Andreas Abel andreas.abel at ifi.lmu.de
Fri Aug 9 08:52:29 CEST 2013

It happens again and again on this list:  Someone request an innocent 
function on tuples or Either and after some initial discussion someone 
else writes "it is in Control.Arrow".  No offense intended, but to me 
"just use Control.Arrow.xxx" is a smart-ass comment in the line of "this 
is just a lax monoidal functor" I hear from category-theorists all the time.

Arrows are on a level of abstraction that many Haskell programmers 
(including me) do not make use of and should *not* be forced to study 
just because they need an innocent function about tuples (or Either).

I bet that most imports of Control.Arrow are to make up for an 
impoverished Data.Tuple, and proper arrow programming is the exception.

Thus, I agree with Henning and David and many more that there is 
something wrong here.  mapEither, mapPair, mapFst, mapSnd and the like 
should be added to Data.Tuple and Data.Either, at the risk of 
duplicating stuff from Control.Arrow.  Otherwise, programmers will just 
roll their own version of these function again (which is duplication 
that comes at a cost) and additionally be alienated at the design of the 
Haskell libraries.

On 08.08.13 4:26 PM, Henning Thielemann wrote:
> On Thu, 8 Aug 2013, Erik Hesselink wrote:
>> On Thu, Aug 8, 2013 at 1:09 PM, Henning Thielemann
>> <lemming at henning-thielemann.de> wrote:
>>> On Thu, 8 Aug 2013, David Luposchainsky wrote:
>>>> On 2013-08-07 19:13, Erik Hesselink wrote:
>>>>>> mapEither :: (l -> l') -> (r -> r') -> Either l r -> Either l' r'
>>>>>> mapEither fl fr = either (Left . fl) (Right . fr)
>>>>> This function is hidden in Control.Arrow as (+++).
>>>> I didn't know about those functions, and it seems everything I
>>>> wanted is
>>>> in there somewhere. However, I'm not sure what to think of
>>>> Control.Arrow, I've always (ab)used it as an extended Data.Tuple,
>>>> and as
>>>> of today probably for its Either API functions.
>>> For me using the arrow functions as Data.Tuple and Data.Either utility
>>> functions is abuse. Control.Arrow is meant to generalize (->). If you
>>> call
>>> maybeLeft and MaybeRight this is certainly not your intention.
>> But the point of generalized functions is to use them in a specialized
>> context, right?
> I think the primary goal is to use them to write generic functions.
>> You can generalize mapEither in multiple ways: you can generalize
>> (->), or you can generalize Either (getting bimap from the bifunctors
>> package). I don't get why it's wrong to specialize the Arrow to (->)
>> and use the functions on Eithers or tuples.
> Why should someone care about Arrows if he only wants to process Either?
>> I do agree that they're hard to find.
> For me this is a hint that there is something wrong.
> If I want to have a function mapEither and do not find it in
> Data.Either, I might have the idea that it is subsumed by a more general
> function somewhere else. In this case I would certainly think about a
> generalization of Either, but not of (->).

Andreas Abel  <><      Du bist der geliebte Mensch.

Theoretical Computer Science, University of Munich
Oettingenstr. 67, D-80538 Munich, GERMANY

andreas.abel at ifi.lmu.de

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