On Tue, Feb 2, 2010 at 20:25, David Menendez wrote:
> On Tue, Feb 2, 2010 at 1:48 PM, Ryan Ingram wrote:
> > Gen slightly breaks the monad laws:
> >
> >> arbitrary >>= return
> > is not the same as
> >> return () >>= const arbitrary
> > because each bind splits the generator, so you end up with a different
> > seed passed to arbitrary in these two cases.
>
Ah yes, that was exactly the problem.
> > If the observable value is "some random object" this is a safe fudge,
> > but if you want repeatable, it doesn't quite work. You need your
> > instances to be exactly identical, down to the associativity of binds,
> > in order to get the same results.
>
> We could avoid that problem by redefining Gen as a state transformer monad.
>
> newtype Gen a = MkGen { unGen :: StdGen -> Int -> (a, StdGen) }
>
> instance Monad Gen where
> return a = MkGen $ \r _ -> (a,r)
> MkGen m >>= k = MkGen $ \r n -> let (a,r') = m r n in unGen (k a) r' n
>
> I'm pretty sure all the Gen primitives can be similarly redefined.
>
And I'm guessing I haven't been or won't be the only one running into this
issue.
Sean
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
