transformers versus mtl

Sittampalam, Ganesh ganesh.sittampalam at
Sat Mar 21 17:53:39 EDT 2009

Henning Thielemann wrote:
> On Sat, 21 Mar 2009, Ross Paterson wrote:
>>> Are there any problems with adding transformers and monads-fd to
>>> extralibs pending the Haskell Platform being ready?
>> That should work, but that wouldn't be till September/October.  I'm
>> not sure what the HP schedule is.  Another possibility is to upload
>> mtl-2.0 as a compatibility layer.
> We already found out, that compatibility to mtl cannot be achieved
> with a package based on transformers.

Can you elaborate on this or point me at the past discussion? I can't
find anything from a quick look through the archives, apart from the
issue that haddock will go wrong if mtl just re-exports things from

> So why shall we rewrite mtl in
> terms of transformers+monad-fd at all? I welcome advertising 
> transformers+transformers+monad-fd,
> but I don't see the benefits of breaking mtl.

If we don't make mtl and transformers use the same types,
we will have some packages with interfaces that use one and some
with interfaces that use the other for quite a long time to come.

Also, if mtl stays in extralibs/the Haskell Platform but remains
incompatible with transformers, then it will be in some way blessed
which will hinder a move to transformers, but if we suddenly remove
it then a lot of people will suddenly have to go and find it from
hackage when previously they got it with GHC.


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