Forward Composition

1 minute read

In the previous post I “remixed” a portion of the composition chapter in the Mostly Adequate Guide. While reading that chapter and later translating the direction of composition kept confusing me. I wanted to convey the idea’s as close as possible.. keeping all the great humor like:

Instead of inside to outside, we run right to left, which I suppose is a step in the left direction (boo!).

And this does make sense, it reads right to left:

let  = exclaim <<< toUpperCase <<< head <<< reverse
loudLastUpper(["jumpkick", "roundhouse", "uppercut"]) //"UPPERCUT!"

But it doesn’t feel like a huge readability gain from:

let loudLastUpper = { (x:[String]) in  exclaim( toUpperCase( head( reverse(x) ) ) ) }

Or the more common:

func loudLastUpper(_ x:[String]) -> String {
    return exclaim( toUpperCase( head( reverse(x) ) ) )
}

It’s less parentheses and I do like the direction arrows pointing out the flow of data. We are still reading right to left which for me doesn’t feel natual in a language that is written left to right.

What I’ve been usually using is a ForwardComposition like this:

precedencegroup ForwardComposition {
    associativity: left
}

infix operator >>>: ForwardComposition

func >>> <A, B, C>(_ f: @escaping (A) -> B, _ g: @escaping (B) -> C) -> (A) -> C {
    return { x in g(f(x)) }
}

I’ve heard this called composeRight or it could be a regular function called forwardCompose instead of an infix:

func forwardCompose<A, B, C>(_ f: @escaping (A) -> B, _ g: @escaping (B) -> C) -> (A) -> C {
    return { a in
        g(f(a))
    }
}

Either way we can chain or pipe the logic from the left to the right, the same direction as the language we are writing in!

// same result as before!

let loudLastUpper = reverse >>> head >>> toUpperCase >>> exclaim
loudLastUpper(["jumpkick", "roundhouse", "uppercut"]) //"UPPERCUT!"

Lastly we can break them up with line breaks to increase readability further… especially in cases of longer method names:

let loudLastUpper = reverse
                    >>> head
                    >>> toUpperCase
                    >>> exclaim