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Flecha

2. Raw memory and the first unsafe program

The safe form should be your default:

safe.fl
i32 main() {
    i32 value = 42;
    reference := &value;
    stdoutln(*reference);
    return 0;
}

The smallest raw equivalent needs an explicit boundary:

unsafe.fl
i32 main() {
    i32 value = 42;
    unsafe {
        *i32 pointer = &raw value;
        stdoutln(*pointer);
    }
    return 0;
}

Both qualified programs print exactly 42 and return zero. The first lets the compiler track the borrow. The second asks the reviewer to prove that pointer is non-null, aligned, points to a live i32, has compatible access, and is not used after value dies. It is longer in proof even though it is similar in syntax.

Run the exact checked pair from the repository:

Terminal window
flecha check curriculum/fixtures/unsafe-first/safe.fl
flecha build curriculum/fixtures/unsafe-first/safe.fl -o build/safe-first
build/safe-first
flecha check curriculum/fixtures/unsafe-first/unsafe.fl
flecha build curriculum/fixtures/unsafe-first/unsafe.fl -o build/unsafe-first
build/unsafe-first
42
42

Raw new<T> also returns an allocation error and creates a cleanup obligation. Pair every successful allocation with exactly one free, including early returns. Prefer an owning safe structure when raw storage is not itself the API.

What to learn next

Section titled “What to learn next”