Haskell - depth for each node in binary tree using Reader monad

Question

I wrote the following code. It is working and using the Reader monad.

Could you give me some hints about code style in Haskell ? Mainly, I mean monads -

Answer 1

There's no need to go in and out of the Reader the way you do it here by using runReader; instead, you can rewrite it as

renumberR :: Tree a -> Reader Int (Tree Int)
renumberR (Node _ l r) = do
    x <- ask
    l' <- local (+1) (renumberR l)
    r' <- local (+1) (renumberR r)
    return (Node x l' r')
renumberR Empty = return Empty

However, you can write it even nicer by just using the applicative interface of Reader:

renumberR (Node _ l r) =
    Node <$> ask <*> local (+1) (renumberR l) <*> local (+1) (renumberR r)
renumberR Empty = pure Empty

Note that I have renamed your function to renumberR to emphasize the fact that it runs in Reader, but not necessarily using its monadic interface.