Does Swift have a null coalescing operator and if not, what is an example of a custom operator?

Question

A common feature in many languages, the Null Coalescing Operator, is a binary operator often used to shorten expressions of the type:

x = possiblyNullValue N         


        

Answer 1

(Also see blog post)

No, Swift doesn't currently support a null coalescing operator.

Defining Custom Null Coalescing Operator for Swift

A custom operator can be defined for Swift, with the following considerations regarding the Swift language:

Swift and Nil

Swift supports a null concept through its Optional type (a Discriminated Union of sorts) which either holds a value of the underlying type or no value (indicated by nil) and must be explicitly defined as being optional:

var a : String?

A non-optional type can never be nil, or assigned nil. Therefore a custom infix binary NCO can be expected to take an optional as its first parameter.

Available Characters for Custom Operators in Swift

In Swift, custom operators can be comprised of the following characters only:

/ = - + * % < > ! & | ^ . ~

The Operator

Given the choice of available characters, ||| (three pipes, no spacing) isn't terrible (similar to the double pipe OR in Javascript which is used like a NCO):

Using the Operator

var a = String?
var z = a ||| "it's nil!"
println(z) //Output: it's nil!

Defining the Operator

operator infix ||| {}

@infix func ||| (left: T?, right: T) -> T  {
  if let l = left { return l }
  return right
}