A shot at an unsafe loop construct Native Elm library. Not necessarily intended to be part of the standard library.

Loop.js

Mayer.John.Native.Loop = function(elm) {

    // Various boilerplate stuff
    'use strict';
    if (elm.Native.Loop) return elm.Native.Loop;
    var Signal = Elm.Signal(elm);

    // This is the unsafe loop construct
    // loop : (b -> a) -> (Signal a -> Signal b) -> b -> Signal b
    var loop = function(convert, transform, initial) {

        // Create the loopback Signal with the initial value
        var loopback = Signal.constant(initial);

        // Lift the convert pure function, and apply to the loopback
        var converted = A2(Signal.lift, convert, loopback);

        // Transform the converted signal using the Signal node
        // This is the eventual
        var output = transform(converted);

        // KEY STEP
        // Whenever this output signal changes, send a message to the dispatcher
        // that the loopback should update in the next "round".
        output.recv(function(val) {
            elm.notify(loopback.id, value);
        })

        // Give the output signal back to the user
        return output;

    };

    return Mayer.John.Native.Loop = { loop: F3(loop) };

}