Const Declarations

View source on GitHub

const_declarations.fl

// Build: build/flecha build examples/const_declarations.fl -o build/const_example
// Run:   build/const_example
// Prints the values described below and exits with 42.

const i32 add(const i32 input) {
    // Parameters arrive initialized; const prevents reassignment here.
    const i32 increment = 2;
    return input + increment;
}

const string echo(const string text) {
    // Owning strings keep their existing copy semantics.
    return text;
}

const i32 zero() {
    // none takes the return type's zero value.
    return none;
}

i32 main() {
    // Const initialization can call a function at runtime.
    const i32 answer = add(40);
    stdoutln(answer);                         // 42

    // Reading a const value does not make its copy const.
    copy := answer;
    copy = copy + 1;
    stdoutln(copy);                           // 43

    // A const return type does not make an inferred binding const either.
    returned := add(40);
    returned = returned + 2;
    stdoutln(returned);                       // 44

    // Both cast spellings accept const-qualified types.
    const i64 wide = answer as const i64;
    const u8 byte = answer.cast<const u8>;
    const f64 fraction = 1.5;
    const bool same = wide == (byte as i64);
    const char letter = 'F';
    stdoutln(wide);                           // 42
    stdoutln(byte);                           // 42
    stdoutln(fraction);                       // 1.5
    stdoutln(same);                           // true
    stdoutln(letter);                         // F

    // A nested mutable binding can shadow a const binding.
    if same {
        i32 answer = 0;
        answer = answer + 1;
        stdoutln(answer);                     // 1
    }
    stdoutln(answer);                         // 42: the outer binding

    // Explicit zero initialization also works with const declarations.
    const i32 empty = none;
    const bool disabled = none;
    const string emptyText = none;
    stdoutln(empty);                          // 0
    stdoutln(disabled);                       // false
    stdoutln(emptyText.len());                // 0

    // none also works in assignment, function arguments, and returns.
    i32 reset = 1;
    reset = none;
    stdoutln(reset + add(none) + zero());      // 2

    // Copying a const owning string produces an independent mutable owner.
    const string original = echo("Flecha");
    mutableCopy := original;
    mutableCopy = "Changed";
    const stringv view = original.view();
    stdoutln(view);                           // Flecha
    stdoutln(mutableCopy);                    // Changed

    // constval is an ordinary identifier, not a qualifier or keyword.
    constval := answer;
    constval = constval + 1;
    stdoutln(constval);                       // 43

    return answer;
}