Revised for loop protocol

16 January 2013

In Rust today, there is a special for syntax designed to support interruptible loops. Since we introduced it, this has proven to be a remarkable success. However, I think we can improve it very slightly.

Current for protocol

The current “for protocol” is best explained by giving an example of how to implement it for slices:

fn each<E>(v: &[E], f: &fn(&E) -> bool) {
    let mut i = 0;
    let n = v.len();
    while i < n {
        if !f(&v[i]) {
        i += 1

As you can see, the idea is that the last parameter to the each() method is a function of type &fn(&E) -> bool, which means that it is given a pointer to an element in the collection and it returns true or false. The return value indicates whether we should continue iterating.

A little known fact is that the for statement returns whatever the each() method returns. This means that each() methods typically have unit return type so that the Rust compiler doesn’t require a semicolon, which would be used to disregard the result of the for expression.


The biggest problem with this protocol is that it is not easily composable. In particular, imagine that I have a simple tree like this:

struct Tree<E> {
    elem: E,
    children: ~[Tree<E>]

Now let’s try to implement the pre-order traversal method for such a tree. You might think you could do it like this:

fn each<E>(t: &Tree<E>, f: &fn(&E) -> bool) {
    if !f(&t.elem) {
    for t.children.each |child| { each(child, f); }

While this will compile, it will not work as expected. For example, this program:

fn main() {
    let t = Tree {
        elem: 0,
        children: ~[
            Tree { elem: 1, children: ~[
                Tree { elem: 2, children: ~[] }
            ] },
            Tree { elem: 3, children: ~[] }

    for each(&t) |e| {
        io::println(fmt!("%d", *e));
        if *e == 1 { break; }

should print “0” and “1”, but it prints “0”, “1”, and “3”. The reason is that while each() does indeed return early when the iteration function returns false, it doesn’t abort the entire iteration, only the current subtree.

One way to fix this is to wrap the each() function with an inner each function that returns a bool to indicate whether execution should stop:

fn each1<E>(t: &Tree<E>, f: &fn(&E) -> bool) {
    each_inner(t, f);

    fn each_inner<E>(t: &Tree<E>, f: &fn(&E) -> bool) -> bool {
        if !f(&t.elem) {
            return false;

        for t.children.each |child| {
            if !each_inner(child, f) {
                return false;

        return true;

Making each() composable

I think that we should change the standard each signature to:

fn each<E>(c: &Coll<E>, f: &fn(&E) -> bool) -> bool

Here the return value of each is always a boolean, and it will be false if the last call to f() returned false, and true otherwise. This makes it easier to write composed each() methods. We would also adjust for statements so that they always return unit and do not return the result of each().

Under this definition, we could write the tree iterator as follows:

fn each2<E>(t: &Tree<E>, f: &fn(&E) -> bool) -> bool {
    f(&t.elem) && t.children.each(|c| each2(c, f))

This is clearly an improvement over each1()!