[Haskell-cafe] irrefutable patterns for existential types / GADTs

Ross Paterson wrote:

The story so far: apfelmus: why are there no irrefutable patterns for GADTs? Conor: because you could use them to write unsafeCoerce Ross: how about irrefutable patterns (or newtypes) for existential types? Simon: Try giving the translation into System F + (existential) data types

I'd like to add that I see no problem with coerce :: Eq a b -> a -> b coerce ~Refl x = x as long as we have coerce _|_ x === _|_ The wish is that f = \refl -> Just . coerce refl = \~Refl x -> Just x should satisfy f _|_ x === Just _|_ f _|_ x =/= _|_ and likewise for any other constructor than Just. Regards, apfelmus