Warning when deriving Foldable/Traversable using -Wall

[Herbert Valerio Riedel]

On 2014-10-08 at 14:00:05 +0200, Alan & Kim Zimmerman wrote:

[...]

> Should this go into Trac?

Fwiw, there is a version "7.9" you can select when writing a Trac ticket
for the very purpose to file bugs against GHC HEAD.

[...]

> The file being compiled is
>
> --------------------------------------------
> {-# LANGUAGE DeriveTraversable #-}
>
> module T9069 where
>
> import Data.Foldable
> import Data.Traversable
>
> data Trivial a = Trivial a
>    deriving (Functor,Foldable,Traversable)
> ---------------------------------------------

There's two simple ways to workaround this;

either

 a) add a 'import Prelude' after the two imports

or

 b) remove the two imports



The a) option has the benefit that it will still work with GHC 7.8.3