03 · Functions

Functions are declared with fn, take typed parameters, and return a typed value. A function with no explicit return type returns () (unit). Effects propagate automatically — if a function calls println!, the compiler infers !io and requires the caller to also carry !io.

functions.cssl

// Pure function — no effects. Safe to call from anywhere.
fn add(a: i32, b: i32) -> i32 {
    a + b
}

// Returns the larger of two floats.
fn max_f32(a: f32, b: f32) -> f32 {
    if a >= b { a } else { b }
}

// Prints a greeting — carries !io because of println!.
fn greet(name: str) !io {
    println!("Hello, {}!", name);
}

fn main() !io {
    let sum = add(2, 3);
    println!("2 + 3 = {}", sum);

    let m = max_f32(1.5, 2.7);
    println!("max   = {}", m);

    greet("Sigil");
}

Output

2 + 3 = 5
max   = 2.7
Hello, Sigil!

What the compiler sees

add and max_f32 are provably pure — calling them from a !gpu shader or a @verify-annotated function is legal. greet is !io; calling it from a pure context is a compile error. The last expression in a block is the implicit return value — no return keyword required.

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Next → 04 · Structs