It is common for a function to accept or return either an array or an object implementing Traversable to be used with foreach. However, because array is a primitive type and Traversable is an interface, there currently is no way to use a type declaration on a parameter or return type to indicate that the value is iterable.

This PR proposes a new iterable pseduo-type. This type is analogous to callable, accepting multiple types instead of one single type.

iterable accepts any array or object implementing Traversable. Both of these types are iterable using foreach and can be used with yield from within a generator.

iterable can be used as a parameter type to indicate that a function requires a set of values, but does not care about the form of the value set (array, Iterator, Generator, etc.) since it will be used with foreach. If a value is not an array or instance of Traversable, a TypeError will be thrown.

function foo(iterable $iterable) {
    foreach ($iterable as $value) {
        // ...
    }
}

iterable can also be used as a return type to indicate a function will return an iterable value. If the returned value is not an array or instance of Traversable, a TypeError will be thrown.

function bar(): iterable {
    return [1, 2, 3];
}

Parameters declared as iterable may use null or an array as a default value.

function foo(iterable $iterable = []) {
    // ...
}

Functions declaring iterable as a return type may also be generators.

function gen(): iterable {
    yield 1;
    yield 2;
    yield 3;
}

This proposal also adds a function is_iterable() that returns a boolean: true if a value is iterable and will be accepted by the iterable pseudo-type, false for other values.

var_dump(is_iterable([1, 2, 3])); // bool(true)
var_dump(is_iterable(new ArrayIterator([1, 2, 3]))); // bool(true)
var_dump(is_iterable((function () { yield 1; })())); // bool(true)
var_dump(is_iterable(1)); // bool(false)
var_dump(is_iterable(new stdClass())); // bool(false)

RFC: https://wiki.php.net/rfc/iterable

so... what about strings? are they iterable? if no, why?
@trowski @nikic

@mokeev1995 PHP cannot iterate strings by characters anyway (only by bytes which makes little sence with multibyte encodings). And you cannot pass a sting to foreach.

Strings cannot become iterables, the fact that you are able to iterate them byte by byte does not qualify them for this pseudo compound type. Especially because you cannot use them in a foreach and the iterable is all about that; you cannot use a Generator in a for. 😉

ok, thanks)

I'm afraid this would be a serious BC break in minor version. It would be perfectly fine for 7.0 (at least as part of reserved types RFC), but since it's already out, the only reasonable version to target is 8.0.
See GH results for:
class Iterable
interface iterable

@Majkl578 From the first page of your first link, there are 10 entries. 8 have extra words (e.g. class IterableFilterIterator) and so wouldn't be affected. 1 of the others is in a namespace and so wouldn't be affected. Which leaves 1 example.

I'm afraid this would be a serious BC break in minor version.

Most people don't consider adding new classes to be a BC break. It's only changing behaviour of existing classes/functions, or removing stuff that is a 'real' BC break.

Which leaves 1 example.

Which is still more than 0. And we are not even considering all the private code out there.

1 of the others is in a namespace and so wouldn't be affected

Not true, real-world applications typically import classes with use which implies breaking the usage of such classes/interfaces.

Most people don't consider adding new classes to be a BC break.

This proposal is not to add new class, but a pseudotype that would break all code using typehint for interface/class named Iterable. And that is a real BC break.

@Majkl578 Behavioral changes are considered serious BC breaks, while this would only cause a BC break due to naming, which can easily be fixed with simple find-and-replace. Yes, it is technically a BC break, but one that can be fixed in a matter of minutes without actually examining any code.

It would be pretty cool if, somehow (I have no idea if this is possible), after this pseudo type has been added, we could start using the array_* functions indiscriminately across all the iterable types.
Is there any discussion going on regarding this that I could read/join?