RE: #4159: move Monad and MonadFix instances for Either from mtl tobase

I find the Either monad is a nice cheap upgrade from the Maybe monad. If I read your objections correctly you would prefer that I define a custom datatype that explicit enumerates the possible errors. That would be rather too heavyweight for casual use, in my opinion. Iavor Diatchki wrote:

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 wrote:

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On Fri, Jul 16, 2010 at 12:58:39AM -0700, Iavor Diatchki wrote:

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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:

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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. _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

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