protoduck

Protoduck

protoduck is a JavaScript library is a library for making groups of methods, called "protocols", that work together to provide some abstract functionality that other things can then rely on. If you're familiar with the concept of "duck typing", then it might make sense to think of protocols as things that explicitly define what methods you need in order to "clearly be a duck".

On top of providing a nice, clear interface for defining these protocols, this module clear, useful errors when implementations are missing something or doing something wrong.

One thing that sets this library apart from others is that on top of defining duck-typed protocols on a single class/type, it lets you have different implementations depending on the arguments. So a method on Foo may call different code dependent on whether its first argument is Bar or Baz. If you've ever wished a method worked differently for different types of things passed to it, this does that!

Install

$ npm install protoduck

Table of Contents

• Example
• Features
• Guide
  • Introduction
  • Defining protocols
  • Simple impls
  • Multiple dispatch
  • Static impls
• API
  • protocol()
  • impls

Example

import protocol from "protoduck"

// Quackable is a protocol that defines three methods
const Quackable = protocol({
  walk: [],
  talk: [],
  isADuck: [() => true] // default implementation -- it's optional!
})

// `duck` must implement `Quackable` for this function to work. It doesn't
// matter what type or class duck is, as long as it implements Quackable.
function doStuffToDucks (duck) {
  if (!duck.isADuck()) {
    throw new Error('I want a duck!')
  } else {
    console.log(duck.walk())
    console.log(duck.talk())
  }
}

// and another place...
class Duck () {}

// Implement the protocol on the Duck class.
Quackable(Duck, [], {
  walk() { return "*hobble hobble*" }
  talk() { return "QUACK QUACK" }
})

// main.js
doStuffToDucks(new Duck()) // works!

Features

• Clear, concise protocol definitions and implementations
• Verifies implementations in case methods are missing
• "Static" implementations ("class methods")
• Optional default method implementations
• Fresh JavaScript Feel™ -- methods work just like native methods when called
• Methods can dispatch on arguments, not just this (multimethods)
• Fast, cached multiple dispatch

Guide

Introduction

JavaScript comes with its own method definition mechanism: You simply add regular functions as properties to regular objects, and when you do obj.method(), it calls the right code! ES6/ES2015 further extended this by adding a class syntax that allowed this same system to work with more familiar syntax sugar: class Foo { method() { ... } }.

protoduck is a similar language extension: it adds something called "protocols" to JavaScript.

The purpose of protocols is to have a more explicit definitions of what methods "go together". That is, if you have a type of task, you can group every method that things definitely need to have under a protocol, and then write your code using the methods defined there. The assumption is that anything that defines that group of methods will work with the rest of your code.

And then you can export the protocol itself, and tell your users "if you implement this protocol for your own objects, they'll work with my code."

Duck typing is a common term for this: If it walks like a duck, and it talks like a duck, then it may as well be a duck, as far as any of our code is concerned.

Defining Protocols

The first step to using protoduck is to define a protocol. Protocol definitions look like this:

// import the library first!
import protocol from "protoduck"

// `Ducklike` is the name of our protocol. It defines what it means for
// something to be "like a duck", as far as our code is concerned.
const Ducklike = protocol([], {
  walk: [], // This says that the protocol requires a "walk" method.
  talk: [] // and ducks also need to talk
  peck: [] // and they can even be pretty scary
})

Protocols by themselves don't really do anything, they simply define what methods are included in the protocol, and thus what will need to be implemented.

Simple impls

The simplest type of definitions for protocols are as regular methods. In this style, protocols end up working exactly like normal JavaScript methods: they're added as properties of the target type/object, and we call them using the foo.method() syntax. this is accessible inside the methods, as usual.

Implementation syntax is very similar to protocol definitions, but it calls the protocol itself, instead of protocol. It also refers to the type that you want to implement it on:

class Dog {}

// Implementing `Ducklike` for `Dog`s
Ducklike(Dog, [], {
  walk() { return '*pads on all fours*' }
  talk() { return 'woof woof. I mean "quack" >_>' }
  peck(victim) { return 'Can I just bite ' + victim + ' instead?...' }
})

So now, our Dog class has two extra methods: walk, and talk, and we can just call them:

const pupper = new Dog()

pupper.walk() // *pads on all fours*
pupper.talk() // woof woof. I mean "quack" >_>
pupper.peck('this string') // Can I just bite this string instead?...

