On: Type-safe Transducers in Clojure, Scala, and Haske...

[jules]

You can do that, but it's more elegant if you define a functional stateful transducer analogous to scanl for lists.

[davdar → jules]

That's like saying "your program is more elegant if it has no monads". It's an incorrect statement. The monadic version is perfectly elegant. Even better: it's the right one.

[jules → davdar]

By the same logic scanl should just have the type:

    (b -> IO a) -> [b] -> IO [a]

instead of

    (a -> b -> a) -> a -> [b] -> [a]

Just putting the whole thing in the IO/ST monad isn't the right solution when you need a very specific form of local state.