[Haskell-cafe] Removing mtl from the Haskell Platform

Ross Paterson ross at soi.city.ac.uk
Wed May 13 11:54:17 EDT 2009

On Wed, May 13, 2009 at 08:04:52AM +0100, Sittampalam, Ganesh wrote:
> We've discussed replacing it with transformers+monads-fd+an mtl
> compatiblity layer on libraries at . Ross and I plan to propose doing this
> for the second release of the platform - it's not fair to disrupt the
> first release at this stage.
> transformers+monads-fd is quite a small evolution over mtl - it factors
> out the fundeps into a separate package, tightens up some of the class
> constraints, and makes State = StateT Identity etc rather than a
> separate type. Even this breaks a few packages, but doing anything more
> substantial would be quite disruptive. That's not to say that we
> shouldn't do it, though.

Indeed it is a conservative step, but splitting the transformers from the
classes makes it easier to experiment with replacements for the classes
while retaining a degree of interoperability.  Of the issues listed, 1)
and 2) are in the type class package (e.g. transformers exports functions
for both liftings of callCC).  I'm not sure what is meant by 3).

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