[Haskell-cafe] Newbie on instance of Monad
jonathanccast at fastmail.fm
Fri Oct 31 16:40:32 EDT 2008
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
> I get this error message:
> Could not deduce (RandomGen g)
> from the context (Monad (RandomMonad g))
> arising from a use of `RandomMonad' at src/encherDB.hs:10:11-21
> Possible fix:
> add (RandomGen g) to the context of the type signature for `return'
> In the first argument of `(.)', namely `RandomMonad'
> In the expression: RandomMonad . const
> In the definition of `return': return = RandomMonad . const
> but I'm not smart enough to understand what
> it means.
Yikes. Just search the archives for `Set not a Monad'; you have the
Hint: data RandomGen g => RandomMonad g a means nothing at all like what
you think it means.
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