For example, given that all functions are known upfront, wouldn't it be possible to define them in their own scope and check for their existence? 

```javascript
var functions = {
  toInt: function(label) {
    return int;
  },
  toPercentage: function(label) {
    // ....
  }
}

var fn = functions[rep[1]];
if (typeof fn === 'function') {
  // call the function
} else {
  // add an error to the new error reporter
}
```