es5 version of count-identifiers

count-identifiers.js

//es5
var parse = require("shift-parser").default;
var reduce = require("shift-reducer").default;
var MonoidalReducer = require("shift-reducer").MonoidalReducer;

//must return a Monoid
function IdentifierCounter() {
  // a monoid over integers and addition
  function Sum() {}
  // by default reduce any node to the identity, zero
  Sum.empty = function() {
    return 0;
  };
  // combine instances by summing their values
  Sum.prototype.concat = function(a) {
    return this + a;
  }
  MonoidalReducer.call(this, Sum);
}

IdentifierCounter.prototype = Object.create(MonoidalReducer.prototype);

// a convenience function for performing the reduction and extracting a result
IdentifierCounter.count = function(program) {
  return reduce(new this, program);
};

// add 1 to the count for each IdentifierExpression node
IdentifierCounter.prototype.reduceIdentifierExpression = function(node, identifier) {
  return 1; //will be summed
}

/*
  In this case, the only node we care about overriding is the
  IdentifierExpression node; the rest can be reduced using the default
  methods from MonoidalReducer.
*/

//test code
var program = "function f() { hello(world); }";
console.dir(IdentifierCounter.count(parse(program)));