mapM_ is actually implemented in terms of Foldable, not Traversable, and its implementation in terms of folding a ByteString is actually rather slow in my experience doing so inside lens and isn't much faster than the naive version that was suggested at the start of this discussion.

But as we're not monomorphizing Foldable/Traversable, this isn't a think that is able to happen anyways.

-Edward

On Wed, Sep 11, 2013 at 2:25 PM, Henning Thielemann <lemming@henning-thielemann.de> wrote:

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.