feat: functions

[callumgran]

Allowing for simple functions would be greatly appreciated.

[LytixDev]

Suggestion for syntax. Using the func keyword as it is what POSIX sh uses now. Open for other keywords, or maybe even the omission of a keyword?

func foo(a, b, c) {
 ...
}

We should also support optional typing (see #33 and #32):

func foo(a: str, b: num, c: num) -> list {
 ...
}

We should also support default values.

func foo(a, b, c = 10) {
 ...
}

Default arguments can optionally be passed when calling the function. I think a regular argument must be passed, and not passing the correct amount of arguments should be an error.

As for functions themselves, I think it makes sense to make them a first-class type. This means we can use them as any other object.

As for calling functions, I see two realistic option here. The first option, which makes sense from a shell script perspective, would be to call it like any other shell command. The other option, perhaps more familiar to most, would be using parentheses.

func concat(a: str, b: str) { ... }

concat "Hello" "World"
# or
concat("Hello", "World")

I think there are good arguments for and against both options. Maybe we could support both? Or would that become confusing? we already support "methods" using foo.bar() syntax.

While on the topic, calling builtin functions (cd, history, etc.) should definitely be done using the standard shell way of doing it: cd ~/Pictures.

[LytixDev]

A basic function implementation has been added.

Example:

var fib_iterative = n {
    var a = 0
    var b = 1
    loop _ in 1..$n+1 {
        var c = $a + $b
        $a, $b = $b, $c
    }
    return $a
}

var fib_recursive = n {
    $n <= 1 && return $n
    return $fib_recursive($n - 1) + $fib_recursive($n - 2)
}

assert $fib_iterative(20) == 6_765
assert $fib_recursive(20) == 6_765