* Yves Parès
One thought occur to me recently when explaining the concept of Monad to non-haskellers: internally, all standard Monads are newtypes wrapping functions: StateT is, WriterT is, ContT is. Even IO and ST are, both conceptually and in their implementation by GHC. ParsecT (not part of mtl, but still) is. And so on...
Writer(T) is not a function, it's just a tuple. Jerzy already mentioned [] and Maybe. Another example is a free monad generated by any polynomial functor. This subsumes Maybe and (almost) [], as described here: http://blog.omega-prime.co.uk/?p=34 To summarise, since there are only so many ways to form types in Haskell (sum, product and exponentiation (functions)), it's no surprise that functions do occur often, but that's not something fundamental to monads. -- Roman I. Cheplyaka :: http://ro-che.info/