"deriving" seems to ignore class context for a type family

[jkff]

class Foo a where
  data Bar a :: *

class (Show (Bar a)) => Qux a
data (Qux a) => Xyzzy a = Xyzzy (Bar a) deriving Show

Here I get the following error:

    No instance for (Show (Bar a))
      arising from the 'deriving' clause of a data type declaration
    Possible fix:
      add an instance declaration for (Show (Bar a))
      or use a standalone 'deriving instance' declaration,
           so you can specify the instance context yourself
    When deriving the instance for (Show (Xyzzy a))

Curiously, I get the same error even if I add ", Show (Bar a)" to the context of "Xyzzy".

[igloo]

Simon, is this the expected behaviour?

[simonpj]

The error message is quite right.

Using a context on a data type declaration, as you are doing, is a mis-feature of Haskell. If you read its specification carefully you'll see that it is practically useless. Any program using it is suspicious. Certainly, it has absolutely no effect on 'deriving' declarations.

You can follow the advice in the error message and use a standalone deriving declaration, thus

{-# LANGUAGE StandaloneDeriving, FlexibleContexts, UndecidableInstances, TypeFamilies #-}

module T5007 where

class Foo a where
   data Bar a :: *

class (Show (Bar a)) => Qux a
data Xyzzy a = Xyzzy (Bar a) 

deriving instance Show (Bar a) => Show (Xyzzy a)