2. Control flow and compound data

Use if for decisions, while when continuation is state-dependent, and for (_, value) := range values to traverse a sequence. Flecha evaluates conditions as booleans; it does not treat arbitrary integers as truth values.

Fixed arrays own inline elements. Checked slices borrow a range and retain its length for bounds checks. string owns text while stringv is a borrowed view. Structures group fields and lifecycle behavior; enums define a closed family of named values. Choose the smallest representation that describes the lifetime you need, then convert explicitly at the boundary.

Work through fixed arrays, checked slices, strings, structures, and enums. Those references are contracts, not extra curriculum steps.

Tested example

Sort a fixed array with bounded loops, mutation through a checked reference, and visible output.

File: curriculum/main-evidence/single/fixed_arrays.fl

// Build: build/flecha build examples/fixed_arrays.fl -o build/fixed_arrays
// Prints 1 through 5, each on its own line, and exits with 0.

void sort(&[5]i32 values) {
    usize pass = 0;
    while pass < (*values).len() {
        usize index = 0;
        while index + 1 < (*values).len() {
            if (*values)[index] > (*values)[index + 1] {
                i32 temporary = (*values)[index];
                (*values)[index] = (*values)[index + 1];
                (*values)[index + 1] = temporary;
            }
            index = index + 1;
        }
        pass = pass + 1;
    }
}

i32 main() {
    [5]i32 original = [5, 1, 4, 2, 3];
    sorted := original; // Copies the elements into independent inline storage.
    sort(&sorted);
    if original[0] != 5 { return 1; }

    usize index = 0;
    while index < sorted.len() {
        stdoutln(sorted[index]);
        index = index + 1;
    }

    [4]u8 zeros = none;
    [4]u8 repeated = [7; 4];
    const [4]u8 fixed = repeated;
    [0]u8 empty = [];
    if zeros[0] != 0 or fixed[3] != 7 or empty.len() != 0 { return 2; }

    [2][3]u8 grid = [[1, 2, 3], [4, 5, 6]];
    cell := &grid[1][2];
    *cell = 42;
    // cell's last use has passed; direct access is permitted again.
    if grid[1][2] != 42 { return 3; }
    return 0;
}

The block above is the complete file executed by the commands below.

The array owns five integers at a fixed size. sort receives a checked mutable reference, and both loops derive their limits from that known length instead of using an unchecked pointer.

Commands

flecha check curriculum/main-evidence/single/fixed_arrays.fl
flecha build curriculum/main-evidence/single/fixed_arrays.fl -o build/fixed_arrays
build/fixed_arrays

Expected output

1
2
3
4
5
exit status: 0

Related reference: /examples/fixed_arrays/.

2. Control flow and compound data

Tested example

Commands

Expected output

What to learn next