[Haskell-cafe] Incompatibility of different (monad transformer) libraries

Arie Peterson ariep at xs4all.nl
Mon Mar 16 10:26:41 EDT 2009

On Sat, 14 Mar 2009 02:14:53 +0100 (CET), Henning Thielemann
<lemming at henning-thielemann.de> wrote:

> I think 'transformers' exports Control.Monad.Trans.State. This should not

> conflict with MTL. However, MTL's State type is different from 
> transformer's one. Is that your problem?

No. The immediate problem is, that a module named Control.Monad.State is
exported by both mtl and monads-tf (and my code needs both packages).

It is indeed unfortunate that mtl:State is a different type from
transformers:State, but that is not biting me right now.

> Since the 'cabal' tool became better and better, today I use a cabal file

> even for small programs. Cabal automatically calls GHC with the right 
> package exposing and hiding options.

I already use cabal; my own package has mtl, transformers and monads-tf
among its dependencies.



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