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