There are systematic ways to layer different monads together. If you want to read about these, a keyword is "monad transformers." Ken Shan discusses them in [his paper](http://arxiv.org/abs/cs/0205026v1) that we recommended earlier.
However, since we haven't studied these, we will just combine the reader monad and the state monad in an ad hoc way. The easiest way to do this is to think of the assignment function and the store of reference cells as a combined state, which gets threaded through the computations in the same way that simple states did in your earlier homeworks.
There are systematic ways to layer different monads together. If you want to read about these, a keyword is "monad transformers." Ken Shan discusses them in [his paper](http://arxiv.org/abs/cs/0205026v1) that we recommended earlier.
However, since we haven't studied these, we will just combine the reader monad and the state monad in an ad hoc way. The easiest way to do this is to think of the assignment function and the store of reference cells as a combined state, which gets threaded through the computations in the same way that simple states did in your earlier homeworks.