Re: leftToMaybe/rightToMaybe

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:

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On Thu, Aug 8, 2013 at 1:09 PM, Henning Thielemann wrote:

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On Thu, 8 Aug 2013, David Luposchainsky wrote:

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On 2013-08-07 19:13, Erik Hesselink wrote:

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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.

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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?

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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@ifi.lmu.de http://www2.tcs.ifi.lmu.de/~abel/