Extending the Swift language is cool, but be aware of what you are using

Features like the @auto_closure keyword in Swift offer a great opportunity to extend the language in ways that might not be possible in other languages. For example, in this article we used it to implement the Ruby ||= operator.

Swift doesn’t yet have a native implementation of unless or until. These are just versions of if or while, but for when the predicate is not true. Obviously you could just stick a ! in front of your if clause, but some programmers prefer the readability of the opposing versions. The implementation of ||= could be rewritten as:

@assignment func ||=<T>(inout lhs: T?, rhs: @auto_closure () -> T) {
    unless(lhs) {
        lhs = rhs()

I know, you think, I can implement unless and until myself! What’s nice is you can make them look almost like the native flow control statements, because closure expressions can be outside the parentheses of function calls if they are the last argument.1 Implementing unless seems pretty straightforward:

func unless<L: LogicValue>(pred: L, block: ()->()) {
    if !pred {

// doSomething only if condition is false
unless(condition) {

until is a little trickier, because you need to execute the predicate multiple times as you loop. But @auto_closure can help you out.

func until<L: LogicValue>(pred: @auto_closure ()->L, block: ()->()) {
    while !pred() {

// doSomething until condition becomes true
until(condition) {

Now, the conditional expression passed as the first parameter to until will be automatically wrapped up into a closure expression and can be called each time around the loop. Any variable used in both the parentheses and the block will be captured by both, so altering it in the block will affect the result of the condition (hence the condition can become true over time).

Armed with your new unless and until functions, you write a function to search a collection for the index to the first occurrence that doesn’t match a predicate:2

func findIfNot
  <C: Collection, L: LogicValue>
  (col: C, pred: (C.GeneratorType.Element) -> L)
  -> C.IndexType? {

    var idx = col.startIndex
    until(idx == col.endIndex) {
        unless(pred(col[idx])) { return idx }
        idx = idx.succ()
    return nil

let a = [1, 2, 3]
let isOdd = { $0%2 == 1}
// find the first non-odd number
let idx = findIfNot(a, isOdd)

Arguments about SESE and other stylistic preferences aside, this function should do the job. Except instead it generates a compiler error, 'C.IndexType' is not convertible to '()'.

Why? Because the return after the unless is a return from that closure expression between the braces, not a return from the findIfNot function. That unless closure expects no value to be returned, so when you return idx it’s an error. But if you were just blithely using this unless function you found in a library as if it were just like an if statement, you might not realize this and the compiler error may come as a shock.

Now say you instead implemented findIfNot to take in an index into the collection as an inout, and advanced that index to the first non-match, instead of returning a value (and returning the endIndex if every item matches). This is pretty bad code, but here it is:

func findIfNot
  <C: Collection, L: LogicValue>
  (col: C, inout idx: C.IndexType,
  pred: (C.GeneratorType.Element) -> L) {

    until(idx == col.endIndex) {
        unless(pred(col[idx])) { return }
        idx = idx.succ()

let a = [1, 2, 3]
let isOdd = { $0%2 == 1}
var idx = a.startIndex 
findIfNot(a, &idx, isOdd)
// idx will not be what you would hope 

Now, no compiler error. Instead, this code will fail much more subtly, always returning as if no non-match was found. If we replace the unless with an if, the code will run forever because the until block will return before moving idx forward, and then loop again.

Unit tests should catch this of course. But as a user of built-in-like functions, you should always be aware they aren’t quite the part of the language they might seem. The bugs resulting might not be as obvious as in this case. Also, this is a good argument for using a more “functional“ approach, by writing general algorithms like findIfNot rarely and testing them thoroughly, and then reusing them as much as possible, rather than writing explicit loops.

Should Apple extend Swift to cover this kind of case? Maybe a keyword that causes a closure that doesn’t return a value to be able to pass the return statement out and act on the calling function?3 Until they do, this is definitely a gotcha to be aware of.

  1. Sadly your implementions will always be lacking one feature that if or while have – leaving off the brackets around the predicate. You can’t write unless so it can be used as if pred { } – unless Apple extends “if the last parameter is a closure expression you can have it outside the parens” to be “if there are two parameters and the last is a closure expression, you can leave the parens off the first parameter”. 
  2. If you don’t follow what the code here is doing, try reading this introduction to algorithms on collections in Swift. 
  3. If I’ve missed something huge here and there’s already a way around this issue, leave a comment! 

Using multiple dispatch to add different types of numbers

This is part of a series on solving Paul Graham’s accumulator problem in Swift. You can find Part 1 here.

In the previous post, we solved the requirement that our accumulator accept any kind of numeric type, and to “upgrade” the type it returns from an integer to floating point only the first time it is called with a floating point number. The solution was to use the Swift Any type:

func foo(var n: Any) -> (Any) -> Any? {
    return { incf(&n, $0) }

There were lots of flaws with this approach, the worst of which was the loss of type-safety that comes with using Any. The user of foo can pass anything into the accumulator – a String, a Range, any user-defined Class. Our solution to this was to have incf return nil for types it didn’t understand. But it would be much better to rewrite foo so that it just wasn’t possible to call it with an unsuitable type in the first place.

One way would be to create a Number class and use that. But then users of the accumulator would have to explicitly convert their variables and literals to this class before using it (as Swift doesn’t support implicit conversion).

