Swift closure type inference

Question

Why is it that this works:

var numbers = [20, 19, 7, 12]
numbers.map({
(number:Int)->Int in
if number % 2 == 0 {return number * 3}
else {return 0}
})

but not this:

var numbers = [20, 19, 7, 12]
numbers.map({
(number) in
if number % 2 == 0 {return number * 3}
else {return 0}
})

which gives the error message: "Cannot convert the expression's type 'Array<U>' to type 'U'"?

Apple's documentation says, "It is always possible to infer parameter types and return type when passing a closure to a function as an inline closure expression."

Answer 1

I can whittle your erroring example down to the simplest version:

let x = numbers.map({
    (number) in
    return number
    })

Note that you're not actually using an inline closure expression. An inline version of what you're trying to accomplish would be:

numbers.map({$0 % 2 == 0 ? $0 * 3 : $0})