What is the difference between functions and closures? [closed]

Question

I have been reading the swift programming guide in iBooks. Could someone explain to me what is the difference between a function and a closure. Is it just that it has no name and can be used in expressions?

Answer 1

First, let's start with definition of Closure, as found in Wikipedia:

In programming languages, a closure (also lexical closure or function closure) is a function or reference to a function together with a referencing environment—a table storing a reference to each of the non-local variables (also called free variables or upvalues) of that function.

Closure is the term that is used to refer to a function along with the variables from its environment that it "closes".

The definition of Closure in Swift is inline with lambdas and blocks in other languages like C# and Ruby.

As for the difference from functions, from the Swift documentation:

Global and nested functions, as introduced in Functions, are actually special cases of closures

So all functions are essentially closures that store references to variables in their context.

Closure expressions are convenient way of writing closures, and provides more terse syntax.

Answer 2

Functions are, in fact, just named closures. The following are at least conceptually equivalent:

let foo = { println("hello") }

func foo()->(){ println("hello") }

This gets a little more complicated in the case of using func to declare methods, as there's some interesting bits of sugar added regarding the automatic insertion of public named parameters, etc. func myMethod(foo:Int, bar:Int, baz:Int) becomes func myMethod(foo:Int, #bar:Int, #baz:Int), for example.

But it's still true that even methods are just a specific case of closures, and if it's true of closures, it's true of functions and methods as well.