Intead, we can create a protocol Number, and extend both Int and Double to support it. Something like this:

protocol Number {
    func plus(rhs: Number) -> Number

extension Int : Number { ... }
extension Double : Number { ... }

Then rewrite the accumulator generator to use it instead of Any:

func foo(var n: Number) -> (Number) -> Number {
    return { n = n.plus($0); return n }

Now foo can be called with an Int or a Double, but can’t be called with any other type that doesn’t implement the Number protocol. The method plus would act polymorphically, adding either kind of number to an existing one, and upgrading from Int to Double when needed.

How do we implement the plus method? It needs to execute different code depending on both the type of the current value and the type of the value being added to it. That is, the behaviour of plus needs to be polymorphic with respect to two different objects, not just one. This requires something called “multiple dispatch” (in this case, specifically double dispatch).

Ordinarily, Swift (and most other OO languages) only alter their behaviour at runtime based on one object – the object who’s method is being called. This is known as single dispatch. But there is a technique that allows us to implement double dispatch. First, we need to add more methods to the Number protocol:

protocol Number {
    func plus(rhs: Number) -> Number
    func plus(rhs: Int) -> Number
    func plus(rhs: Double) -> Number

Then, we implement them for Int and Double like this:

extension Int : Number {
    func plus(rhs: Number) -> Number {
        // re-call plus with an Int parameter this time
        return rhs.plus(self)

    func plus(rhs: Int) -> Number {
        // an Int plus an Int is an Int
        return self + rhs

    func plus(rhs: Double) -> Number {
        // an Int plus a Double is a Double
        return Double(self) + rhs

extension Double : Number {
    // re-call plus with a Double parameter this time
    func plus(rhs: Number) -> Number {
        return rhs.plus(self)

    func plus(rhs: Double) -> Number {
        // a Double plus a Double is a Double
        return self + rhs

    func plus(rhs: Int) -> Number {
        // a Double plus an Int is a Double
        return self + Double(rhs)

How does this work? The interesting part is the implementation of plus that takes a Number. This calls plus again, but not recursively – instead it calls it on the right-hand side object, passing self in as a paramter. Because at this point, the code is inside a definitive type (either an Int or a Double), self will be one of those types rather than the abstract Number, and it will call one of the methods lower down.

We can easily test this works for all combinations:

let combos: Array<(Number,Number)> = [
    (1, 1),
    (1.1, 1.1),
    (1, 1.1),
    (1.1, 1),

for (a, b) in combos {
    println("\(a) + \(b) = \(a.plus(b))")

prints out:

    1 + 1 = 2
    1.1 + 1.1 = 2.2
    1 + 1.1 = 2.1
    1.1 + 1 = 2.1

Fun as this technique is, as Bjarne Stroupsrup says, “if you consider this elegant, you need to raise your standards, but it gets the task done”. Accumulators generated with this code can take either integers or floating point numbers, and ones that have only seen integers return integers, and you can’t call it with any other unsupported types.

It still suffers from similar problems to the incf solution – every time you add a new type of Number you need to alter the code quite extensively. And implementing additional operations like minus or multiply would require even more code. Looking at a solution to that will be the subject of the next in the series. Follow @AirSpeedSwift to catch it.

Swift’s for loops avoid an easy mistake with variable capture

This article from a while back points out “the biggest mistake everyone makes with closures” in Ruby, Python, Perl and Javascript.1

Try running this code in Ruby, which counts i up from 1 to 3 creating a closure each time which returns the value of i:

a = []
for i in 1..3
  a.push(lambda { i })

for f in a
  print "#{f.call()} "

and it prints out the possibly surprising value 3 3 3.

Using more “functional” constructs doesn’t always save you. This Python also prints all 3s:2

a = [lambda: i for i in xrange(1, 4)]
for a in f:
  print f()

The article goes on to suggest some workarounds.

Here is the equivalent in Swift:

var a: Array<()->Int> = []

for i in 1...3 {
  a.append( { i })

for f in a {
  println("\(f()) ")

and happily, this prints 1 2 3.

Note, this also works with references to objects, so if you subsituted a for over a sequence of objects instead of Ints in the code above, each closure would capture a reference to a different object.

Be careful though, this applies to for loops but not to other loops like while or the C-style for(;;), so:

var i: Int = 0
while(++i <= 3) {
    a.append( { i })

for f in a {
    print("\(f()) ")

prints out 3 3s.

Why is this? Is this a special hack to make for...in behave less surprisingly? To understand why it does this, you need to realize that this:

for i in 1...3 {
  a.append( { i })

is just a shorthand equivalent to writing this code using the source sequence’s generator:

var g = (1...3).generator()
while let i = g.next()
  a.append( { i })

The answer to our question lies in the let i =, which declares a fresh variable i for each iteration of the loop, hence if you capture i, it will retain its value at the end of the block. If you use this form of while loop, you’ll get the same results as with the for...in version.

  1. Go here for an epic list of how other languages handle this issue. 
  2. The Ruby each will do something similar in version 1.8, though it was altered in 1.9 to behave differently which is somehow more scary. 

Writing Algorithms on Collections in Swift

edit: this post has been updated to Swift as of Xcode 6.1 (Swift 1.1).

Suppose you want a new algorithm that checks if every member of an array fulfills a given criteria.

Your first thought might be to add it as an extension of Array:1

extension Array {
    func all_of<B: BooleanType>
        (predicate: (Element) -> B) -> Bool {
            for e in self {
                if !predicate(e) {
                    return false
            return true

This function takes a predicate (a function that takes an element and returns true or false), and applies it to every member of the array, only returning true if it was true for every member.

Note we specify a template argument, B: BooleanType, that allows the predicate to return not just a Bool but any type that can behave like a Bool (such as ObjCBool)2.

