transformers versus mtl

Sittampalam, Ganesh ganesh.sittampalam at
Tue Mar 24 17:42:23 EDT 2009

Sittampalam, Ganesh wrote:
> Ross Paterson wrote:
>> On Sun, Mar 22, 2009 at 06:31:08AM -0000, Sittampalam, Ganesh wrote:
>>> Essentially we have the choice of producing full compatibility by
>>> keeping the separate type in mtl, in which case State from mtl is
>>> not State from transformers, or partial compatibility by
>>> re-exporting the transformers one.
>> Yes, we must choose between full compatibility with mtl and full
>> compatibility with transformers.  A quick implementation of the
>> former is 
>> The separate versions of State etc are present, but deprecated.
> Cool. I've been been playing around with regression testing hackage
> following Duncan's blog post on the subject - I've got a baseline
> test running now and will try your mtl-compat out next, 

I've tried this now. Various new failures, which I don't have time
to investigate all of tonight, but the first one I looked at is in
the 'cgi' package, and is because the Functor instances for
ReaderT r m and WriterT w m now (correctly) depend on Functor m,
whereas in mtl they depend on Monad m.


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