replace Monoid dependency with Alternative dependency for ‘filterM’

[davidchambers]

⚠️ This is a breaking change.

It became apparent in sanctuary-js/sanctuary#359 that Z.filterM(odd, Just(4)) is invalid due to the Monoid (m a) constraint. Just(4) cannot satisfy this constraint as it cannot satisfy the requirements of Semigroup. Just(4) can, though, satisfy the requirements of Alternative.

This will allow S.filterM to operate on values of type Maybe a even with type checking enabled.

[safareli]

In haskell alt for List is concat:

instance Alternative [] where
    empty = []
    (<|>) = (++)

[davidchambers]

Good catch. I find this counter-intuitive, so I tend to forget. Thanks for the reminder. :)

[safareli]

Even though, both of the implementations will conform to Alternative laws

[safareli]

⛱

[syaiful6]

We maybe need to relax the constraint, instead of Alternative, we can just take Plus constraint. List can implements Plus, but it can't implements Alternative, as it violate the distribute laws of Alternative as Fantasy Land defined. Because x.ap(f.alt(g)) is not equivalent to x.ap(f).alt(x.ap(g)) in List.

[safareli]

@syaiful6 for Arrayx.ap(f.alt(g)) is equivalent to x.ap(f).alt(x.ap(g))

for example:

const x = [1]
const f = [a => a+1]
const g = [a => a*3]
f.alt(g) = [a => a+1, a => a*3]
x.ap(f.alt(g)) = [(a => a+1)(1), (a => a*3)(1)]
x.ap(f.alt(g)) = [2, 3]
x.ap(f) = [2]
x.ap(g) = [3]
x.ap(f).alt(x.ap(g)) = alt([2], [3]) = [2,3] 

i.e. List is valid Alternative when alt is concat

[syaiful6]

ah, correct..

[davidchambers]

I modified Irakli's example to convince myself that the equivalence is not limited to singleton lists:

> ([(+ 1),(+ 9)] <|> [(* 3),(* 9)]) <*> [1,2,3]
[2,3,4,10,11,12,3,6,9,9,18,27]

> [(+ 1),(+ 9)] <*> [1,2,3] <|> [(* 3),(* 9)] <*> [1,2,3]
[2,3,4,10,11,12,3,6,9,9,18,27]