[Haskell-cafe] Removing mtl from the Haskell Platform

David Leimbach leimy2k at gmail.com
Wed May 13 11:11:05 EDT 2009

Nothing controversial said here... I'm just agreeing with Russel.

On Tue, May 12, 2009 at 4:03 PM, <roconnor at theorem.ca> wrote:

> I wanted to pass this idea around the cafe to get some thoughts before
> submitting a trac on this topic.
> I'd like to see the mtl removed from the Haskell Platform.
> The mtl was a tremendous step forward when it was developed.  However, we
> have learned a few things about monad transformers since the development of
> the mtl, and it is time that we moved forward.
> There are at least 3 significant problem with the mtl.
> 1) `pass' should not be a member functions of the MonadWriter class.  It is
> my understanding that there is no `MonadWriter w m => MonadWriter w (ContT s
> m)' instance because the `pass' function cannot be implemented. I'm also
> highly suspicious of some other methods too (I'm looking at you `local').
> 2) The `StateT s (Cont r a)' instance of callCC is wrong.  The paper on
> modular monad transformers <http://www.cs.nott.ac.uk/~mjj/pubs/mmt/mmt.pdf>
> describes why this is wrong.
> 3) I am told by many people that the order of the state and value pair in
> `State' is backwards.  Actually, I'm not entirely sure what the issue is
> here, but I trust the people who say this.
> 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.
> --
> Russell O'Connor                                      <http://r6.ca/>
> ``All talk about `theft,''' the general counsel of the American Graphophone
> Company wrote, ``is the merest claptrap, for there exists no property in
> ideas musical, literary or artistic, except as defined by statute.''
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