@DLosc Update: I ended up reimplementing my Function type as Constant Value | Function Int (Value -> Function). Applying a Function to a Value always results in another Function. If the Function has arity 1, the result is a Constant; applying a Constant leaves it unchanged. The neat thing about this approach is that I can make Function an instance of Monoid (which I just read about). Function composition is just mappend, and function application is just composition with a Constant.

@DLosc <*> is ap, <$> is map, >>= is bind. I don't know of one for =<< but I tend to think of it as "reverse bind"