poly.js

var poly = require('poly')

// The Haskell Eq Typeclass
var Eq = poly({
  eq: function(a, b){ return !Eq.notEq(a, b) },
  notEq: function(a, b){ return !Eq.eq(a, b) }
})


// tag the prototype with a unique value 
// that poly can dispatch against later
poly.tag(String.prototype)

Eq.implement(String.prototype, { 
  eq: function(a, b) { return a === b }
})

Eq.eq("a", "b")    // false
Eq.notEq("a", "b") // true

poly.tag(Object.prototype)

Eq.implement(Object.prototype, { 
  eq: function(a, b){ 
    var ks = Object.keys(a).concat(Object.keys(b))

    return ks.every(function(key){
      return eq(a[key], b[key])
    })
  }
})

Eq.eq({ "x": "foo"}, { "x": "bar" })    // false
Eq.notEq({ "x": "foo"}, { "x": "bar" }) // true

// poly tag twice is a no-op 
poly.tag(Object.prototype)

Eq.eq({ "x": "foo"}, { "x": "bar" })    // false
Eq.notEq({ "x": "foo"}, { "x": "bar" }) // true

// ad-hoc typing can happen
var primitive = poly.type();
poly.tag(String.prototype, primitive)
poly.tag(Number.prototype, primitive)
poly.tag(Boolean.prototype, primitive)

Eq.implement(String.prototype, { 
  eq: function(a, b) { return a === b }
})

Eq.eq(1, "f") // false
Eq.eq(1, 1)   // true