[Haskell-cafe] Removing mtl from the Haskell Platform

Sittampalam, Ganesh ganesh.sittampalam at credit-suisse.com
Wed May 13 03:04:52 EDT 2009

roconnor at theorem.ca wrote:

> I think that use of the mtl should be deprecated so that we move on
> to improved monad transformer libraries.  Having the mtl in the
> Haskell Platform does the opposite by further entrenching its use,
> possibly to the point where we may not be able to get rid of it for
> years.    

> If I had to recommend a replace library, I would pick monadLib. 
> However, there are other libraries, such as the mmtl and transformers
> and it's related packages that I haven't looked at, and may also make
> fine replacements for the mtl.   

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.

I guess the libraries@ list will continue to be the place that such
decisions get made, but I think it's a bit up in the air at the moment.



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