x.js

// map:: (A -> B) -> Signal<A> -> Signal<B>
function map(lambda) { return function duplex(source) {
    return function signal(next, end) {
        signal(function(value) {
            var sink = next(value)
            return sink ? rest(duplex, sink) : null
        }, end)
    }
} }

// rest :: (Signal<A> -> Signal<B>) -> Sink<B> -> Sink<A>
function rest(duplex, sink) {
    return function continuable(signal) { 
        sink(duplex(signal)) 
    }
}