I see this in GHC.TypeNats
sameNat :: (KnownNat a, KnownNat b) =>
Proxy a -> Proxy b -> Maybe (a :~: b)
sameNat x y
| natVal x == natVal y = Just (unsafeCoerce Refl)
| otherwise = Nothing
The unsafeCoerce says that sameNat is part of the trusted code base. And indeed, it’s only because SNat is a private newtype (i.e its data constructor is private to GHC.TypeNats) that you can’t bogusly say (SNat 7 :: SNat 8)
You want exactly the same thing, but for a comparison oriented data CompareEv, rather than its equality counterpart :~:. So the same approach seems legitimate.
I always want code with unsafeCoerce to be clear about (a) why it’s necessary and (b) why it’s sound.
Simon
From: Glasgow-haskell-users
data CompareEv u v = (u < v) => CompareLT | (u ~ v) => CompareEQ | (u > v) => CompareGT
Then I'd like to define a comparison operation (to be used with `AllowAmbiguousTypes` and `TypeApplications`, alternatively taking proxy arguments):
compareEv :: (KnownNat m, KnownNat n) => CompareEv u v
With `compareEv`, we can bring evidence into scope in `case` expressions. I don't know how to implement `compareEv`. The following attempt fails to type-check, since `compare` doesn't produce evidence (which is the motivation for `compareEv` over `compare`):
compareEv = case natVal (Proxy @ u) `compare` natVal (Proxy @ v) of LT -> CompareLT EQ -> CompareEQ GT -> CompareGT
Can `compareEv` be implemented in GHC Haskell? Is there already an implementation of something similar? Any other advice? Thanks, -- Conal