Iterator conveniences

This proposal complements the iterators strawman with an API of conveniences for constructing iterators.

Library

Iterator.create: function(trap: function() -> iterator) -> iterator
Iterator.for:    function(x: Object, trap: function() -> iterator) -> iterator
keys:            function(x: Object, own: boolean = false) -> iterator
values:          function(x: Object, own: boolean = false) -> iterator
items:           function(x: Object, own: boolean = false) -> iterator

Iterator.create: Creates a new iterator proxy using the given trap.
Iterator.for: Creates a new iterator proxy using the given trap for the iterate trap and otherwise behaving as a forwarding proxy.
keys: Creates a new iterator proxy that iterates over the property names of the object x. If own is provided and is a truthy value, then the iterator only produces the names of the object’s own properties.
values: Creates a new iterator proxy that iterates over the property values of the object x. If own is provided and is a truthy value, then the iterator only produces the values of the object’s own properties.
items: Creates a new iterator proxy that iterates over the properties of the object x, providing at each iteration a two-element array with the property name at index 0 and the property value at index 1. If own is provided and is a truthy value, then the iterator only produces the object’s own properties.

Sample implementation

This code works in the Firefox 4 betas, using the script type “application/javascript;version=1.8”.

// The module pattern; this ought to be a standard module.
var Iteration = (function() {
 
Iterator = {
    create: function(trap) {
        // Proxy.forwardingHandler is given an implementation on the harmony:proxies page.
        var handler = Object.create(Proxy.forwardingHandler({
                                        toString: function() { return "[object Iterator]" }
                                    }),
                                    { iterate: { value: trap } });
        return Proxy.create(handler);
    },
    for: function(obj, trap) {
        function g() {
            for (var x in obj) {
                yield x;
            }
        }
        // Proxy.forwardingHandler is given an implementation on the harmony:proxies page.
        var handler = Object.create(Proxy.forwardingHandler(obj),
                                    { iterate: { value: function() {
                                          return g();
                                      } } });
        return Proxy.create(handler);
    }
};
 
function values(obj) {
    function makeIterator() {
        for (x in obj) {
            yield obj[x];
        }
    }
    return Iterator.create(function() {
        return makeIterator();
    });
}
 
function keys(obj) {
    function makeIterator() {
        for (x in obj) {
            yield x;
        }
    }
    return Iterator.create(function() {
        return makeIterator();
    });
}
 
function items(obj) {
    function makeIterator() {
        for (x in obj) {
            yield [x, obj[x]];
        }
    }
    return Iterator.create(function() {
        return makeIterator();
    });
}
 
return {
    Iterator: Iterator,
    values: values,
    keys: keys,
    items: items
};
 
})();