Control flow

if

if is a statement and an expression. As an expression it must have an else, and both branches must have the same type:

Str size(Int n) {
    return if (n < 10) { "small" } else if (n < 100) { "medium" } else { "large" };
}

Int main() {
    println(f"{size(3)} {size(42)} {size(500)}");
    return 0;
}

small medium large

Conditions must be Bool; there is no truthiness. && and || short-circuit.

match

match takes apart a sum type (Option, Result, or your own) and is an expression:

type Shape = Circle(Int) | Rect(Int) | Empty;

Int area(Shape s) {
    return match s {
        Circle(r) => 3 * r * r,
        Rect(side) => side * side,
        Empty => 0,
    };
}

Int main() {
    println(f"{area(Circle(2))} {area(Rect(3))} {area(Empty)}");
    return 0;
}

12 9 0

Each arm binds the variant’s payload. Arms can themselves branch, match again, or use else.

for

for walks a range or a list. The loop variable is a fresh binding on every iteration:

Int main() {
    for (Int i in 0..3) {
        println(f"row {i}");
    }
    List(Str) fruit = ["fig", "pear"];
    for (Str f in fruit) {
        println(f);
    }
    return 0;
}

row 0
row 1
row 2
fig
pear

a..b is half-open and a..=b includes b. break and continue work as in C.

A for loop cannot accumulate into an outer binding, because there is no assignment. To compute a result, use recursion or a linear builder passed through a recursive function:

Int sum_list(List(Int) xs, Int i, Int acc) {
    if (i >= xs.len()) { return acc; }
    return sum_list(xs, i + 1, acc + xs[i]);
}

Int main() {
    List(Int) xs = [4, 8, 15, 16, 23, 42];
    println(f"{sum_list(xs, 0, 0)} {xs.sum()}");
    return 0;
}

108 108

if-let and while-let

A pattern in a condition binds on success:

Int main() {
    Option(Int) found = Some(7);
    if (Some(v) = found) {
        println(f"found {v}");
    } else {
        println("nothing");
    }
    return 0;
}

found 7

while (Some(x) = next()) { ... } loops while the pattern matches.