On Tue, Aug 28, 2012 at 12:08 PM, Nicolas Frisby <nicolas.fri...@gmail.com>wrote:
> On Tue, Aug 28, 2012 at 3:34 AM, Simon Marlow <marlo...@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
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