One of these things is not like the other.

gistfile1.txt

-- Currying in Haskell
add :: Int -> Int -> Int -> Int -> Int
add a b c d = a + b + c + d
-- use `add 3 7 9 11`

// Currying in Swift
func add(a: Int) -> (Int -> Int) {
    func add1(b: Int) -> (Int -> Int) {
        func add2(c: Int) -> (Int -> Int) {
            func add3(d: Int) -> Int {
                return a + b + c + d
            }
            return add3
        }
        return add2
    }
    return add1
}
// use `add(3)(7)(9)(11)`

// JavaScript Currying
function add(a) {
    return function(b) {
        return function(c) {
            return function(d) {
                return a + b + c + d;
            }
        }
    }
}
// use `add(3)(7)(9)(11)`

Swift also supports:

func add(a:Int)(b:Int)(c:Int)(d:Int) -> Int {
  return a + b + c + d
}

👍 …which is impressively close to Haskell and syntactically equivalent to what I remember in Scala:

def add(a:Int)(b:Int)(c:Int)(d:Int) = a + b + c + d

Shouldn't the first definition be like this ?

func add(a: Int) -> Int -> Int -> Int -> Int{
    func add1(b: Int) -> Int -> Int -> Int {
        func add2(c: Int) -> Int -> Int {
            func add3(d: Int) -> Int {
                return a + b + c + d
            }
            return add3
        }
        return add2
    }
    return add1
}