[darcs-devel] [patch1880] decouple RepoPatchV3 impl from NamedPrim

[Ganesh Sittampalam]

Ganesh Sittampalam <ganesh at earth.li> added the comment:

> Sure. It still bugs me that we have to go through (unsafely) unpacking
> and then repacking the Sealed here. I have tried lots of alternatives
> but nothing works that isn't essentially the same as what we have now.
> IMO the strictness restriction for pattern matching existentials (and
> GADTs in general) is really annoying and I wish they would lift that. I
> opened an issue here: https://gitlab.haskell.org/ghc/ghc/issues/17130

If the GADT embeds a proof then matching on it has to be strict, or you
can get an unsound program, e.g. you could write

 case undefined :: Int :~: Bool of
  ~Refl -> 5 :: Bool

But that's not true of a pure existential, so some kind of restricted
lazy match would be possible in theory.

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