On Tue, Aug 28, 2012 at 12:08 PM, Nicolas Frisby <nicolas.frisby@gmail.com> wrote:

On Tue, Aug 28, 2012 at 3:34 AM, Simon Marlow <marlowsd@gmail.com> wrote:
>>>> 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