We can test it works easily:

let isEven = { ($0 % 2) == 0 }
let mixed = Array(1...4)     // [1, 2, 3, 4]
let doubled = map(1...4) { $0 * 2 }
let evens = Array(doubled)   // [2, 4, 6, 8]

mixed.all_of(isEven)  // false
evens.all_of(isEven)  // true

Incidentally, the above shows a nice trick that’s useful for testing collection algorithms, which is that arrays can be initialized with sequences, such as ranges. So if you quickly want an array of increasing numbers, you can write Array(begin...end). If you want something more complicated, you can pass a range to map or filter to alter the range first.

This implementation is nice because it’s now built into Array, but there are a few downsides.

Firstly, what if you wanted a slightly different version all_of_equal, that just took a value and confirmed if every entry was equal to that value? You’ll need to test for equality, so Element will need to be Equatable. You might think you’d specify a function that goes like this:

extension Array {
    func all_of_equal<Element: Equatable>
    (value: Element) -> Bool {
        return self.all_of { $0 == value }

Nope, that won’t work. The problem is that you can’t further constrain Element within Array to be Equatable. What if you created an array of something that didn’t conform to Equatable? Would this no longer be allowed now you’ve added this method? Or should this method not be callable? Neither of these are palatable so the language just won’t let you do it.

Instead, you need to define a non-member function that takes an Array and an Element. Then you can constrain that element as much as you like:

func all_of<E, B: BooleanType>
    (a: Array<E>, predicate: (E) -> B) -> Bool {
        for e in a {
            if !predicate(e) {
                return false
        return true

func all_of_equal<E: Equatable>
    (a: Array<E>, rhs: E) -> Bool {
        return all_of(a) { $0 == rhs }

In fact, because of Swift’s operator overloading, you can even give it the same name:

func all_of<E: Equatable>
    (a: Array<E>, rhs: E) -> Bool {
        return all_of(a) { $0 == rhs }

// calls the version that takes an equatable value
all_of([1,1,1], 1)

// calls the version that takes a closure
all_of([1,1,1]) { $0%2 == 1 }

The second benefit of defining this as a non-member function is you can generalize it to apply not just to arrays, but any kind of collection.3 To do this, you need to extend the generic parameter clause in a couple of ways:

func all_of<E, C: CollectionType, L: BooleanType
    where E == C.Generator.Element>
    (c: C, predicate: (E) -> L) -> Bool {
        for e in c {
            if(!predicate(e)) {
                return false
        return true

First, we swap out Array for a type of Collection, by specifying a generic placeholder C that must be a collection. Then, we further constrain the collection to contain types E, which are the type of input our predicate will take.

Strictly speaking, we don’t need to include the placeholder E. You could replace it in the predicate definition with C.Generator.Element. But you might find it’s cleaner to create a placeholder for it, especially if you apply further constraints to E.

Now, we can use this function on not just arrays, but any type of collection, such as ranges directly:

let isEven = { ($0 % 2) == 0 }
all_of(1...4, isEven)  // returns false

or other types of collection:

let doubled = map(1...4) { $0 * 2 }
all_of(doubled, isEven)  // returns true

There’s one final improvement that could be made. Collections implement the Sequence protocol, which also gives us enough to iterate over the elements, checking the predicate. So we could rewrite all_of even more generally:

func all_of<E, S: SequenceType, B: BooleanType
where E == S.Generator.Element>
(sequence: S, predicate: (E) -> B) -> Bool {
    for e in sequence {
        if(!predicate(e)) {
            return false
    return true

Because Sequences behave differently, we don’t need to mandate a ForwardIndex to make them work with the for.

There are some times you still want to use collections rather than sequences. If we were writing a different algorithm, say find_first_of, that we wanted to return an index into the collection of the first occurrence, we’d want to stick with an algorithm that works on collections. Also, collections imply deterministic results over multiple iterations, and consistent size, which some algorithms might rely on. But not in the case of all_of, so SequenceType is a better choice.

  1. The most important question facing the Swift community today: how do you split long function footprints over multiple lines? 
  2. In fact, the Swift standard library docs explicitly state that Bool and ObjCBool should really only be the two types that ever implement BooleanType, so maybe you could say screw it and just make the predicate return a Bool. That’s what the predicates for std lib functions like filter do. But then, that wouldn’t make for quite so educational a blog post. 
  3. You might be thinking, I know, I’ll extend CollectionType! But CollectionType is a protocol, and you can’t extend protocols with implementations. What you want is something like traits or mixins. Swift doesn’t have those (yet, hopefully). 

Closures and Closure Expressions are two different things

Bit of quick monday morning pedantry. fuckingclosuresyntax.com, while fun and useful, is perpetuating a subtle conflation of two things, that most Swift programmers have silently adopted, that in turn is probably confusing a lot of beginners.

This function returns a closure:

func return_a_closure() -> () -> (Int) {
  acc = i
  func the_closure() -> Int {
    return ++acc

Even though func was used to define the closure, not { }, it is still a closure, because it’s a function that captures an environment of variables (in this case acc).

The { } syntax alternative to func is called a “closure expression” but it doesn’t make the function returned any more closurey than the example above.

If you already know the difference, it’s natural to drop the word “expression” and just call them closures, and this is what most people writing about Swift are doing. This was inevitable as soon as Apple decided to call them closure expressions, rather than blocks or lambdas or anonymous functions. But this conflation seems to be confusing a lot of people coming to Swift without a background in first-class function and closures. The opening of the closures chapter in the official Swift book makes the difference quite clear, but still the confusion remains.

