[Haskell-cafe] Newbie on instance of Monad
lemming at henning-thielemann.de
Fri Oct 31 16:53:17 EDT 2008
On Fri, 31 Oct 2008, Jonathan Cast wrote:
> On Fri, 2008-10-31 at 18:43 -0200, Mauricio wrote:
>> 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.
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