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Protocol based polymorphism for javascript.
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examples Implement some code examples.
tests Make difference between Object and Default


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Protocol JS library is inspired by idea of clojure protocols. Protocols provide a powerful way for decoupling abstraction interface definition from an actual implementation per type, without risks of interference with other libraries.

There are several motivations for protocols:

  • Provide a high-performance, dynamic polymorphism construct as an alternative to existing object inheritance that does not provides any mechanics for guarding against name conflicts.
  • Provide the best parts of interfaces:
    • specification only, no implementation
    • a single type can implement multiple protocols
  • Protocols allow independent extension of the set of types, protocols, and implementations of protocols on types, by different parties.


A protocol is a named set of function signatures:

var protocol = require('protocol/core').protocol

var Sequential = protocol({
  first: ('Returns first item of this sequence', [ protocol ]),
  rest: ('Returns sequence of items after the first', [ protocol ]),
  join: ('Returns sequence of items where head is first, and this is rest', [ Object, protocol ])
  • No implementations are provided
  • Docs can be optionally specified for the protocol and the functions, via elegant JS hack.
  • The above yields a set of polymorphic functions and a protocol object
  • The resulting functions dispatch on the type of their protocol argument, and thus must have it in the list of arguments.

protocol will generate an interface containing a corresponding functions. returned interface may be used to extend data types with it's implementations:

Sequential(Array, {
  first: function(array) { return array[0] || null },
  rest: function(array) { return, 1) },
  join: function(item, array) {
    return[ item ], array)

Once protocol is implemented for a given type it can be used with a given data types:

Sequential.first([ 1, 2, 3 ]) // => 1[ 1, 2, 3 ])  // => [ 2, 3 ]

Sequential.first('hello')     // TypeError: Protocol not implemented: first

Protocol may be implemented for any other data types by any other party:

Sequential(String, {
  first: function(string) { return string[0] || null },
  rest: function(string) { return, 1) },
  join: function(item, string) { return item + string }

Sequential.first('hello')     // => 'h'

Protocol implementation may be provided to all the data types by ommiting type argument:

  first: function(_) { return _ },
  rest: function(_) { return null }

Sequential.head(5)          // => 5
Sequential.tail(3)          // => null

Since protocol implementations are decoupled from the actual protocol definition there maybe multiple implementations, but user will be in charge of deciding which one to pull in.

Argument pattern based dispatch

This library previously was doing argument pattern based method dispatch. If you are looking into something more in that line, check out dispatcher library that was forked from protocol to explore that direction.

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