transformers: remove instance MonadFix ExceptT and ErrorT

[Levent Erkok]

Henning: I don't think this is the right forum to discuss examples of mfix.
Granted, it's not something everyone needs every day; think about how often
you write recursive data-values, and imagine how often you need those to be
happening in some monad. Not very often. But the phenomenon has been well
studied, and there are a few illustrative "real-world" examples in Chapter
9 of: https://sites.google.com/site/leventerkok/erkok-thesis.pdf

-Levent.


On Tue, Aug 12, 2014 at 2:34 PM, Henning Thielemann <
schlepptop at henning-thielemann.de> wrote:

> Am 12.08.2014 um 21:17 schrieb Edward Kmett:
>
>
>  so it seems the logical extension of your request would be to strip all
>> such instances,
>>
>
> one by one, please :-)
>
>
> Can someone provide me examples of useful applications of mfix on Maybe,
> ExceptT etc.?
>
> I mean, all those mfix instances contain an "error" - wouldn't it be
> better if the user would do such dirty things and insert a more specific
> error message?
>
> Admittedly, I have used mfix only once so far, at all, namely on an RWS
> monad, as far as I remember. Eventually I chose another design, because the
> mfix way was too fragile.
>
>
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