### Define and Implement a Protocol

Source: https://github.com/zkat/protoduck/blob/latest/README.md

This example demonstrates defining a protocol with default implementations, enforcing it in a function, and implementing it for a class.

```javascript
const protoduck = require('protoduck')

// Quackable is a protocol that defines three methods
const Quackable = protoduck.define({
  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())
  }
}

// ...In a different package:
const ducks = require('./ducks')

class Duck () {}

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

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

--------------------------------

### Implement a multi-dispatched protocol

Source: https://github.com/zkat/protoduck/blob/latest/README.md

Demonstrates defining a protocol with a target type and providing multiple implementations for different types.

```javascript
import protoduck from 'protoduck'

// Multi-dispatched protocols
const Comparable = protoduck.define(['target'], {
  compare: ['target'],
})

class Foo {}
class Bar {}
class Baz {}

Comparable.impl(Foo, [Bar], {
  compare (bar) { return 'bars are ok' }
})

Comparable.impl(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'
```

--------------------------------

### Implement a singly-dispatched protocol

Source: https://github.com/zkat/protoduck/blob/latest/README.md

Demonstrates defining a protocol without types and implementing it for a class Foo.

```javascript
import protoduck from 'protoduck'

// Singly-dispatched protocols
const Show = protoduck.define({
  show: []
})

class Foo {
  constructor (name) {
    this.name = name
  }
}

Show.impl(Foo, {
  show () { return `[object Foo(${this.name})]` }
})

const f = new Foo('alex')
f.show() === '[object Foo(alex)]'
```

--------------------------------

### Implement a Protocol for a Class

Source: https://github.com/zkat/protoduck/blob/latest/README.md

Use the .impl method to add protocol methods to a class. These methods become available as properties on instances of the class.

```javascript
class Dog {}

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

--------------------------------

### Define and Implement Multimethods

Source: https://github.com/zkat/protoduck/blob/latest/README.md

Multimethods allow dispatching based on multiple arguments. Define matching strings in the protocol definition and provide the specific types during implementation.

```javascript
const Playful = protoduck.define(['friend'], {// <---\
  playWith: ['friend'] // <------------ these correspond to each other
})

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

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

// And we define it *again* for a different combination
Playful.impl(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
```

--------------------------------

### Define a Protocol

Source: https://github.com/zkat/protoduck/blob/latest/README.md

Use protoduck.define to create a protocol definition that specifies the required methods for an object.

```javascript
// import the library first!
const protoduck = require('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 = protoduck.define([], {
  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
})
```

--------------------------------

### Define a protocol with protoduck.define

Source: https://github.com/zkat/protoduck/blob/latest/README.md

Defines a new protocol named Eq that expects an implementation for the eq function.

```javascript
const Eq = protoduck.define(['a'], {
  eq: ['a']
})
```

--------------------------------

### Call Implemented Protocol Methods

Source: https://github.com/zkat/protoduck/blob/latest/README.md

Once a protocol is implemented for a class, instances can call the defined methods directly.

```javascript
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?...
```

--------------------------------

### proto.impl(target, types, implementations)

Source: https://github.com/zkat/protoduck/blob/latest/README.md

Adds a new implementation to a defined protocol for specific types.

```APIDOC
## proto.impl(target, types, implementations)

### Description
Adds a new implementation to the given protocol across `types`. If the protocol is derivable, the `implementations` object can be omitted.

### Parameters
- **target** (Object) - Required - The target class or object to implement the protocol for.
- **types** (Array) - Optional - The types to map to method arguments.
- **implementations** (Object) - Optional - Functions matching the protocol's API.

### Example
```javascript
Show.impl(Foo, {
  show () { return `[object Foo(${this.name})]` }
})
```
```

--------------------------------

### define(types, spec, opts)

Source: https://github.com/zkat/protoduck/blob/latest/README.md

Defines a new protocol based on specified types and function requirements.

```APIDOC
## define(types, spec, opts)

### Description
Defines a new protocol across arguments of types defined by `types`, which will expect implementations for the functions specified in `spec`.

### Parameters
- **types** (Array) - Optional - The types to be used in the protocol.
- **spec** (Object) - Required - The functions and their associated types that the protocol requires.
- **opts** (Object) - Optional - Configuration options including `name`, `where` (constraints), and `metaobject`.

### Example
```javascript
const Eq = protoduck.define(['a'], {
  eq: ['a']
})
```
```

--------------------------------

### Define Protocol Constraints

Source: https://github.com/zkat/protoduck/blob/latest/README.md

Constraints ensure that any type implementing a protocol also implements required parent protocols. This prevents errors by enforcing dependencies.

```javascript
const Show = proto.define({
  // This syntax allows default impls without using arrays.
  toString () {
    return Object.prototype.toString.call(this)
  },
  toJSON () {
    return JSON.stringify(this)
  }
})

const Log = proto.define({
  log () { console.log(this.toString()) }
}, {
  where: Show()
  // Also valid:
  // [Show('this'), Show('a')]
  // [Show('this', ['a', 'b'])]
})

// This fails with an error: must implement Show:
Log.impl(MyThing)

// So derive Show first...
Show.impl(MyThing)
// And now it's ok!
Log.impl(MyThing)
```

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