I recently discovered an interesting way of closing typeclasses while
playing with type-level peano naturals:
class Nat n where
caseNat :: forall r. n -> (n ~ Z => r) -> (forall p. (n ~ S p,
Nat p) => p -> r) -> r
I usually use this one:
class Nat n where
caseNat :: p Z -> (forall m. Nat m => p (S m)) -> p n
instance Nat Z where
caseNat pz _ = pz
instance Nat n => Nat (S n) where
caseNat _ psm = psm
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