Swift function to return function

Question

I try to implement a sample function to return a function.

The intended code in JS is as follows:

var class1 = function(val)
{
  var val1 = val;

  var obj = {
    say: function(val)
    {
      console.log(val1 + ' ' + val);
    }
  };
  return obj;

};

var obj1 = class1('hello');

obj1.say('ken');
obj1.say('tom');

In Swift, the function is a first-class object, but sort of hard to write like in JS. Thanks!

Are you expecting any other solution before marking one as accepted? — Jean-Philippe Pellet, Aug 06 '14 at 19:57
possible duplicate http://stackoverflow.com/questions/29022985/how-to-comprehend-the-first-class-function-in-swift/32528268#32528268 — Marco Dinatsoli, Sep 11 '15 at 22:49

score 6 · Answer 1 · answered Jun 18 '14 at 10:41

6

Here it is, a bit tweaked:

func mkGreeter(greeting: String) -> (String) -> () {
    return { name in println("\(greeting) \(name)") }
}

let greeter = mkGreeter("Hello")

greeter("Ken") // prints "Hello Ken"
greeter("Tom") // prints "Hello Tom"

mkGreeter returns a function which “remembers” the greeting param and which can be called directly.

answered Jun 18 '14 at 10:41

Jean-Philippe Pellet

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Thanks Jean-Philiipe, what I don't understand is why mkGreeter is defined `String` -> `String`. Since the return value is a function, the type should be function, correct? – Jun 18 '14 at 19:47
The type of `mkGreeter` is `String -> (String -> ())`, i.e., it takes a `String`, and returns a function of type `String -> ()`, which takes a `String` too and returns nothing. – Jean-Philippe Pellet Jun 19 '14 at 12:08

Wilson · Answer 2 · 2016-09-11T15:14:11.807

If you want to replicate your Javascript code in Swift, it will be sort of like the following:

struct Class1 {
  let val1: String

  init(_ val1: String) {
    self.val1 = val1
  }

  func say(val: String) {
    print(val1 + " " + val)
  }
}

let class1: (String) -> Class1 = { val1 in
  let obj = Class1(val1)

  return obj
}


let obj1 = class1("hello")

obj1.say("ken")
obj1.say("tom")

Where class1 variable holds a function to be invoked which accepts a String as parameter and returns an object of Class1 type.

Tweaking a litle bit the solution of Jean-Philippe Pellet, in order to make it more clear hopefully:

typealias ReturnType = (String) -> ()

func class1(val: String) -> ReturnType {
  let returnFunction: ReturnType = { (val1: String) in
    print(val + " " + val1)
  }

  return returnFunction
}

let obj1 = class1("hello")

obj1("ken")
obj1("tom")

where the typealias ReturnType represents the signature of a function which accepts a String and returns void.

Swift function to return function

2 Answers2