Thanks for your answers. I still have a question, though...
I think the reasoninging here is the following: If MonadState is part of the standard library and it needs this extension to work, then all compilers must support this extension.
Yes, exactly, that was my point.
I'm not completely sure how "standard" the MonadState class is though. Also, it might be that there could be implementations of MonadState that do not use the extension?
And indeed, I understand from the next (Luke Palmer's) answer that the community does not fully agree on what should be standard:
However, your question is rather moot here, as you are using the *mtl. *It uses UndecidableInstances, whose blessing into the de facto standard would require as a precondition the batshit-insanity of the de facto community.
I personally use *transformers* as my monad library, as it is (as far as I know) the only Haskell 98 monad library on Hackage (you'll hardly notice the more explicit lifts, I promise!).
I just looked through Transformers and it does not define MonadState, indeed. Well... I am a beginner in haskell. I have detailed in the first post the very simplistic example I was trying to set up, the level of a beginner's tutorial. As anyone discovering a new language, I certainly did not intend to look so soon at compiler extensions. Now, trying to avoid duplicate code at this very level of simplicity seems to require compiler extensions! Here it is: - The MonadState type seems required to reuse the same code for "State s a" and "StateT s IO a". - MonadState needs compiler extensions and is implemented using the mtl library which seems to have drawbacks. So 1) How can I use transformers instead of the mtl? This is in no tutorial, and searchinf for "mtl" on the haskell wiki yields no result at all. 2) What should be the Haskell98-compatible type signature for my play2 function, instead of (MonadState [a] m, Eq a) => a -> m Bool ? Thanks again.