[Haskell-cafe] Re: Monad.Reader with updates

[Mauricio]

 > Hi Mauricio.  What you want actually already exists in QuickCheck as
 > the "Gen" monad.
 >
 > newtype Gen a
 >   = Gen (Int -> StdGen -> a)
 >
 > instance Monad Gen where
 >   return a    = Gen (\n r -> a)
 >   Gen m >>= k =
 >     Gen (\n r0 -> let (r1,r2) = split r0
 >                       Gen m'  = k (m n r1)
 >                    in m' n r2)
 >
 > (...)

Nice. I think that's exactly what I was trying to do.

 > You could also implement this as a variation on the State monad if you
 > wanted to avoid using split: (...)

Yes.  After Brent's explanation I finally realized State was the perfect
option. Maybe  it should  also be better  for QuickCheck. I  just didn't
know it…  There are  many things  in the standard  library that  do nice
things, but  I don't understand them  until I write a  few hundred lines
trying to do what they do :)

Thanks for your support and patience,
Maurício