[Haskell-cafe] Non-traversable foldables

[Henning Thielemann]

On Fri, 9 Oct 2009, Martijn van Steenbergen wrote:

> Hallo café,
>
> Can anyone think of datatypes that are Foldable but not Traversable?
>
> If not, what is the purpose of having a separate Foldable class?

Recently I wondered, why Foldable is superclass of Traversable, since I 
have examples where 'traverse' makes sense, but 'fold' not.

http://www.haskell.org/pipermail/libraries/2009-February/011361.html