optional is too constrained.

[HuwCampbell]

I don't think optional needs the Plus part of Alternative.

optional :: forall f a. Alt f => Applicative f => f a -> f (Maybe a)
optional a = map Just a <|> pure Nothing

I have had to locally fork this for an experiment I'm doing.

[HuwCampbell]

The reason is that Plus requires that an empty element exists, but that's not always true.

[hdgarrood]

I think this makes sense. For a concrete use case, I can see the less constrained version being useful for a parsing library, where you might want to provide an Alt instance for trying one parser and then trying some other parser if the first one doesn't succeed, but where you don't want to provide a Plus instance, because that requires you to introduce the concept of an "empty" (i.e. useless) error message.

Haskell's semigroupoids library provides the less constrained version of optional too: https://hackage.haskell.org/package/semigroupoids-5.3.4/docs/Data-Functor-Alt.html#v:optional

[hdgarrood]

Ha, I just read your optparse PR and realised that's pretty much exactly your reasoning too! I came across this same issue independently in hdgarrood/aeson-better-errors#12.

[hdgarrood]

Resolved by #47. Thanks!