That depends on the monad library. In transformers it would be:

(Monad m, Eq a) => a -> StateT [a] m Bool

State s is just a type synonym for StateT s Identity, so this works for State as well.

I like it, it is simpler than the one using MonadState. I had read many tutorials and wiki articles, and saw nowhere that there were different implementations of what seemed to be standard libraries.

On a higher level, in case you are interested, here's a description of how I would model your problem.

Actually using a monad transformer was the goal of it all, tutorial-like. The example was contrived. Another question would be: when is it relevant to use monad transformers? I'm not sure to have enough haskell practice to be ready for that one, though. :^) And for sure, my example didn't need it. Thanks Eric