[Haskell-cafe] Why MTL and Transformers?

Sittampalam, Ganesh ganesh.sittampalam at credit-suisse.com
Tue Oct 13 03:24:19 EDT 2009

Gregory Crosswhite wrote:
> Something that I have been wondering for a while is:  why are there
> *still* two monad transformer libraries with seemingly identical
> functionality, especially given that they have conflicting
> namespaces?  It creates stupid problems like the one that Erik
> encountered and had to work around.   
> I recognize that this situation most likely came about via historical
> accident rather than by intention, but why does it continue to be
> true?  

The intention is to migrate to transformers, which has a slightly better
design and separates out the functional dependencies into a separate
package (monads-fd), thus allowing use of the type families variant
(monads-tf) instead.

This has been discussed to some extent on the libraries list and a
workable strategy involving making a new mtl 2.0 that re-exports
transformers has been suggested. However the migration from extralibs to
the Haskell Platform put the brakes on further progress for a bit.

Now that this migration is essentially complete I have been intending to
revive the proposal but haven't got round to it yet. Please do jump in
and do so yourself if you feel motivated!



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