The bright side is this might be forcing new developers to learn about closures, by throwing them in the terminology deep end. If Apple had stuck with Objective-C’s term, blocks, maybe fewer programmers would be learning about variable capture (plus, fuckingblocksyntax.com was already taken).

What would have been really cool? If Apple had done away with the func keyword altogether, leaving closure expressions assigned to variables as the only way to declare functions. But that would have pushed way too many people out of their comfort zone.

If all this is nonsense-speak to you, try reading this tutorial.

Implementing some Ruby builtins in Swift

Type 1. into irb and hit tab, and it’ll ask you if you want to see all 105 possibilities. Try it with [ ] and you’ll be offered 155. Ruby comes with a lot of built-in methods on its default types.

It’s debatable how useful all of these are. The idiom in Swift seems much more to be like in the C++ STL, where there are algorithms that operate on collections, rather than collections having methods that operate on themselves. To see more about this, read this article on writing algorithms against collections.

But for the sake of looking at a few Swift language features, let’s implement some of the Ruby popular ones. Maybe they’d be useful making a Ruby refugee feel at home.

First off, the Int loops. Lots of “hey look, ruby is cool” intros kick off with this one:

10.times do |i|
   "ruby is cool!"

This is pretty easy to add to the Swift Int using an extension:

extension Int {
    func times(f: () -> () ) {
        for _ in 0..self {

10.times { "swift is cool too!" }

Note the _ instead of a variable in the for, because the value isn’t actually needed.

Next, two for alternatives, upto and downto:

1.upto(3) {|i| print "#{i}.. "} # prints 1.. 2.. 3..
3.downto(1) {|i| print "#{i}.. "} # prints 3.. 2.. 1..

in Swift would be:

extension Int {
    func upto(n: Int, f: (Int) -> () ) {
        for i in self...n {
    func downto(n: Int, f: (Int) -> () ) {
        for i in reverse(n...self) {

Note the use of the inclusive ... this time, unlike with times which used the convention of a zero base up to but not including n so needed non-inclusive .., because this time we want to start at self and count inclusively to n. Humorously, Ruby also has inclusive and exclusive ranges but .. and ... are the opposite way around.

For downto, the range is wrapped in a reverse in order to count backwards.

Next, some extensions to array, in a similar vein to map or reduce.

Ruby’s each is like map, but doesn’t generate any return value:

extension Array {
    func each(f: (Element) -> ()) {
        for e in self {

Element is a Type Property representing the type of value the Array contains, to this function works generically for type contained by array.

Ruby’s reverse_each does the same but in the opposite direction:

extension Array {
    func each(f: (Element) -> ()) {
        for e in Swift.reverse(self) {

Again, we use reverse to go backwards through the array. Wait, you might think, isn’t it inefficient to reverse the array just to iterate over it? But that’s not what reverse does – instead it generates a lazy collection that serves up the values in reverse order.

Note, we have to prefix it Swift.reverse to disambiguate it from the Array member function reverse – which would result in creating a whole new array in reversed order, if we used it.

Ruby’s delete_if is the opposite of Swift’s filter – it takes a discriminator function and returns any elements that don’t match. We could easily just implement it from scratch with a for loop, but instead let’s try and implement it by re-using filter.

One way to do this is to first create what’s called a “decorator”. Decorators are functions that take other functions, and return a new function that has changed the behaviour of the original function in some useful way.

What we want is a decorator that takes a function that returns a boolean, and then returns the opposite of that. Here it is:

// function that takes a function, f, that takes an element
// and returns a bool, and returns a function that applies f
// then returns the opposite
func not_f<T>(f: (T)->Bool) -> (T)->Bool {
    // this returns a closure that captures f
    return { !f($0) }

If you’re a bit hazy on closures and variable capture, read this.

Armed with not_f, we can define delete_if:

extension Array {
    func delete_if(f: (Element) -> Bool) -> Element[] {
        return self.filter(not_f(f))

Finally, Ruby’s equivalent of filter is called select. It’s identical, it’s just called by a different name. Is there some way we could alias filter if we wanted?

Yup, like this:

extension Array {
    var select: ((Element) -> Bool) -> Element[] { 
        return Array.filter(self) 

You can now use select in exactly the same way you use filter. This is quite a silly thing to do, but it does demonstrate a couple of language features: read-only computed properties that return closures, and references to member functions.

Read-only computed properties are used like regular properties i.e. can be accessed using the dot syntax, but behave like functions that take no arguments and return a value. In this case we have defined a property select that returns a function when you call it. We’re just returning an existing member function (more on that next) but you could have more complex logic that created and returned different kinds of closures.

The reason it’s nice to implement these as computed properties is you don’t need to use a () after the property name. This means if you want to compute a function for someone to use, they can use it like this: x.computedfun(). If computedfun was itself a function, you’d have to write this: x.computedfun()(), which looks odd. Other than that, they behave just like functions that return functions.

