[Haskell-cafe] Monad instance for Data.Set

[oleg at okmij.org]

The following code solves exactly the problem of implementing
(restricted) MonadPlus in terms of Data.Set:

	http://okmij.org/ftp/Haskell/DoRestrictedM.hs

The code is written to demonstrate the do-notation. We write the
monadic code as usual:

> test1s_do () = do
>   x <- return "a"
>   return $ "b" ++ x

and then instantiate it for Maybe String:

> test1sr_do :: Maybe String
> test1sr_do = unRM $ test1s_do ()
> -- Just "ba"

or for Data.Set:

> test2sr_do :: Set.Set String
> test2sr_do = unSM $ test1s_do ()
> -- fromList ["ba"]

It seems GHC 6.10 will support the do-notation even for the
generalized monads.