mapM_ for bytestring

[Henning Thielemann]

On Wed, 11 Sep 2013, Duncan Coutts wrote:

> For mapM etc, personally I think a better solution would be if
> ByteString and Text and other specialised containers could be an
> instance of Foldable/Traversable. Those classes define mapM etc but
> currently they only work for containers that are polymorphic in their
> elements, so all specialised containers are excluded. I'm sure there
> must be a solution to that (I'd guess with type families) and that would
> be much nicer than adding mapM etc to bytestring itself. We would then
> just provide efficient instances for Foldable/Traversable.

I'd prefer to keep bytestring simple with respect to the number of type 
extensions. Since you must implement ByteString.mapM anyway, you can plug 
this into an instance definition of Traversable ByteString.