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2.0.1 • Public • Published

Protocols Travis npm version license

protocols is a JavaScript library for defining and implementing typeclass-like protocols that support dispatch across multiple arguments. See also: Clojure protocols but on any number of types across all arguments of the individual functions.

protocols is built on top of genfun, a fast, prototype-based multimethod library, but this is mostly all behind the scenes.

On top of providing a nice, clear interface for defining interfaces, this module makes the best effort to verify both protocols and implementations and tries to provide clear, useful errors for common mistakes.


$ npm install @zkat/protocols

Table of Contents


protocol(<types>?, <spec>)

Defines a new protocol on across arguments of types defined by <types>, which will expect implementations for the functions specified in <spec>.

If <types> is missing, it will be treated the same as if it were an empty array.

The types in <spec> must map, by string name, to the type names specified in <types>, or be an empty array if <types> is omitted. The types in <spec> will then be used to map between method implementations for the individual functions, and the provided types in the impl.

const Eq = protocol(['a', 'b'], {
  eq: ['a', 'b']

proto(<target>?, <types>?, <implementations>)

Adds a new implementation to the given proto across <types>.

<implementations> must be an object with functions matching the protocol's API. The types in <types> will be used for defining specific methods using the function as the body.

Protocol implementations must include either <target>, <types>, or both:

  • If only <target> is present, implementations will be defined the same as "traditional" methods -- that is, the definitions in <implementations> will add function properties directly to <target>.

  • If only <types> is present, the protocol will keep all protocol functions as "static" methods on the protocol itself.

  • If both are specified, protocol implementations will add methods to the <target>, and define multimethods using <types>.

If a protocol is derivable -- that is, all its functions have default impls, then the <implementations> object can be omitted entirely, and the protocol will be automatically derived for the given <types>

import protocol from '@zkat/protocols'
// Singly-dispatched protocols
const Show = protocol({
  show: []
class Foo {}
Show(Foo, {
  show () { return `[object Foo(${this.name})]` }
var f = new Foo()
f.name = 'alex'
f.show() === '[object Foo(alex)]'
import protocol from '@zkat/protocols'
// Multi-dispatched protocols
const Comparable = protocol(['target'], {
  compare: ['target'],
class Foo {}
class Bar {}
class Baz {}
Comparable(Foo, [Bar], {
  compare (bar) { return 'bars are ok' }
Comparable(Foo, [Baz], {
  compare (baz) { return 'but bazzes are better' }
const foo = new Foo()
const bar = new Bar()
const baz = new Baz()
foo.compare(bar) // 'bars are ok'
foo.compare(baz) // 'but bazzes are better'


npm i @zkat/protocols

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