Alternative typeclasses when parameter constrained

[rjmk]

Related to #24.

As well as the getSemigroup method, one can also

export function concat<A>(semigroup: Semigroup<A>): (fx: Maybe<A>, fy: Maybe<A>) => Maybe<A> {
  return function concat(fx, fy) {
    const x = prj(fx)
    const y = prj(fy)
    if (x == null) {
      return fy
    }
    if (y == null) {
      return fx
    }
    return of(semigroup.concat(x, y))
  }
}

And type check with (<A>(a: Semigroup<A>) => ({ concat: concat(a) }: Semigroup(<Maybe<A>>))

I slightly prefer this API, but was wondering if you had strong feelings about it / reasons to avoid it

[gcanti]

Maybe export both APIs?

export function concat<A>(semigroup: Semigroup<A>): (fx: Maybe<A>, fy: Maybe<A>) => Maybe<A> {
  return function concat(fx, fy) {
    const x = prj(fx)
    const y = prj(fy)
    if (x == null) {
      return fy
    }
    if (y == null) {
      return fx
    }
    return of(semigroup.concat(x, y))
  }
}

export function getSemigroup<A>(semigroup: Semigroup<A>): Semigroup<Maybe<A>> {
  return {
    concat: concat(semigroup)
  }
}

export function getMonoid<A>(semigroup: Semigroup<A>): Monoid<Maybe<A>> {
  return {
    empty,
    concat: concat(semigroup)
  }
}

[rjmk]

That's really nice (I love the typechecking you get from getSemigroup)

[gcanti]

Are there other APIs which we should export in your opinion? For example concat in Either

[rjmk]

I think it makes sense to aim for parity with the Haskell & Purescript preludes

[rjmk]

Sorry for the terseness yesterday! I'll make some PRs (if they'd be well received) for some further instances in the coming days.

Also, how about chainRec? The stack safety can't be typechecked AFAIK, but the signature at least could be

[gcanti]

Sure, in the meanwhile I pushed #34

[rjmk]

🔥

[rjmk]

What do you think about multiple instances of semigroup for Either? There's the one with no constraints as well:

instance Semigroup (Either a b) where
  Left _ <> b = b
  a <> _ = a

[gcanti]

Ah yes, we are free to implement several instances of Semigroup, it's the beauty of static-land. We have to just find suitable names :)

[gcanti]

Tracking chainRec here #35

[gcanti]

@rjmk FYI released in https://github.com/gcanti/flow-static-land/releases/tag/0.2.2