How to use function currying in Swift 4

Question

I try to understand the function currying tutorial but that code seem out of date. And it's still not really clear about function currying.

I try with this function:

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

And it runs ok with Playground (Xcode 9 beta 6). But the problem is I cannot use this function as the tutorial:

let add = curry(+)
let xs = 1...100
let x = xs.map(add(2))

The code above return error:

Playground execution failed:

    error: FunctionsCurrying.playground:31:17: error: ambiguous use of operator '+'
    let add = curry(+)
                    ^

Please correct me and help me to clear about function currying.

Answer 1

That problem is not Swift 4 related, you would get the same error message in Swift 3.

There are many overloaded + operators, therefore in

let add = curry(+)

the compiler does not know which one to choose. With an explicit type cast

let add = curry((+) as ((Int, Int) -> Int))

or an explicit type annotation

let op: (Int, Int) -> Int = (+)
let add = curry(op)

the code compiles and runs as expected.