Friends George Karachalas, Tom Schrijvers, Dimitrios Vytiniotis, and I are working on finally cracking the problem of accurately reporting pattern-match overlap and redundancy warnings, in the presence of GADTs. You know the problem; consider vzip :: Vect n a -> Vect n b -> Vect n (a,b) vzip VN VN = VN vzip (VC x xs) (VC y ys) = VC (x,y) (vzip xs ys) data Vect :: Nat -> * -> * where VN :: Vect Zero a VC :: a -> Vect n a -> Vect (Succ n) a Are there any missing equations in vzip? No! But GHC complains anyway. We have lots of Trac tickets about this; e.g. https://ghc.haskell.org/trac/ghc/ticket/3927. We now have a draft paper (wait a week) and a prototype implementation, that fixes the problem. But we need your help. We'd like to try our prototype on real code, not just toy examples. So, could you send George a pointer to any packages you have, or know of, that * use GADTS (or other fancy type features) and * would benefit from accurate pattern-match overlap/redundancy warnings? Specifically: * Where you have had to add a catch-all f _ _ = error "impossible" to silence GHC from saying "missing patterns" * Or where you have added {-# OPTIONS_GHC -fno-warn-missing-patterns #-} to silence the warnings. * Or something else like that. George's email is in cc. Time is short - the ICFP deadline is on 27 Feb. So sooner is better than later for us. But later is better than never. Thank you! Simon