parser.js

void function(){
  var p = require('combigreater')
  var liberate = require('liberate')
  var call = liberate(Function.prototype.call)
  var slice = liberate(Array.prototype.slice)

  var R = require('rationals')
  var ZERO = R(0)
  var ONE = R(1)

  var digit = p.oneOf('0123456789')
  var digits = p.many1(digit)
  var integer = p.map(function (array) { return R(array.join('')) }, digits)


  function createOperation(op, def){
    return function(){ return function(a, b){ return (a||def)[op](b||def) } }
  }

  var plus  = p.symbol('+')
  var dash  = p.symbol('-')
  var star  = p.symbol('*')
  var slash = p.symbol('/')
  var add   = p.map(createOperation('add', R(0)), plus)
  var sub   = p.map(createOperation('sub', R(0)), dash)
  var mul   = p.map(createOperation('mul', R(1)), star)
  var div   = p.map(createOperation('div', R(1)), slash)


  //var whitespace = p.map(function(){return ''}, p.many1(p.oneOf(
  //      '\u0009\u000B\u000C\u000D\u0020\u0085\u00A0\u1680\u180E\u2000\u2001\u2002\u2003\u2004\u2005\u2006\u2007\u2008\u2009\u200A\u2028\u2029\u202F\u205F\u3000'
  //)))
  var whitespace = p.map(function(){return ''}, p.many1(p.symbol(' ')))
  var ws = p.many(p.symbol(' '))

  function leftExpParser(op, rightExp){
    return p.map(
      function(s){
        return s[1].reduce(function(l, n){ return n[0](l, n[1]) }, s[0])
      }
      , p.all([rightExp, p.many(p.all([op, rightExp]))])
    )
  }


  var additive = p.any([add, sub])
  var multiplicative = p.any([mul, div])

  var e0 = p.lazy(function(){
    return p.any([
        integer
      , p.map(function(a){return a[1]}, p.all([p.symbol("("), exp, p.symbol(")")]))
      , p.map(function(a){return a[0](0, a[1])}, p.all([additive, e0]))
      , p.map(function(a){return a[1]}, p.all(ws, e0))
      ])
  })

  var exp = leftExpParser(additive, leftExpParser(multiplicative, e0))

  function arith(str){
    var r = exp(str)
    console.log('\n\n\n',str, r)
    return r.value
  }

  module.exports = arith
}()