Sketched out example of protocol library built on top of genfun

genfun-protocol.js

var Protocol = require('genfun-protocol')

var Eq = Protocol(['a', 'b'], {
  eql: ['a', 'b'],
  // Extra set of [] and you can add a default implementation!
  // This will dispatch only for existing instances
  // (that is, it doesn't fall through to [Object, Object] in this case)
  neq: [['a', 'b'], (a, b) => !Eq.eql(a, b)]
})

Eq.impl([Number, Number], {
  eql: [[Number, Number], (a, b) => a === b]
  // neq [Number, Number] is automatically implemented
})

var Show = Protocol(['a'], {
  show: ['a']
})

class Thing {}
class Stream {}

Show.impl([Thing], {
  out: [[Thing], (x) => JSON.stringify(x)]
})

Show.impl([Stream], {
  out: [[Stream], (stream) => {
    var buffer = ''
    stream.on('data', (chunk, enc, next) => {
      buffer += chunk.toString(enc)
      next()
    })
    return new Promise((resolve, reject) => {
      stream.on('end', () => resolve(buffer))
      stream.on('error', (e) => reject(e))
    })
  }]
})

var Read = Protocol([Show('obj'), 'exemplar'], {
  in: ['obj', 'exemplar']
})

Read.impl([String, Thing], {
  in: [[String, Thing], (obj) => JSON.parse(Show.show(obj))]
})

Read.impl([Stream, Thing], {
  in: [[Stream, Thing], (stream) => {
    return Show.show(stream).then(JSON.parse)
  }]
})

// Error, because Number doesn't have a Show impl
Read.impl([Number, Thing], {
  in: [[Number, Thing], (num) => '' + num]
})