So what is select actually returning? To understand that, you need need to realize you can reference a classes’ member functions. See the following code:

class MyClass {
    let n: Int

    init(_ i: Int) { n = i }

    func multiply(i: Int) { 
        println("n: \(n) * i: \(i) = \(n*i)")

    func add(i: Int) { 
        println("n: \(n) + i: \(i) = \(n*i)")

let a = MyClass(2)
let b = MyClass(5)

// prints "n: 2 * i: 10 = 20"

// prints "n: 5 + i: 10 = 50"

// this takes a reference to the member 
// function multiply of MyClass
var memfunref = MyClass.multiply

// you can't use memfunref directly.
// you have to apply an instance of
// MyClass to it:
var instancefunref = memfunref(a)

// now you can call it, and it's
// like calling a.multiply, so
// this prints "n: 2 * i: 10 = 20" 

// MyClass.add has the same type as
// MyClass.multiply, they're both (Int)->(),
// so you can assign it to memfunref too:
memfunref = MyClass.add

// and apply a different instance of MyClass
instancefunref = memfunref(b)

// this prints "n: 5 + i: 10 = 50"

// or you can skip the instancefunref step
// prints "n: 5 + i: 10 = 50" again

Now we can see what that computed property is doing. It’s returning the member function filter of Array, then applying self to it, and returning that. The caller then can call the returned function () and it runs filter against the object they fetched it from. Like I said, bit silly, but it demonstrates a feature that could probably be put to better use in other cases.

If you liked this post, this one is also about implementing Ruby features in Swift, in this case implementing the ||= operator using the @auto_closure Swift feature.

Default Parameters in Swift – Dynamically or Statically Bound?

edit: after this post originally went up, the Swift dev team confirmed on the forums that default parameters should be dynamically bound. However, as of Swift 1.1, they’re still statically bound.

Quick quiz. What does the following code do?

class Shape {
  func draw(colour: String = "Red") { 
    println("\(colour) shape")

let s1 = Shape()

It prints Red shape, right? No argument supplied, so the default value of Red is used.

Ok what about this?

class Circle: Shape { 
  override func draw(colour: String = "Blue") { 
    println("\(colour) circle") 

let s2 = Circle()

It should Blue circle. No surprises there. Again no argument, but the overridden function defaults to a different color and prints a different shape.

Ok finally, what about this?

let s3: Shape = Circle()

Blue circle again, right? s3‘s static type is the base class, but it points to an inherited class. Polymorphism means the overridden function runs, so it should be just like the previous example.

If Swift bound default parameters dynamically, sure. But as of Swift 1.1, it behaves like C++,1 and you actually get Red circle. This is because parameters are bound statically not dynamically. Similarly, which overloaded function is chosen is also done statically – as seen in our series on Swift overload resolution

Why is C++ like this? Because default arguments are determined at compile time, not at run time. So while the correct function is applied, the default argument is looked up via the static type of s3, which is a Shape not a Circle. This could lead to some quite surprising behavior, which is why this is Item 37 of the Effective C++ book, which has more info on why it is this way (it’s related to run-time performance) and some workarounds to avoid it.

  1. I wrote the same code in C++ to double-check this and, holy cow, writing some simple C++ was painful after a week of Swift. 

A Basic Tutorial on Functions and Closures in Swift

Note: this is an article for people new to functional programming style, and to closures. Unlike the rest of this blog, it is very much an explantion for beginners, so if you’re already familiar with these topics you might want to skip it.1 You may find these other articles interesting though – Implemeting an Accumulator in Swift, and A Straw-man Argument for Trying More Functional Programming in Swift.

If you like this post, consider buying a copy of our book, Advanced Swift, which includes introductory material like this, and then builds on it to cover many more advanced Swift topics.

Reading the /r/swift subreddit and stack overflow, there seems to be a lot of questions about functions and closures from programmers learning Swift. There’s confusion between what a closure is, what a function is, and what a closure expression is. And many people are new to the whole idea of functions being first class objects. This article is an attempt to help out.

To understand functions and closures in Swift you really need to understand three things, in roughly this order of importance:

  1. Functions can be assigned to variables and passed in and out of other functions as arguments, just as an Int or a String can be.
  2. Functions can “capture” variables that exist outside of their local scope.
  3. There are two ways of creating functions: with the func keyword, or with { }. Swift calls the latter “closure expressions”.

I suspect people new to the topic of closures are coming at it in reverse order, and maybe missing one of these points, or conflating the terms “closure” and “closure expression”, and that this can cause a lot of confusion. It’s a three-legged stool, and if you miss one of the three points above, you’ll fall over when you try to sit down.

1. Functions can be assigned to variables and passed in and out of other functions as arguments, just as an Int or a String can be.

In Swift, as in many modern languages, functions are referred to as “first-class objects”. You can assign functions to variables, and you can pass them in and out of other functions, to be called later.

This is the most important thing to understand. “Getting” this for functional programming is akin to “getting” pointers in C. If you don’t quite grasp this part, everything else will just be noise.

Here is an example of assigning functions to variables and passing them to functions:

// This is a function that takes an Int and prints it. 
func printInt(i: Int) {
    println("you passed \(i)")

// This assigns the function you just declared
// to a variable.  Note the absence of () after the 
// function name.
let funVar = printInt

// Now you can call that function using your variable. 
// Note the use of () after the variable name.
funVar(2)  // will print out "you passed 2" 

// You can also write a function that takes a
// function as an argument
func useFunction(funParam: (Int) -> () ) {
    // call the passed-in function:

// You can call this new function passing in either 
// the original function:
// or the variable

Why is being able to treat functions like this such a big deal? Because it allows you to easily write “higher-order” functions, which take functions as arguments and apply them in useful ways.

For example, arrays have a member function map that takes a function and applies it to each member of the array, returning a new array containing the results.

