swift - Default argument not permitted in a tuple type when defining function type

Question

I’ve got a function like this:

func next(step: Int = 1){
    //....
}

And now I would like to define a type alias so that I can easily pass around this function.

However I am not able to find a way to declare that the first argument has a default value. I tried things like this

typealias ActionNext = (step: Int = default) -> ()

var nextFunc: ActionNext = next

but they all give me a error message like

Default argument not permitted in a tuple type

Is there any way to define a type for this kind of function?

Answer 1

No, that is not possible

Explanation:

A type alias declaration introduces a named alias of an existing type into your program.

(step: Int = 1) -> () is not a proper type. A type is for example (step: Int) -> (), a default value is not allowed there.

If you write

typealias ActionNext = (Int) -> ()

var nextFunc: ActionNext = next

It works. Or even when you write (step: Int) -> ()

But I assume what you want to achieve is being able to call nextFunc() omitting the parameter and using its default value. That is not possible. To understand why, you can follow the Type Alias Declaration grammar - in the type you can not specify default values.