I gave this a try but ended up going with tmhedberg's suggestion because it (as he said) allowed me to stay with lookup. I would use a richer data structure except for the parseHashLiteral function just needs to return a Parser LispVal anyway, so there's nothing "richer" just yet.

However, if I decide to go forward with some of the more advanced things in the tutorial like threading good error messages and such, I'm pretty sure some new data types are in my future.

Also, when you say "underscore for lenses", did you mean that your example would be pretty clean with the Lenses library? I've been looking for an excuse to learn that so if so I'd love to learn more!

This simplifies the type signature, but doesn't avoid the need for `return $ f "unneeded"`, which seems to be the ugly part he was really hoping to eliminate.

Also, now you have to write your own lookup function instead of just using Prelude.lookup.