Functional RPN Calc

rpn.js

var R = require('ramda');

var rpn = R.compose(R.head, R.reduce(foldingFn, []), R.split(' '));

function foldingFn(acc, v) {
  if(isOperator(v)) {
    var vs   = R.take(2, acc),
        rest = R.drop(2, acc);

    return R.prepend(useOperator(v, vs), rest);
  } else {
    return R.prepend(parseInt(v), acc);
  }
}

var OPERATORS = {
  '+': R.add,
  '-': R.subtract,
  '/': R.divide,
  '*': R.multiply
};

var isOperator = R.partialRight(R.has, OPERATORS);

function useOperator(op, vs) {
  var operator = R.prop(op, OPERATORS);

  return R.reduce(operator, R.head(vs), R.tail(vs));
}

rpn('4 2 + 3 *');
// => 18