// a simple function that doubles an int
func doubler(i: Int) -> Int { return i * 2 }

// the following runs doubler on each number,
// returning a new array of the results
let a = [1, 2, 3, 4].map(doubler)

// a now contains [2, 4, 6, 8]

There are several other member functions in array that take functions – filter (takes a function that examines each element for inclusion in the returned array), sort (takes a function that determines a custom ordering for two elements), reduce (takes a function to incorporate each element in turn into a running value – e.g. plus to sum them, max to find the highest value element).

2. Functions can “capture” variables that exist in the context they were declared

You can also return functions from other functions:

// This is a function that returns another function.
// That other function takes an Int and returns nothing.
func returnFunc() -> (Int) -> () {
  func innerFunc(i: Int) {
    println("you passed \(i) to the returned func")
  return innerFunc

let f = returnFunc()
f(3)  // will print "you passed 3 to the returned func"

(The funcName() ->(Int) ->() syntax can be a little confusing at first. Try not to get too hung up on it for now, come back to it in detail later when you understand more.)

If, when you declare a function, it references variables outside the function’s scope, those variables are “captured”, and stick around after they would otherwise fall out of scope and be destroyed.

To see this, let’s revisit our returnFunc function, but add a counter that increases each time we call it:

func returnFunc() -> (Int) -> () {
  var counter = 0  // local variable declaration
  func innerFunc(i: Int) {
    counter += i   // counter is "captured"
    println("running total is now \(counter)")
  return innerFunc
  // normally counter, being a local variable, would  
  // go out of scope here and be destroyed. but instead, 
  // it will be kept alive for use by innerFunc

let f = returnFunc()
f(3)  // will print "running total is now 3"
f(4)  // will print "running total is now 7"

// if we call returnFunc() again, a fresh counter
// variable will be created and captured
let g = returnFunc()
g(2)  // will print "running total is now 2"
g(2)  // will print "running total is now 4"

// this does not effect our first function, which
// still has it's own captured version of counter
f(2)  // will print "running total is now 9"

Think of these functions combined with their captured variables as similar to instances of classes with a method (the function) and some member variables (the captured variables).

A combination of a function and an environment of captured variables is called a “closure” in computer programming terminology. So f and g above are examples of closures, because they capture and use a non-local variable (counter) that was declared outside them.

3. The { } syntax for closure expressions

In Swift, you can declare functions in two ways. One is with the func keyword demonstrated above. The other way is to use a “closure expression”.

Here is the doubler function from above, written using the closure expression syntax:

let doubler = { (i: Int) -> Int in return i*2 }
// doubler can be used just the same way as before
[1, 2, 3].map(doubler)  

Functions declared as a closure expression can be thought of as function “literals”, in the same way as 1 and "hello" are Int and String literals. They are also anonymous – they aren’t named, unlike with the func keyword. The only way they can be used is if you assign them to a variable when they are created, as we do here with doubler.

The doubler declared using the closure expression, and the one declared earlier using the func keyword, are completely equivalent. They even exist in the same “namespace”, unlike in some languages i.e. if you declared doubler using func, and then doubler using let or var in the same scope, you’ll get an error that you are redeclaring something that has already been used (similar to if you tried to declare a variable with the same name twice).

Why is the { } syntax useful then? Why not just use func every time? Well, it can be a lot more compact, especially when writing quick functions to pass into other functions such as map. Here is our doubler map example written in a much shorter form:

[1, 2, 3].map { $0 * 2 }

This looks very different, because we’ve leveraged several features of Swift to compress it down. Here they are one by one:

  1. If you are passing the closure in as an argument, and that’s all you need it for, there’s no need to store it in a local variable first. Think of this like passing in a numeric expression, such as 5*i, to a function that takes an Int as a parameter.
  2. If all the types can be “inferred” from the context by the compiler, you don’t need to specify them. In this case, the function passed to map is taking in an Int (that’s what’s in the array), and returning an Int (because an Int times an Int is an Int), so we can leave the types off.
  3. If the closure expression is a single expression, you can leave off the return and it will automatically return the value of the expression.
  4. Swift automatically provides shorthand names for the arguments to the function – $0 for the first, $1 for the second etc.
  5. If the last argument to a function is a closure expression, you can move the expression outside the parenthesis of the function call. This is nice if you have a multi-line closure expression, as it more resembles a regular function definition, or other block statement such as if(expr) { }.
  6. Finally, if a function has no arguments other than a closure expression, you can leave off the parenthesis after the function name altogether.

Using each of the above rules, we can boil down the expression to the form above:

  1. [1, 2, 3].map( { (i: Int) ->Int in return i * 2 } )
  2. [1, 2, 3].map( { i in return i * 2 } )
  3. [1, 2, 3].map( { i in i * 2 } )
  4. [1, 2, 3].map( { $0 * 2 } )
  5. [1, 2, 3].map() { $0 * 2 }
  6. [1, 2, 3].map { $0 * 2 }

If you’re new to functional programming in general, and to Swift syntax, then these compact function declarations might seem daunting at first. But as you get more comfortable with the syntax, and with using functional programming style, they will start to feel more natural and you’ll be grateful for the ability to remove the clutter so you can see more clearly just what the code is doing. Once you get used to reading code written like this, it’ll be much clearer to you at a glance than code doing the same thing in a conventional for loop.

One final point on naming. It’s important to remember that functions declared with func can be closures, just like ones declared with { }. Remember, a closure is a function combined with any captured variables. While functinos created with { } are called “closure expressions”, people often refer to this syntax as just “closures”. But don’t get confused and think that functions declared with the closure expression syntax are different from other functions – they aren’t.2 They are both functions, and they can both be closures.

