amb in javascript

gistfile1.js

// following https://gist.github.com/911094

function amb_run(program, failure_value) {
    var index = 0, steps = [];
    while (true) try {
        return program(amb, fail);
    } catch(ex) {
        if (ex !== fail) throw ex;
        for (var i = steps.length; --i >= 0;) {
            var a = steps[i];
            if (++a[0] < a[1]) break;
        }
        if (i < 0) return failure_value;
        index = 0;
        steps.length = i + 1;
    }
    function amb(values) {
        if (values.length == 0) fail();
        if (index == steps.length)
            steps[index] = [ 0 ];
        steps[index][1] = values.length;
        return values[steps[index++][0]];
    }
    function fail() { throw fail }
}