do you know a relatable scenario where one would u...

# arrow

[sitepodmatt]

do you know a relatable scenario where one would use ap on an Either

[Derek Berner]

Suppose you have a curried function of type (A) -> (B) -> C. Passing that to Either<X, A>#map would return a value of type Either<X, (B) -> C>. If you want to get an Either<X, C> out of it, you can it into Either<X, B>#ap. This is arbitrarily chainable too.

Example:

fun makePerson(name: String): (Int) => Person = { age: Int -> Person(name,age) }
val name: Either<X,String> = /*...*/
val age: Either<X,Int> = /*...*/ 
val person: Either<X,Person> = age.ap(name.map(::makePerson))

Applicative#map makes this syntax much cleaner, and is implemented using ap.

[sitepodmatt]

Thanks, will (attempt to) digest that

[Derek Berner]

ap gets a lot of use in something like Haskell (where map and ap are called <$> and <*>, respectively) because you can use that syntax to go Person <$> name <*> age (equivalent to Applicative#ap(Functor#map) in Arrow), which, once you understand the operators, is a little nicer than liftA2 Person $ name age (equivalent to Applicative#map in Kotlin).

[sitepodmatt]

Thanks. It's a little clearer now

[sitepodmatt]

val output = fn1.map({ it(2) })
    val output2 = Some(2).ap(fn1)
    val output3 = Some(2).flatMap({ v -> fn1.map({ it(v) }) })
    println(output)
    println(output2)
    println(output3)

[sitepodmatt]

What happens if fn1 return Some("Matt" + i) is there like a flatMap version of ap

[Derek Berner]

Yes, for monads

[Derek Berner]

In fact the default implementation of ap for monads uses flatMap