Trying to understand by from the examples

[avillega]

Hi! thanks a lot for the effort you are putting in this language, it looks fantastic!
I was going through the language tour and kept seeing a patter where the name of the argument of a functions is preceded by a _ or by by I couldn't find what does this "labels" mean. By how it is used, it seems to be a way of referring to that argument from the caller, but I am not sure.
a full example, from the language tour looks like:

fun scale(_ v: Vector2, by factor: Vector2) -> Vector2 {
  (x: v.x * factor.x, y: v.y * factor.y)
}

Note the by factor: ... and the _ v: ...

[kyouko-taiga]

Those are argument labels and they are part of a function's (or subscript's) API. When you declare a function, for each parameter you get to specify an argument label, which must be used at call site, and a parameter name, which is used in the body of the function. Those can be distinct:

fun foo(label name: Int) {
  // here we must use `name`
}

fun bar() {
  foo(label: 1) // here we must use `label`
}

You can use _ to specify that there is no label at call site, as in the example you presented. Finally, as a shorthand, you can omit the label in the declaration, in which case it gets the same value as the parameter name. In other words, fun f(x: Int) is short for fun(x x: Int).

All of these rules have been shamelessly copied from Swift.