New for loops in Rust 2012-04-05

How to handle iteration in Rust has been a hotly debated topic for many months. We have several loop constructs but generally prefer to loop using higher order functions. A recent change by marijn makes for aware of how to iterate using higher-order functions and stack-based lambdas, while still allowing loop control flow using break, cont and even ret.

The bottom line is that you can now write:

for vec::each(v) {|elt|
    alt elt {
      some(143) { cont; }
      some(val) { io::println(#fmt("%?", val)) }
      none { ret "done"; }

and it will behave like you expect: cont proceeds to the next iteration, break ends the loop, and ret returns from the outer function scope.

If, instead of a vector, you had a list to iterate over, then you would write something like for list::each(l) {.... Whereas previously for only worked for specific builtin types (vec and str), the new for will work on any higher-order function with the appropriate signature (in this case vec::each has the right signature).

How does it work?

The for keyword is now followed by a call to a function where the last paramater is a lambda block, using the lambda block call sugar. As a refresher, Rust permits some syntactic sugar to make higher order functions look more like control structures so each(v) {|i| ... } is the sugared form of each(v, {|i| ... }).

To be used in a for loop the lambda block must have a bool return type which is used to indicate whether the loop should continue (true) or break (false).

The following is a typical higher-order each that breaks early by returning false:

each(v) {|elt| if elt == 0 { true } else { false } }

The for loop syntax adds even more sugar to that though and will let you write cont and break which behave as though the lambda block returned true or false. This is equivalent to the above example:

for each(v) {|elt| if elt == 0 { cont; } else { break; } }

In the absense of a value to return, for loops will modify the the lambda block as if it resulted in true (or cont):

for each(v) {|elt|
    // implicit `cont`

But that’s not all!

The new for syntax even allows you to write ret statements that return not from the lambda but from the outer function just like you expect from any other loops. It does this by implicitly stashing the return value into a captured stack variable, causing the for loop to break, then returning the stashed value. Writing

for each(v) {|elt|
    if elt == 0 { ret "foo"; }

behaves (more or less) as if you had written

let retval = none;
for each(v) {|elt|
    if elt == 0 { retval = "foo"; break }
if retval { ret retval.get(); }

As an example of how to write functions that are compatible with for here is how vec::each is currently implemented:

fn each<T>(v: [const T], f: fn(T) -> bool) unsafe {
    let mut n = len(v);
    let mut p = ptr::offset(unsafe::to_ptr(v), 0u);
    while n > 0u {
        if !f(*p) { break; }
        p = ptr::offset(p, 1u);
        n -= 1u;

The old form of for is deprecated and will likely be removed from the language.