How to use Swift @autoclosure

Question

I noticed when writing an assert in Swift that the first value is typed as

@autoclosure() -> Bool

with an overloaded method

Answer 1

Consider a function that takes one argument, a simple closure that takes no argument:

func f(pred: () -> Bool) {
    if pred() {
        print("It's true")
    }
}

To call this function, we have to pass in a closure

f(pred: {2 > 1})
// "It's true"

If we omit the braces, we are passing in an expression and that's an error:

f(pred: 2 > 1)
// error: '>' produces 'Bool', not the expected contextual result type '() -> Bool'

@autoclosure creates an automatic closure around the expression. So when the caller writes an expression like 2 > 1, it's automatically wrapped into a closure to become {2 > 1} before it is passed to f. So if we apply this to the function f:

func f(pred: @autoclosure () -> Bool) {
    if pred() {
        print("It's true")
    }
}

f(pred: 2 > 1)
// It's true

So it works with just an expression without the need to wrap it in a closure.