# match eval t2 with Leaf (Num f) -> f 2 4;;
- : int = 13
-2. Based on the evaluator code from the assignment from week 7, and what you've learned about the Reader monad,
+2. Based on [[the evaluator code from assignment 7|/exercises/assignment7/#index3h2]], and what you've learned about the Reader monad,
enhance the arithmetic tree code to handle an arbitrary set of free variables. Don't use Juli8 libraries for this; just do it by hand.
Return to the original code (that is, before the modifications required by the previous problem).
-- substitute your own choices for the type Env and value env0
let { xx :: ReaderT Env Maybe Int; xx = return 1 } in runReaderT xx env0
- Okay, here are some questions about various monad transformers. Use OCaml to help you answer them. Which combined monad has the type of an optional list (that is, either `None` or `Some [...]`): an Option transformer wrapped around an underlying List monad, or a List transformer wrapped around an underlying Option monad? Which combined monad has the type of a function from `store`s to a pair `('a list, store)`: a List transformer wrapped around an underlying State monad or a State transformer wrapped around an underlying List monad?
+ Okay, here are some questions about various monad transformers. Use OCaml or Haskell to help you answer them. Which combined monad has the type of an optional list (that is, either `None` or `Some [...]`): an Option transformer wrapped around an underlying List monad, or a List transformer wrapped around an underlying Option monad? Which combined monad has the type of a function from `store`s to a pair `('a list, store)`: a List transformer wrapped around an underlying State monad or a State transformer wrapped around an underlying List monad?
The last two problems are non-monadic.