Re: Functor, Applicative, Monad, Foldable, Traversable instances for (, , ) a b

I did see those, but was hoping people could provide real code in the context of a library or code solving a particular problem. Real world Haskell, if you will :) Tom

El 9 abr 2017, a las 15:33, Francesco Ariis escribió:

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On Sun, Apr 09, 2017 at 03:27:02PM -0500, amindfv@gmail.com wrote: Genuinely curious: can people point to specific examples in their (or others') code where the ((,) a) instance for Foldable is really helpful and difficult to replace?

I asked in the other thread, these are some of the answers I got:

Oliver Charles:

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Personally, I think it would be a shame to lose foldMap on EIther. I frequently foldMap over Maybe values (where mempty is suitable in case of "failure"), and I can certainly see myself doing the same thing with Either.

Oleg Grenrus:

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Anecdotally, today I wrote `foldMap toList`, to use as `Maybe (NonEmpty a) -> [a]`. Also I rely heavily in lens-based code on the `folded :: Fold (Maybe a) a`.

There haven't been replies on specific usage of functions length/sum/ maximum/etc.

I can't recall where, but someone else was making a point that a {-# Poison something #-} pragma would fracture the ecosystem. I don't think this holds much weight though, as library authors would most likely toggle the flag in development (to avoid calling length Either, etc.) and toggle it back on release.

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