[Haskell-cafe] StateT and ContT

Ivan Lazar Miljenovic ivan.miljenovic at gmail.com
Tue Jan 20 13:09:13 UTC 2015

On 20 January 2015 at 23:09, Edward Kmett <ekmett at gmail.com> wrote:
> You pinged me on it while I was in the middle of a trip, and when I went to
> dig into the issue, the rabbit hole went a lot deeper than I could devote at
> the time.

Apologies if I sounded harsh about the "no response" in my previous
email; it certainly wasn't my intent.

> Excuses aside, it does appear that the primary reason for the current
> behavior is to match existing prior behavior.
> That said, I'm not sure there is a good way to offer an upgrade path to
> change the behavior, as it is the sort of thing that would just silently
> break very large chunks of code in very subtle ways with the user having no
> reason to suspect that a change in the underlying semantics of the mtl was
> to blame, so I'm rather reticent about just diving in and "fixing" it as
> basically every existing user of the instance is pretty much guaranteed to
> have breakage.

This is what I thought: I just wanted to make sure that I wasn't
missing something (especially since there's no real documentation that
I could find as to _how_ liftCallCC' fails to satisfy the laws of a
monad transformer).

> -Edward
> On Sun, Jan 18, 2015 at 5:56 AM, Ivan Lazar Miljenovic
> <ivan.miljenovic at gmail.com> wrote:
>> In the transformers library, there are two implementations for callCC
>> for use with StateT (both lazy and strict) [1,2], the second of which
>> is documented to not satisfy the laws of a monad transformer.
>> [1]:
>> http://hackage.haskell.org/package/transformers-
>> [2]:
>> http://hackage.haskell.org/package/transformers-
>> However, in mtl the MonadCont class [3] uses the second definition for
>> the StateT instance.  Is there a good reason for this, or just to
>> match the behaviour of pre-transformers mtl [4] ?
>> [3]:
>> http://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Cont-Class.html
>> [4]:
>> http://hackage.haskell.org/package/mtl-
>> (I did raise an issue for this on mtl's Github issue tracker, but it
>> hasn't had any responses for two months.)
>> --
>> Ivan Lazar Miljenovic
>> Ivan.Miljenovic at gmail.com
>> http://IvanMiljenovic.wordpress.com
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Ivan Lazar Miljenovic
Ivan.Miljenovic at gmail.com

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