Closures: why are they so useful?

Question

As an OO developer, maybe I have difficulty seeing its value. What added value do they give? Do they fit in an OO world?

Answer 1

It's a pity, people no longer learn Smalltalk in edu; there, closures are used for control structures, callbacks, collection enumeration, exception handling and more. For a nice little example, here is a worker queue action handler thread (in Smalltalk):

|actionQueue|

actionQueue := SharedQueue new.
[
    [
        |a|

        a := actionQueue next.
        a value.
    ] loop
] fork.

actionQueue add: [ Stdout show: 1000 factorial ].

and, for those who cannot read Smalltalk, the same in JavaScript syntax:

var actionQueue;

actionQueue = new SharedQueue;
function () {
    for (;;) {
        var a;

        a = actionQueue.next();
        a();
    };
}.fork();

actionQueue.add( function () { Stdout.show( 1000.factorial()); });

(well, as you see: syntax helps in reading the code)

Edit: notice how actionQueue is referenced from inside the blocks, which works even for the forked thread-block. Thats what makes closures so easy to use.