Lazy wrap/value comonad from lodash/underscore

comonad_wrapper.js

// Helpers
//=================
var compose = function(f,g) {
  return function(x) {
    return f(g(x))
  }
}

var id = function(x) { return x }


// Wrapper
//=================

// make the type that holds our eventual computation f and our value x
var _ = function(x, f) {
  this.x = x;
  this.f = f;
}

// start up with x and the identity fn
_.from = function(x) {
  return new _(x, id);
}

// run the f on the x
_.prototype.extract = function() {
  return this.f(this.x);
}

// compose our previous f that drops out of the wrapper, then put it back in with from
// the eager version would just be _.from(f(this));
_.prototype.extend = function(f) {
  return new _(this.x, compose(f, compose(_.from, this.f)));
}

// in lodash these typically dispatch on w.x or just w depending on if it's wrapped or not
_.toUpper = function(w) { return w.x.toUpperCase() }
_.last = function(w) { return w.x[w.x.length-1] }

var result = _.from("I am not a holla back girl").extend(_.toUpper).extend(_.last).extract()

console.log('result', result);
// L