Convincing GHC that `Maybe Void` is a unit type
I have a type family IfEq (n :: Nat) (m :: Nat) o that evaluates to o or Void depending on wether n :== m . Since Maybe Void is only inibited by Nothing I should be able to define a function Maybe (IfEq nmo) -> Maybe o but I can't figure it out.
Follow up: Can this generalized from the Maybe monad to a more general type? (eg. all MonadPlus s)
EDIT: I had initially rolled my own Nat but (thank's to @chi) with the GHC Nat kind KnownNat constrait it pretty straightforward to do what I describe above. However GHC can not infer KnownNat a => KnownNat (1+a) which is imperative to me so I am back at square 1.
尝试类似
foo :: forall n m o . (KnownNat n, KnownNat m) =>
IfEq n m o -> Maybe o
foo x = case sameNat (Proxy :: Proxy n) (Proxy :: Proxy m) of
Just Refl -> Just x
Nothing -> Nothing
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