MTL vs Transformers. Any status update?

Ganesh Sittampalam ganesh at
Sat Nov 28 16:14:51 EST 2009

On Sat, 28 Nov 2009, Thomas DuBuisson wrote:

> On Sat, Nov 28, 2009 at 8:56 AM, Ross Paterson <ross at> wrote:
>> On Sat, Nov 28, 2009 at 04:40:49PM +0000, Duncan Coutts wrote:
>>> 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?
>> The problem is that transformers and mtl use the same module names,
>> such as Control.Monad.Identity.  Unless one of these packages is hidden,
>> people using ghci or ghc --make will have problems.
> I've happily used LANGUAGE PackageImports to handle transformer / mtl
> conflicts.  I don't see why either one must change their module names
> and there are obvious disadvantages.

Is there long-term value in having both packages, if mtl ends up as just 
re-exporting pieces of transformers+monads-fd? I guess it reduces the 
dependencies people have to declare, but that seems marginal to me.


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