Add Alt ParseT instance

[alexbiehl]

I use cabal to build purescript and I found that your newest version 0.9.1.0 exports a custom (<|>) which clashes with Protolude.<|> which is a reexport from Control.Applicative. I noticed because Language.PureScript.Docs.Types failed like crazy. Of course an upper bound on aeson-better-errors helped but I think using the standard Alternative instance prevents such incompatibilities in the first place.

[hdgarrood]

Sadly we can't give <|> an Alternative instance because it wouldn't be law-abiding - there's no identity element for it.

[hdgarrood]

Sorry, I mean we can't give ParseT an Alternative instance using the current <|>.

[alexbiehl]

Ok, I see. As a side note, some parsers use the failing parser as identity for Alternative have you considered that?
(And: I can't find the Alternative laws in base where are they written down?)

[hdgarrood]

That wouldn't be an identity unfortunately, since we need p <|> empty to be the same as p. This won't be the case if p fails, because you'll get a parser which fails with whatever message empty has, as opposed to one which fails with p's message.

Also there's no way of failing with an "empty" message right now - any failure must come with a specific reason. I think this is a good thing, as "empty" is always going to be a useless error message.

The laws are documented at https://hackage.haskell.org/package/base-4.9.1.0/docs/Control-Applicative.html

[taktoa]

I just went through the exact same process as @alexbiehl and I would like to suggest the Alt typeclass from semigroupoids. It has laws that fit the bill, decent syntax (<!> is even a pretty clever choice; ! is almost |, but not quite 😄), and semigroupoids is a popular, well-maintained library. In any case, I think exporting an alias of a popular operator like <|> is a bad idea (I always hate using ansi-wl-pprint for this very reason). Note that if you do switch to Alt, you should probably re-export the typeclass, since I don't think it's a good idea to make everyone who uses the library add a dependency.

[hdgarrood]

Alt looks ideal, I'd be happy with that.