I'll rephrase and expand what I wrote on IRC in case that didn't reach you because of Freenode's problems: I'd just pass the parser to Runner#initialize and make the attribute in Runner immutable.

Passing in the initializer makes this an instance of dependency injection which is generally an useful pattern to adhere to.

The immutability is generally a good thing because it lowers the number of states an object can be in. This usually leads to simpler and less buggy code.

@dmajda Updated