Labeled and Optional Arguments

Functions can have labeled and also optional arguments. Labeled arguments need not be specified in order, and optional arguments don’t need to be specified at all. When declaring a function you must specify the labeled and optional arguments before any non labeled arguments.

let f = |#lbl1, #lbl2, arg| ...

In this case lbl1 and 2 are not optional, but are labeled. You can call f with either labeled argument in either order. e.g. f(#lbl2, #lbl1, a).

let f = |#opt = null, a| ...

opt need not be specifed when f is called, if it isn’t specified then it will be null. e.g. f(2) is a valid way to call f. You can also apply type constraints to labeled and optional arguments.

let f = |#opt: [i64, null] = null, a| ..

Specifies that opt can be either an i64 or null and by default it is null. A default is checked where the function is defined: it must fit the argument’s type, and when that type is a declared type variable, the variable’s constraint. So 'a: [Int, Float] |#start: 'a = 0.0, x: 'a| is fine; a call that omits start takes 'a from the default, and a call that passes #start: 1 takes it from the call. The compiler implements subtyping for functions with optional arguments. For example if you write a function that takes a function with a labeled argument foo, you can pass any function that has a labeled argument foo, even if it also has other optional arguments. The non optional and non labeled arguments must match, of course. For example,

let f = |g: fn(#foo: i64, x: i64) -> i64, x: i64| g(#foo: x, x);
let g = |#foo:i64, #bar: i64 = 0, x: i64| foo + bar + x;
// valid call
f(g, 42)

outputs

$ graphix test.gx
84