Optional assignment operator implementation. `foo ?= bar` will evaluate and assign bar to foo if and only if foo is nil. Equivalent to `||=` in Ruby and other languages.

optional-assignment.swift

infix operator ?= {
    associativity right
    precedence 90
    assignment
}

func ?=<T>(inout variable: T?, expr: @autoclosure () -> T) {
    if variable == nil {
        variable = expr()
    }
}