MTL vs Transformers. Any status update?

Duncan Coutts duncan.coutts at
Sat Nov 28 11:40:49 EST 2009

On Fri, 2009-11-27 at 00:33 +0000, Ross Paterson wrote:
> As I understand it, the Haskell Platform timetable means this will
> take some time.
> I think the ultimate aim should be that everyone moves to transformers
> and monads-fd (probably with some renaming of modules in the latter),
> and mtl disappears.  If we decide on that, this is we could get there:
> 1. rename the modules in monads-fd to their final names.
>    (We also need suggestions for what those names should be.)
> 2. release mtl-2.0 as a deprecated compatibility layer over transformers
>    and monads-fd, and get all users to hide all versions of mtl, including
>    the new one.   19 packages on hackage must be updated to work with
>    the new version.
> 3. encourage authors to upgrade their packages to depend on transformers
>    and (possibly) monads-fd in place of mtl.
> 4. after a couple of cycles, remove mtl from the platform.

Could someone remind me why it's better for use to ask everyone to
switch to monads-fd / transformers rather than to mtl-2? Does that make
the transition easier?

If it makes little difference I'd suggest making mtl-2 the new
recommendation (with whatever content you've all agreed) simply because
it's easier to explain to everyone else.


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