Currying, composing and trace.swift

Inspired from https://drboolean.gitbooks.io/mostly-adequate-guide/content/ch5.html

// Curries by partially applying 1 out of 2 arguments
func curry<A, B, C>(fn: (A, B) -> C, _ val: A) -> (B -> C) {
    return { b in
        fn(val, b)
    }
}

// "compose" operator
infix operator >> {
    associativity left
}
func >> <A, B, C>(f: A -> B, g: B -> C) -> (A -> C) {
    return { a in g(f(a)) }
}

// Trace at any point of function composition
func trace<T>(tag: String, _ val: T) -> T {
    print("\(tag) -> \(val)")
    return val
}

func doubled(number: Int) -> Int { return number * 2 }
func odd(number: Int) -> Bool { return number % 2 == 1 }

let oddAfterDoubling = doubled >> odd
let oddAfterDoubling_withTrace = doubled >> curry(trace, "after add") >> odd
oddAfterDoubling_withTrace(13) // after add -> 26