02 · Variables and Types

Sigil bindings are immutable by default; let mut opts into mutability. Every binding has an explicit type annotation or one that the compiler infers — there is no var or auto that hides the type. The four primitive scalar types are i32, f32, bool, and str.

variables.cssl

fn main() !io {
    let x: i32  = 42;
    let y: f32  = 3.14;
    let flag: bool = true;
    let name: str  = "Sigil";

    // Mutable binding — must be declared up front.
    let mut count: i32 = 0;
    count += 1;

    println!("x     = {}", x);
    println!("y     = {}", y);
    println!("flag  = {}", flag);
    println!("name  = {}", name);
    println!("count = {}", count);
}

Output

x     = 42
y     = 3.14
flag  = true
name  = Sigil
count = 1

What the compiler sees

Attempting to reassign an immutable binding is a compile error, not a runtime panic. The compiler also tracks whether a mut binding is actually mutated — unused mutability is a warning. Type annotations can be omitted when the initializer makes the type unambiguous (e.g., let x = 42; infers i32).

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