Pattern Matching

Source: examples/pattern_matching.ion in the repo.

// Pattern matching showcase

// Option handling
fn find_user(name) {
    let db = #{alice: 1, bob: 2, charlie: 3};
    db.get(name)
}

match find_user("alice") {
    Some(id) => io::println(f"Found user with id: {id}"),
    None => io::println("User not found"),
}

// Result handling with ? operator
fn parse_config(input) {
    let port = input.to_int()?;
    if port < 1 || port > 65535 {
        Err("port out of range")
    } else {
        Ok(#{port: port, host: "localhost"})
    }
}

match parse_config("8080") {
    Ok(config) => io::println(f"Server at {config.host}:{config.port}"),
    Err(e) => io::println(f"Config error: {e}"),
}

// List pattern matching
fn describe(items) {
    match items {
        [] => "empty list",
        [x] => f"single item: {x}",
        [x, y] => f"pair: ({x}, {y})",
        [first, ...rest] => f"starts with {first}, {rest.len()} more items",
    }
}

io::println(describe([]));
io::println(describe([42]));
io::println(describe([1, 2]));
io::println(describe([10, 20, 30, 40]));

// Tuple matching
fn classify_point(point) {
    match point {
        (0, 0) => "origin",
        (x, 0) => f"on x-axis at {x}",
        (0, y) => f"on y-axis at {y}",
        (x, y) => f"at ({x}, {y})",
    }
}

io::println(classify_point((0, 0)));
io::println(classify_point((5, 0)));
io::println(classify_point((0, 3)));
io::println(classify_point((2, 7)));

// Guards
fn grade(score) {
    match score {
        s if s >= 90 => "A",
        s if s >= 80 => "B",
        s if s >= 70 => "C",
        s if s >= 60 => "D",
        _ => "F",
    }
}

for score in [95, 82, 73, 61, 45] {
    io::println(f"Score {score}: {grade(score)}");
}