#15434: DerivingVia (and perhaps even GND) works badly with DeriveGeneric -------------------------------------+------------------------------------- Reporter: konn | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: Resolution: | Keywords: 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: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * cc: dfeuer (added) Comment: As a counterpoint, I quite dislike the idea of GHC demanding that users write instances a certain way. In fact, one of the very reasons I authored `DerivingStrategies` in the first place was because one couldn't derive classes like `Read` or `Show` by any means except `stock` deriving. True, 90% of the time, this is what you want, but for the other 10%, having explicit control through `DerivingStrategies` is quite handy. Replying to [comment:2 konn]:
I can't imagine the situation where `Generic` should give the different information than its true structure.
There are folks out there who write `Generic` instances which differ from what `stock` deriving would give you for various reasons. One reason is that some people like to define data types abstractly and only allow creating/matching on values of that data type through pattern synonyms. While I don't have any examples of people `newtype`/`via`-deriving `Generic` instances on hand, it's not inconceivable that folks might want to do this.
Hence, I think this behaviour should be enabled by default, even without `Safe` extension.
I [https://ghc.haskell.org/trac/ghc/ticket/13065 used to share this opinion], but I no longer do. I don't think GHC (or `Safe`) should be in the business of enforcing the structure of `Generic` instances. You'll never segfault from using a hand-written `Generic` instance (unlike, say, `Typeable`). I think what you want is for GHC to be able to verify that a particular `Generic` instance's structure actually matches the data type it was derived for. This is a reasonable desire, since there are certain properties about data types that can easily be discerned from a `Rep` instance, but only if you have confidence that the `Rep` instance isn't "lying". dfeuer (cc'd) has toyed with some ideas for how this could be achieved in GHC—perhaps he could chime in here. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15434#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler