Re: Comparing StableNames of different type

Great. It was the sharing of polymorphic values like [] I was worried about. Thanks for confirming. On Tue, Aug 28, 2012 at 1:07 PM, Edward Kmett wrote:

On Tue, Aug 28, 2012 at 12:08 PM, Nicolas Frisby wrote:

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On Tue, Aug 28, 2012 at 3:34 AM, Simon Marlow wrote:

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On 24/08/2012 07:39, Emil Axelsson wrote: > > Are there any dangers in comparing two StableNames of different > type? > > stEq :: StableName a -> StableName b -> Bool > stEq a b = a == (unsafeCoerce b)

Ok, I've added it. It will be in GHC 7.8.1.

Cheers, Simon

Might we benefit from having a variant that returns Maybe (a :=: b)? Is that safe? I have limited experience with StableNames, but that intuitively seems safe. But polymorphism and references deserve more thought than I've given this yet.

I'm referring to "data (:=:) :: * -> * -> * where Refl :: (a :=: a)", just to be clear.

No.

You can't safely determine that a ~ b given that two stablenames are equal.

If you give Nothing a stableName, it'll have one stable name, regardless of if you use it as a Maybe Int or a Maybe Bool. Maybe Int and Maybe Bool are clearly not equal. This is admittedly an implementation detail. GHC would be perfectly within its rights (if somewhat silly) to construct a fresh Nothing every time, but it doesn't.

The reasoning you applied only works for fully monomorphic types.

-Edward