#4159: move Monad and MonadFix instances for Either from mtl to base

[Iavor Diatchki]

Hello,
I don't mind the instance being in MTL because then I can choose if I
should use it or not.
Adding it to the base library will force it on everyone.
I agree that this is a matter of taste but I also think that being
careful about what to _omit_ is an important part of good design.
So far, I have not seen any convincing reasons about why we might need
this instance.
-Iavor

PS: About (+ e) being used in papers on monads: the instance we are
adding is actually "Either e" which would be confusing enough for
someone who is just learning about these things.  It would be even
more confusing when---having finally figured out the
correspondence---they tried to define the instance from the paper and
got an "overlapping instance" error.

PPS:  I don't mean to sound too negative here (I am told I have some
skill for it though :-).  Examples of the utility of this instance
would be most welcome though.


On Wed, Jul 21, 2010 at 7:54 AM, Ross Paterson <ross at soi.city.ac.uk> wrote:
> On Fri, Jul 16, 2010 at 12:58:39AM -0700, Iavor Diatchki wrote:
>> I am against adding the instances for Monad Either to the base package
>> for the same reasons as the ones in the thread "proposal #4095: add
>> Applicative instance for Either".
>> http://www.haskell.org/pipermail/libraries/2010-June/013715.html
>
> namely:
>> The benefit of omitting some instances is that when programmers find them
>> lacking, they might consider an alternative way to achieve their goal.
>> In the case of Monad and Either, my feeling is that an alternative would
>> almost always lead to code which is easier to understand.
>
> I don't believe that witholding valid canonical instances to force a
> different style is viable in general.  In this particular case there
> is already an instance in mtl, and (+ e) is used as an example is just
> about every paper on monads.
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