord, Show )

since the Show instance of Attr.T needs an (Name.Attribute name) constraint. JHC does not seem to generate that automatically. GHC does so, but I do not know, whether this is Haskell 98. I can fix this by writing a custom Show instance, but there are more types with that problem.

Probably a bug, otherwise data T a = T [a] deriving (Show) wouldn't work because instance Show a => Show [a] where ... -- Felipe.