#14070: Allow ‘unsafe’ deriving strategy, deriving code with ‘unsafeCoerce’ -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: | QuantifiedContexts, deriving Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Replying to [comment:12 Iceland_jack]:
ah! So this means we can also derive optic methods once `QuantifiedContexts` lands
I'm not sure what's special about "optic methods", but yes, I'm pretty sure `QuantifiedContexts` should let us be able to use `GeneralizedNewtypeDeriving` in any situation where we need to `coerce` underneath a higher-kinded type parameter (like the `f` in `Lens (f a) a`).
and (possibly / hopefully) that we can derive everything derivable?
I mean, saying that you can derive everything that is derivable is a bit of a tautology. But I think the answer to that question is yes :)
It does require `ImpredicativeTypes` written in the Haskell surface language, wasn't that removed in 8.2?
Alas, `ImpredicativeTypes` hasn't been removed yet. Simon laid out a proposal for doing so, while still preserving the ability to use `TypeApplications` to instantiate polytypes (as in `coerce @(forall a. Lens (t a) a)`) [https://mail.haskell.org/pipermail/ghc- devs/2016-September/012824.html here], but no one has drafted that up into a concrete GHC proposal yet. I'd do this myself, except I'm far too unfamiliar with the typechecker details to come up with a comprehensive document explaining how this change would work. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14070#comment:13 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler