notimplemented

n.js

var _slice = Array.prototype.slice;
 
function Maybe (fn) {
  return function () {
    return fn !== void 0 ? fn.apply(this, arguments) : 'Notimplemented'
  }
}
 
function N (f) {
  var name = f.name;
  return function (o) {
    return f.call(this, o) || Maybe(o[name]).call(o,this)
  }
}
 
function type1 (v) {
  this.v = v;
}
 
type1.prototype.equal = N(function equal (o) {
  if (o instanceof type1) {
    return this.v === o.v 
  }
})
 
function type2 (v) {
 this.v = v; 
}
 
type2.prototype.equal = function equal (o) {
  if (o instanceof type2) {
    return this.v === o.v
  } else if (o instanceof type1) {
    return this.v === o.v 
  }
}
 
a = new type1(1)
b = new type2(1)

a.equal(b) /=> true / "Not implemented" depending on b has a method equal
a.equal(a) /+> true
b.equal(b) /=> true
b.equal(a) /=> true