Once you think you understand everything in this article, try reading some more advanced articles that involve closures on this blog – Implemeting an Accumulator in Swift, and Implementing Ruby’s ||= in Swift.

  1. Or read it for the purpose of giving me nitpicking feedback, which would be great. 
  2. Well, OK, they are. For example you can’t do the skipping the return statement trick with a one-line func. But that’s just syntactic sugar, the point is they don’t differ in the fundamental way they behave. 

Implementing Ruby’s ||= operator in Swift using @autoclosure

edit: this has been updated for Swift as of Xcode 6.1 (Swift 1.1)

A common idiom in Ruby is to assign a value to a variable only if that variable isn’t already set, using the ||= operator:

s ||= "default value"

Swift doesn’t provide such an operator out of the box, but using some of the features of the language, it’s possible to implement one.

Simple first implementation

First let’s try implementing it just for string types. We need to define the operator itself first:

infix operator ||= {
    associativity right
    precedence 90

And then implement it with a function:

func ||=(inout lhs: String?, rhs: String) {
    if(lhs == nil) {
        lhs = rhs

Note the use of the assignment attribute on the operator definition, and the inout parameter on the left-hand side, which is the variable being assigned to.

This works. If you try the following

var s: String?
s ||= "first assignment"
s ||= "second assignment" 

the second assignment will have no effect on s – it will keep the value “first assignment”.

Using generics to apply to any type

What about implementing this with generics, so it will work not just for String but for any type? That’s pretty simple:

func ||=<T>(inout lhs: T?, rhs: T) {
    if(lhs == nil) {
        lhs = rhs

Now you can use our ||= on any type optional type – String?, Int?, or any user-defined class.

Using autoclosure to avoid unncessary evaluation

Our ||= still doesn’t quite match the Ruby version. If the value on the left-hand side is already set, the statement on the right-hand side is never executed. This is important if, for example, the right-hand side were a function with other side-effects, or an expensive computation.

But by default in Swift, any statement passed to a parameter is fully executed first. To replicate the Ruby functionality, we have to use an attribute in Swift called autoclosure. This is used like this:

func ||=<T>(inout lhs: T?, rhs: @autoclosure () -> T) {
    if(lhs == nil) {
        lhs = rhs()

What autoclosure does is wrap the arguments suppled in a closure, for later execution. Then, if they aren’t needed, they are never executed. If they are needed (in this case, because lhs is nil), the closure can be called (note the new type for rhs, and the parenthesis after rhs, indicating it is now calling a function).

To check this works, try the following, which should only print “I’ve been run” once to the console:

func printlnWhenRun() -> Int {
    println("I've been run")
    return 0
var i: Int?
i ||= printlnWhenRun()
i ||= printlnWhenRun()

Implementing for Boolean values

This works great for optional types, but what about the more conventional behaviour, a compound logical-OR-and-assign operator for boolean values?

Despite listing it in the Expressions precedence section of the language reference, this doesn’t appear to be implemented in Swift natively.

edit: it’s now been removed from the precedence section. You can still declare it as below though.

No problem though, we can just implement it ourselves. Here it is:

func ||=<T: BooleanType>(inout lhs: T, rhs: @autoclosure () -> T) {
    if(!lhs) {
        lhs = rhs()

Note the BooleanType type constraint after the type parameter. This is necessary because we need to guarantee that you can pass whatever type is used into the if statement. This wasn’t necessary with the other version because it was the optional qualifier supplying this ability.1

But doesn’t this clash with the other definition? Nope. A combination of the type constraint on the logical version, and the presence of the optional parameter on the optional-assignment version, means the possible uses of these two functions are entirely disjoint, and the Swift compiler will pick the right one for you. Even a Bool? type will go to the optional-assignment version.2

Using lazy properties instead of ||=

Having done all of this, you could argue ||= isn’t all that useful in Swift because of different feature, lazy properties. These are properties of a class or struct that are only initialized for the first time when they are used – in much the same way as Ruby devs use ||=. Below is some code showing this in action:

class MyClass {
    lazy var complexThing = CreateComplexThing()
var c = MyClass()
// ... maybe some time later
var thing = c.complexThing  // CreateComplexThing() runs now

Obviously this only applies when your property is part of a class, not when you’re declaring a variable in a function, so maybe there’s still a place for ||=. But if not, at least we learned something implementing it.

  1. And there’s no need to constrain to objects with the ability to assign to each other – they all have that ability. There’s no “Assignable” protocol. Be nice if there were – that would imply you could overload “=”. But this ain’t C++. 
  2. It is possible to write two generic functions that overlap in their inputs, in which case you will get an “use of unresolved identifier” error from the compiler when you call the function (note, not when you define the overlapping function, which is a break from other features of generics when you get a compiler error at the time of definition rather than use). 

Rundown of how each Effective C++ item relates to Swift

In this post we talked about how some of the items in Effective C++ might apply to Swift.

Many of the entries highlighted some limitations on the part of Swift, so some might think this post was a criticism of Swift.  Actually far from it – many of the items in Effective C++ are pointing out dangers of C++ where you can shoot yourself in the foot if you aren’t careful.  In most cases, when you look at the equivalent issue in Swift, it’s a non-issue.

