Thank you Adam.
One example is https://github.com/haskell/vector/issues/34 Yes, this looks like an example where injectivity will work. One question here: how does one build vector with GHC HEAD? I tried but failed because of dependencies.
I see lots of potential uses in HList. For example in HZip.hs there's a Zip using type families:
type family HZipR (x::[*]) (y::[*]) :: [*] type instance HZipR '[] '[] = '[] type instance HZipR (x ': xs) (y ': ys) = (x,y) ': HZipR xs ys Bad news: my current implementation won't allow to declare HZipR as injective. That's because my implementation is conservative and does not permit calling type families by injective type family.
I don't know how realistic this is but a constraint (HLength x ~ HLength y) would ideally have the same result as SameLength x y. I'm not sure if I understand that part. HLength is not injective. How would injectivity help here?
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