Re: [Haskell-cafe] Newbie on instance of Monad

On Fri, 31 Oct 2008, Jonathan Cast wrote:

On Fri, 2008-10-31 at 18:43 -0200, Mauricio wrote:

...

Hi,

After a lot of thinking, I can't get what I am doing wrong in this code:

------ data ( RandomGen g ) => RandomMonad g a = RandomMonad (g -> a)

instance Monad (RandomMonad g) where return = RandomMonad . const RandomMonad f1 >>= f2 = RandomMonad f3 where f3 a = f2f1 a (next a) RandomMonad f2f1 = f2 . f1

Yikes. Just search the archives for `Set not a Monad'; you have the same issue.

I think that this would be the answer if he is after a constraint for 'a', but he wants to constraint 'g'.

Hint: data RandomGen g => RandomMonad g a means nothing at all like what you think it means.