using-optional-labeled-function-arguments.md

Using Optional Labeled Function Arguments

If you are constructing a function that takes some arguments, but one of those arguments has a reasonable default value, then you can use an optional labeled argument. Labeled arguments are those arguments prefixed with a ~. If you give the argument a default value, then it becomes optional.

let thing = (~a=1, b: int, c: int) => {
  a + b + c;
};

In this case ~a is a labeled argument. It is also optional and will default to 1 if not specified. The other two arguments, b and c, are positional arguments and thus required in order for the function to evaluate.

Here are two ways of using this function either by specifying ~a or excluding it so that it defaults to 1.

thing(~a=2, 1, 1)
|> string_of_int
|> print_endline /* 4 */

thing(1, 1)
|> string_of_int
|> print_endline /* 3 */

See more details here.