Andrew Wagner wrote
data Foo a = Bar a data (Ord a) => Baz a = Bah a
Note that both of these have kind * -> *. However, Baz could never be an instance of monad, because there is a restriction on the types it can operate on.
There is a wide-spread opinion that one ought not to give context to a data type declaration (please search for `restricted datatypes Haskell'). Someone said that in GHC typechecker such contexts called stupidctx. There has been a proposal to remove that feature from Haskell, although I like it as a specification tool. John Hughes wrote a paper about a better use for that feature: John Hughes. 1999. Restricted datatypes in Haskell. In Proceedings of the 1999 Haskell workshop, ed. Erik Meijer. Technical Report UU-CS-1999-28, Department of Computer Science, Utrecht University. http://www.cs.chalmers.se/~rjmh/Papers/restricted-datatypes.ps That proposal has not been implemented. One should point out that restricted monads are available in Haskell right now: http://www.haskell.org/pipermail/haskell-prime/2006-February/000498.html It seems one can even use the do-notation for them, with the help of `rebindable syntax' feature of GHC. This is because the types of restricted bind and return are exactly the same as those of regular bind and return. Only the `Monad' constraint is a bit different. Restricted monads are the strict super-set of the ordinary monads, so the backwards compatibility is maintained. One almost wishes for a fuller integration of restricted monads into the language...