On 2008-02-20, Ross Paterson
conventions won't be usable in my ByteString code, for instance. [...] http://software.complete.org/listlike/static/doc/ListLike/Data-ListLike.html
As Henning pointed out, multiple parameter type classes are problematic for core libraries at present.
An alternative might be explicit dictionaries. For example, a partial solution would be to provide coinductive views, i.e. for all these types to provide functions of a type like
full -> Maybe (item, full)
Hrm, what exactly is the return data here? Is is the head and the tail if the list has >= 1 item, or Nothing otherwise? Or...? The problem with this approach, if my guess is correct, is that you can't achieve native speed because you will have to be re-implementing everything in terms of these functions. For instance, I'd be using a re-implementation of length instead of a native ByteString length, which may be much faster. I notice that Data.Foldable does some similar things but does not use multi-parameter type classes. I seem to recall that I attempted to do this in the same manner, but got tripped up somewhere. I can't remember now exactly what the problem was, but I can go back and look if nobody knows off-hand. What is the problem with MPTC in base?