Am Mittwoch, 13. Mai 2009 01:03 schrieb roconnor@theorem.ca:
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.
4) The identifiers State and StateT are flawed. Something of value State s a doesn’t denote a state but a state transformer or however you want to name it. Best wishes, Wolfgang