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