x.js

var observable = require("observable")
var foldp = require("signal/foldp")
var merge = require("signal/merge")
var transform = require("signal/transform")

var MAX = 100

// take an arbitrary source
var input = SomeSource()

// rewinder is a clock :3
var rewinder = observable(0)

var applyRewind = transform(rewinder, function (back) {
    return function (state) {
        return {
            history: history
            , rewind: back
            , current: history[back] || null
        }
    }
})

var catchHistory = transform(input, function (value) {
    return function (state) {
        var history = state.history
        history.unshift(value)
        if (history.length > MAX) {
            history.pop()
        }

        return {
            history: history
            , rewind: state.rewind
            , current: history[state.rewind]
        }
    }
})

var fns = merge([applyRewind, catchHistory])
var currentInputState = input()
var inputWithHistory = foldp(fns, function (state, fn) {
    return fn(state)
}, { history: [currentInputState], rewind: 0, current: currentInputState })

// rewind lol
rewinder(5)

// now when input updates it always shows the 5th most current value