While and Loop

Move offers two constructs for looping: while and loop.

while loops

The while construct repeats the body (an expression of type unit) until the condition (an expression of type bool) evaluates to false.

Here is an example of simple while loop that computes the sum of the numbers from 1 to n:

fun sum(n: u64): u64 {
    let sum = 0;
    let i = 1;
    while (i <= n) {
        sum = sum + i;
        i = i + 1
    };

    sum
}

Infinite loops are allowed:

fun foo() {
    while (true) { }
}

break

The break expression can be used to exit a loop before the condition evaluates to false. For example, this loop uses break to find the smallest factor of n that's greater than 1:

fun smallest_factor(n: u64): u64 {
    // assuming the input is not 0 or 1
    let i = 2;
    while (i <= n) {
        if (n % i == 0) break;
        i = i + 1
    };

    i
}

The break expression cannot be used outside of a loop.

continue

The continue expression skips the rest of the loop and continues to the next iteration. This loop uses continue to compute the sum of 1, 2, ..., n, except when the number is divisible by 10:

fun sum_intermediate(n: u64): u64 {
    let sum = 0;
    let i = 0;
    while (i < n) {
        i = i + 1;
        if (i % 10 == 0) continue;
        sum = sum + i;
    };

    sum
}

The continue expression cannot be used outside of a loop.

The type of break and continue

break and continue, much like return and abort, can have any type. The following examples illustrate where this flexible typing can be helpful:

fun pop_smallest_while_not_equal(
    v1: vector<u64>,
    v2: vector<u64>,
): vector<u64> {
    let result = vector::empty();
    while (!vector::is_empty(&v1) && !vector::is_empty(&v2)) {
        let u1 = *vector::borrow(&v1, vector::length(&v1) - 1);
        let u2 = *vector::borrow(&v2, vector::length(&v2) - 1);
        let popped =
            if (u1 < u2) vector::pop_back(&mut v1)
            else if (u2 < u1) vector::pop_back(&mut v2)
            else break; // Here, `break` has type `u64`
        vector::push_back(&mut result, popped);
    };

    result
}

fun pick(
    indexes: vector<u64>,
    v1: &vector<address>,
    v2: &vector<address>
): vector<address> {
    let len1 = vector::length(v1);
    let len2 = vector::length(v2);
    let result = vector::empty();
    while (!vector::is_empty(&indexes)) {
        let index = vector::pop_back(&mut indexes);
        let chosen_vector =
            if (index < len1) v1
            else if (index < len2) v2
            else continue; // Here, `continue` has type `&vector<address>`
        vector::push_back(&mut result, *vector::borrow(chosen_vector, index))
    };

    result
}

The loop expression

The loop expression repeats the loop body (an expression with type ()) until it hits a break

Without a break, the loop will continue forever

fun foo() {
    let i = 0;
    loop { i = i + 1 }
}

Here is an example that uses loop to write the sum function:

fun sum(n: u64): u64 {
    let sum = 0;
    let i = 0;
    loop {
        i = i + 1;
        if (i > n) break;
        sum = sum + i
    };

    sum
}

As you might expect, continue can also be used inside a loop. Here is sum_intermediate from above rewritten using loop instead of while

fun sum_intermediate(n: u64): u64 {
    let sum = 0;
    let i = 0;
    loop {
        i = i + 1;
        if (i % 10 == 0) continue;
        if (i > n) break;
        sum = sum + i
    };

    sum
}

The type of while and loop

Move loops are typed expressions. A while expression always has type ().

let () = while (i < 10) { i = i + 1 };

If a loop contains a break, the expression has type unit ()

(loop { if (i < 10) i = i + 1 else break }: ());
let () = loop { if (i < 10) i = i + 1 else break };

If loop does not have a break, loop can have any type much like return, abort, break, and continue.

(loop (): u64);
(loop (): address);
(loop (): &vector<vector<u8>>);