unpromisify.js

// Convert a promise-oriented API into a callback-oriented API

function identity(x) { return x; }

function slice(arrayish, arg1, arg2) {
  return Array.prototype.slice.call(arrayish, arg1, arg2);
}

// Makes a transformer from promise-returning functions to
// callback-accepting functions.
function to_cb_accepting_fn(transform) {
  transform = transform || identity;
  return function(proc) {

    function f(self) {
      return function(/* args..., callback*/) {
        var callback = arguments[arguments.length - 1];
        proc.apply(self, slice(arguments, 0, -1))
          .then(function(value) {
            callback(null, transform(value));
          }, callback);
      };
    }

    var func = f(proc);
    func.as_property_of = f;

    return func;
  };
}

// Makes a transformer that takes functions returning values to
// functions returning transformed values. Used when you want to
// transform return values into something unpromisified.
function to_plain_func(transform) {
  // NB in principle, if there's no transform, our function ought to
  // just return the procedure itself; however, because of the way
  // I've done methods (`as_property_of`), I need to cons a fresh
  // closure to return.
  transform = transform || identity;
  return function(proc) {

    function f(self) {
      return function(/* , args... */) {
        return transform(proc.apply(self, arguments));
      };
    }

    var func = f(proc);
    func.as_property_of = f;

    return func;
  };
}

// Makes a transformer from objects to objects; the transformed
// objects are built according to the spec given, which puts
// transformers againsts property names.
function object(spec) {
  var keys = Object.keys(spec);
  var len = keys.length;
  return function(obj) {
    var res = {_original: obj}; // original included for debugging
    for (var i=0; i < len; i++) {
      var k = keys[i];
      res[k] = spec[k](obj[k]).as_property_of(obj);
    }
    return res;
  };
}

module.exports.func = to_cb_accepting_fn;
module.exports.plain_func = to_plain_func;
module.exports.object = object;