Multiple Dispatch

You may have noticed before that we have these [] in various places that don't seem to have any obvious purpose.

These arrays allow protocols to be implemented not just for a single value of this, but across all arguments. That is, you can have methods in these protocols that use both this, and the first argument (or any other arguments) in order to determine what code to actually execute.

This type of method is called a multimethod, and isn't a core JavaScript feature. It's something that sets protoduck apart, and it can be really useful!

The way to use it is simple: in the protocol definitions, you put matching strings in different spots where those empty arrays were, and when you implement the protocol, you give the definition the actual types/objects you want to implement it on, and it takes care of mapping types to the strings you defined, and making sure the right code is run:

const Playable = protocol(['friend'], {
  playWith: ['friend']
})

class Cat {}
class Human {}
class Dog {}

// The first protocol is for Cat/Human combination
Playable(Cat, [Human], {
  playWith(human) {
    return '*headbutt* *purr* *cuddle* omg ilu, ' + human.name
  }
})

// And we define it *again* for a different combination
Playable(Cat, [Dog], {
  playWith(dog) {
    return '*scratches* *hisses* omg i h8 u, ' + dog.name
  }
})

// depending on what you call it with, it runs different methods:
const cat = new Cat()
const human = new Human()
const dog = new Dog()

cat.playWith(human) // *headbutt* *purr* *cuddle* omg ilu, Sam
cat.playWith(dog) // *scratches* *hisses* omg i h8 u, Pupper

In general, most implementations you write won't care what types their arguments are, but when you do? This may end up saving you a lot of trouble and allowing some tricks you might realize you can do that weren't possible before!

Static impls

Finally, there's a type of protocol impl that doesn't involve a this value at all: static impls. Some languages might call them "class methods" as well. In the case of protoduck, though, these static methods exist on the protocol object itself, rather than a regular JavaScript class.

Static methods can be really useful when you want to call them as plain old functions without having to worry about the this value. And because protoduck supports multiple dispatch, it means you can get full method functionality, but with regular functions that don't need this: they just operate on their standard arguments.

Static impls are easy to make: simply omit the first object type and use the arguments array to define what the methods will be implemented on:

const Eq = protocol(['a', 'b'], {
  equals: ['a', 'b']
})

const equals = Eq.equals // This isn't necessary, but it shows that we
                         // don't need a `.` to call them, at all!

Eq([Number, Number], {
  equals(x, y) {
    return x === y
  }
})

Eq([Cat, Cat], {
  equals(kitty, cat) {
    return kitty.name === cat.name
  }
})

equals(1, 1) // true
equals(1, 2) // false
equals(snookums, reika) // false
equals(walter, walter) // true
equals(1, snookums) // Error! No protocol impl!

API

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.

Example

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>

Example

import protocol from 'protoduck'

// 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 'protoduck'

// 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'

Changelog

4.0.0 (2017-04-17)

Bug Fixes

• test: .name is inconsistently available (3483f4a)

Features

• api: Fresh New API™ (#2) (534e5cf)
• constraints: added optional where-constraints (16ad124)
• defaults: allow default impls without arrays in defs (6cf7d84)
• deps: use genfun@4 (f6810a7)
• meta: bringing project stuff up to date (61791da)

BREAKING CHANGES

• api: The API was significantly overhauled.

• New protocol creating is now through protoduck.define() instead of protoduck()

• Implementations are through Duck#impl instead of Duck(...)

• The private option was removed

• Static protocols were removed -- only method-style protocols are available now.

• As part of that: the target argument to impl can no longer be omitted

• The main export object is now the metaobject. protoduck.impl can be used to extend to MOP

• .isDerivable is now a property on Duck instances, not a static method

• .hasImpl is now a method on Duck instances, not a static method

• Protoduck will now genfunnify existing functions as default methods for genfuns declared in a protocol when implementing

• Error messages have been overhauled to be more helpful

• deps: nextMethod is now an extra argument to methods

• meta: node@<4 is no longer supported