Add Applicative *>, <*, and Alternative <|> operators

[inamiy]

This pull request adds new Applicative *>, <*, and Alternative <|> operators, which are needed feature in tryswift/TryParsec#11 and all other functional libraries ✨ (See also: #65)

By the way, Argo is currently using the same <|> operator in Alternative.swift#L13 with precedence 140, but this causes continuous monadic operations not working nicely. For example, m1 *> m2 <|> m3 *> m4 doesn't work and must use parenthesis to work around like (m1 *> m2) <|> (m3 *> m4), which is very cumbersome. Since Haskell uses lower precedence for <|>, I have dropped it down to precedence 120 (or, precedence 125 might be a better choice as discussed in tryswift/TryParsec#11).

I also added comments about Haskell's infix operator precedences for comparison with Runes 👀

[houndci-bot]

Opening Brace Spacing Violation: Opening braces should be preceded by a single space and on the same line as the declaration. (opening_brace)

[inamiy]

Hmm, it seems below workaround is needed for using Argo when <|> is precedence 120:

return curry(self.init)
  <^> j <| "id"
  <*> (j <| ["userinfo", "name"] <|> j <| "name")  // <- needs parenthesis to work around...
  <*> j <|? "email"

It works better when <|>'s precedence is in range 131...139, but I'm not sure if that value is appropriate since <|> becomes higher priority than <^> and <*>.

[gfontenot]

@inamiy Is that a problem with the precedence of Argo's <| family of operators? I don't think we've put nearly as much thought into the precedence of those as we have these. I'd be interested to know what the precedence is for .: in Aeson.

It's probably worth noting that we'll probably revisit all of these precedence settings in Swift 3.

[inamiy]

After looking back and simplifying my above old snippet, I found the problem was only in between <*> and <|>.
(So, it's not a problem of Argo's <| operator. <| has 150 and Aeson.(.:) has default infixl 9, which are both high enough)

let d1: Decoded<String> = j <| ["userinfo", "name"]
let d2: Decoded<String> = j <| "name"

return curry(self.init)
  <^> j <| "id"
  <*> (d1 <|> d2)  // <- parenthesis is still required
  <*> j <|? "email"

Because of infixl 4 <*> and infixl 3 <|> in Haskell, adding parenthesis around alternatives is always a requirement to perform applicative style.
That means, ArgoTests/Models/User.swift#L14 should also have parenthesis instead if we adopt this pull request.

This will be a breaking change, so it sounds good to revisit all precedence settings before Swift 3 ;)

[gfontenot]

Cool, so the gist is that the precedence for <|> is actually currently incorrect in Argo. I think it's reasonable for us to move forward with this since we're going to be bumping major versions across the board with Swift 3 (and the precedence refactoring here) anyway.

[gfontenot]

@inamiy Do you want to rebase/squash this down on top of current master and then I'll work at merging it?

[inamiy]

@gfontenot Sure! Rebased in 7a6da19. (CI failure seems unrelated)

[gfontenot]

@inamiy yeah, the CI failure is because of Xcode 8, which doesn't exist on Circle. Totally expected on my part.

[gfontenot]

💥 merged, thanks!