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Re: MTL vs Transformers

Another alternative is to simply force the user to explicitly select the appropriate package in the source code, with the syntax:

import "mtl" Control.Monad.Trans

Ugly, but the packaging is easier.

On Thu, Nov 26, 2009 at 11:01 PM, Erik de Castro Lopo <mle+hs@mega-nerd.com> wrote:
Hi all,

Apparently the Haskell Platform is transitioning from one implementation
of monad transformers (MTL) to another implementation (Transformers).


I have a preliminary package for Transformers but have noticed a problem;
both MTL and Transformers provide a module named (eg Control.Monad.Trans).
The symptom of this problem is that when both are installed ghc can't
figure out which version of Control.Monad.Trans to use.

When just using ghc directly, this problem can be overcome using the
-hide-all-packages and then explicity adding packages back in using
the -package option (this is also the solution that cabal-install

The above solution however does not work when debian packages for
mtl and transformers are both installed and a new debian package is
being built that depends on one of these transformers libraries.

The obvious solution is to modify haskell-devscripts so that when
it calls ghc, it uses -hide-all-packages and then explicity adds
in the required packages using the -package option.

Does this make sense? If so, I can take a look at haskell-devscripts
and see if I can implement this new behaviour.

Erik de Castro Lopo

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