Promises: Repeat operation until it succeeds?

Question

I want to perform an operation repeatedly, with an increasing timeout between each operation, until it succeeds or a certain amount of time elapses. How do I structure this

Answer 1

I think you can't do it on promise level - a promise isn't an operation, but is just a value that's going to arrive in the future, so you can't define a function typed Promise -> Promise that will achieve it.

You'd need to go one level down and define a function typed Operation -> Promise where Operation is itself typed () -> Promise. I assume the operation doesn't take any parameters - you can partially-apply them beforehand.

Here's a recursive approach that doubles the timeout on every run:

function RepeatUntilSuccess(operation, timeout) {
    var deferred = Q.defer();
    operation().then(function success(value) {
        deferred.resolve(value);
    }, function error(reason) {
        Q.delay(timeout
        .then(function() {
            return RepeatUntilSuccess(operation, timeout*2);
        }).done(function(value) {
            deferred.resolve(value);
        });
    });
    return deferred.promise;
}

Demo: http://jsfiddle.net/0dmzt53p/