For completeness, here is a rundown of every item, what kind of article it is, and how it applies to Swift:

Item Danger or benefit? In Swift
1: View C++ as a federation of languages Probably a benefit? Applies the same
2: Prefer consts, enums and inlines to defines Pre-processor dangerous! No preprocessor!  No reflection or lisp-style macros either tho.
3: Use const whenever possible Benefit let and mutating are there, but they only go so far. 
4: Make sure that objects are initialized before they’re used Danger Fully locked down, you have to initialize variables properly.
5: Know what functions C++ silently writes and calls Bit of both Swift writes some init methods automatically, though it’s nowhere near as potentially confusing as C++
6: Explicitly disallow the use of compiler-generated functions you do not want Benefit Doesn’t really apply to Swift
7: Declare destrutors virtual in polymorphic base classes Danger Not an issue, every method is virtual in Swift classes.
8: Prevent exceptions from leaving destructors Danger No exceptions in Swift!
9: Never call virtual functions during construction or destruction Danger Swift’s enforcement of initializing the super and all member variables helps prevent this problem
10: Have assignment operators return a reference to *this More of a convention No assignment (or copy constructors) in Swift
11: Handle assignment to self in operator= Danger No assignment (or copy constructors) in Swift
12: Copy all parts of an object Danger Still dangerous, but without as good tools to fix it (no copy constructors on structs!)
13: Use objects to manage resources Benefit ARC + deinit = same benefit!  So long as you’re careful about variable capture.
14: Think carefully about copying behavior in resource-managing classes Good advice Good advice applies, take it
15: Provide access to raw resources in resource-managing classes Good advice Good advice applies, take it
16: Use the same form in corresponding uses of new and delete Bit of both No need for explicit memory management in Swift.  That’s good, right?
17: Store newer objects in smart pointers in standalone statements Bit of both ARC means every pointer in Swift is a smart pointer, so all good.
18: Make interfaces easy to use correctly and hard to use incorrectly Good advice Good advice applies, take it
19: Treat class design as type design Good advice Good advice applies, take it
20: Prefer pass-by-reference-to-const to pass-by-value Bit of both Doesn’t really apply to Swift, though I wish it had proper const support.
21: Don’t try to return a reference when you must return an object. Danger ARC means every pointer in Swift is a smart pointer, so all good.
22: Declare data members private Benefit No public/private in Swift – yet.  It’s coming apparently.
23: Prefer non-member non-friend functions to member functions Bit of both Seems to apply the same to Swift.
24: Declare non-member functions when type conversions should apply to all parameters Bit of both Type conversion in Swift is very different, and rarely implicit, so this doesn’t really apply.
25: Consider support for a non-throwing swap Bit of both No exceptions in Swift, so your swap will definitely be non-throwing.
26: Postpone variable definitions as long as possible Good advice Good advice applies, take it
27: Minimize casting Danger Swift type casting is a lot safer/saner/more powerful than C++. 
28: Avoid returning “handles” to object internals. Good advice Good advice applies, take it
29: Strive for exception-safe code Danger Swift code is definitely exception-safe.
30: Understand the ins and outs of inlining Danger If compilers aren’t better at humans at inlining these days, something has gone wrong.
31: Minimize compilation dependencies between files Danger No more headers!
32: Make sure public inheritance models “is-a” Good advice Good advice applies, take it
33: Avoid hiding inherited names Danger Swift mandates you override a method if it has the same signature as a parent class version
34: Differentiate between inheritance of interface and inheritance of implementation Good advice Good advice (mostly) applies, take it
35: Consider alternatives to virtual functions Good advice I need to research this one…  It probably applies.
36: Never redefine an inherited non-virtual function Danger No non-virtual functions in Swift
37: Never redefine a function’s inherited default parameter value Danger Swift behaves the same right now but this has been declared a bug in the compiler
38: Model “has-a” or “is-implemented-in-terms-of” through composition. Good advice Good advice applies, take it
39: Use private inheritance judiciously Danger Private inheritance is a weird C++ thing, don’t sweat it.
40: Use multiple inheritance judiciously. Danger No multiple implementation inheritance in Swift.  Most people think this is the right idea.
41: Understand implicit interfaces and compile-time polymorphism Benefit Swift has compile-time polymorphism through generics much like C++ templates but they differ a lot (C++ templates are crazy powerful crazy complicated)
42: Understand the two meanings of typename Confusing, if not dangerous You know how I said C++ templates got crazy complicated?  This is part of that.
Item 43: Know how to access names in templatized base classes Confusing, if not dangerous I need to research this one… probably doesn’t apply.
Item 44: Factor parameter-independent code out of templates Danger Is it possible Swift generics use could result in code bloat?  Would really need some serious LLVM digging to find out.
Item 45: Use member function templates to accept “all compatible types.” Benefit Swift generics provide much of the same features.  Needs a bit more research.
Item 46: Define non-member functions inside templates when type conversions are desired Benefit Swift is not very implicit type-conversion friendly (opinion probably divided on whether this is a good or bad… I think bad, most probably think good)
Item 47: Use traits classes for information about types Benefit I need to research this one… Associated types and type constrains in Swift seem related to this.
Item 48: Be aware of template metaprogramming A bizarre C++ rabbit hole Probably best not to think about it.
Items 49-52: New and delete Various Swift does have UnsafePointer and malloc so maybe these are kind of topics are not as much of a non-issue as you might think at first.
53: Pay attention to compiler warnings Good advice Good advice applies, take it
Item 54: Familiarize yourself with the standard library, including TR1 Benefit Objective-C, like C++, has a huge library behind it, all of which can be used from Swift.  Question is, how much will be ported to a more Swift-like form?  Time will tell
Item 55: Familiarize yourself with Boost Beneft Boost is fantastic.  But early indications are, there’s a huge community of Swift developers looking to do similar things.