Monet.js Free monad DSL and interpreter example

interpreter-kvs.js

const { compose, identity, add, always, dissoc, assoc } = require('ramda')
const { taggedSum, tagged } = require('daggy');
const { Free } = require('monet');

const KVS = taggedSum('KVS', {
  Put: ['key', 'val', 'next'],
  Del: ['key', 'next'],
  Get: ['key', 'val']
});

KVS.prototype.map = function(f) {
  return this.cata({
    Put: (k, v, n) => KVS.Put(k, v, f(n)),
    Del: (k, n) => KVS.Del(k, f(n)),
    Get: (k, v) => KVS.Get(k, compose(f, v))
  })
}

const unit = tagged('unit', ['x'])
const Unit = () => unit('')

const put = (key, val) => Free.liftF(KVS.Put(key, val, Unit()))

const del = key => Free.liftF(KVS.Del(key, Unit()))

const get = key => Free.liftF(KVS.Get(key, identity))

const modify = (key, f) =>
  get(key).chain(v => put(key, f(v)))

const interpreters = {
  string: (free, store) => {
    function go(store, free, strs) {
      return free.resume().fold(
        x => x.cata({
          Put: (k, v, n) => go(assoc(k, v, store), n, strs.concat(`Putting ${k} with ${v}`)),
          Del: (k, n) => go(dissoc(k, store), n, strs.concat(`Deleting ${k}`)),
          Get: (k, f) => go(store, f(store[k]), strs.concat(`Getting ${k}`))
        }),
        always(strs.join('\n'))
      )
    }

    return go(store, free, [])
  },
  pure: (free, store) =>
    free.resume().fold(
      x => x.cata({
        Put: (k, v, n) => interpreters.pure(n, assoc(k, v, store)),
        Del: (k, n) => interpreters.pure(n, dissoc(k, store)),
        Get: (k, f) => interpreters.pure(f(store[k]), store)
      }),
      always(store)
    )
}

const script = get('swiss bank account id').chain(id => {
  return modify(id, add(1000))
    .chain(() => put('bermuda airport', 'getaway car'))
    .chain(() => del('tax records'))
})

const store = {
    'swiss bank account id': 'xxxx1',
    'xxxx1': 5032.12,
    'tax records': [
        {'date': '2/20/15', 'subject': 'aaaaa', 'amount': 2322.90 },
        {'date': '2/20/15', 'subject': 'bbbbb', 'amount': 3412.90 },
        {'date': '2/20/15', 'subject': 'ccccc', 'amount': 4502.90 },
        {'date': '2/20/15', 'subject': 'ddddd', 'amount': 5692.90 }
    ]
}

console.log('--------------------------------');
console.log(interpreters.string(script, store));
console.log('--------------------------------');
console.log(interpreters.pure(script, store));
console.log('--------------------------------');

Key value store DSL and interpreter example

This is a Javascript implementation of the KVS example linked in the monet.js docs. It is a rewrite of the KVS example in the fantasy-frees repository.

Run this example on Repl.it