[Haskell-cafe] Injective type families for GHC

Jan Stolarek jan.stolarek at p.lodz.pl
Tue Feb 10 11:38:10 UTC 2015

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 

> 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?


Politechnika Łódzka
Lodz University of Technology

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