#9575: -XAutoDeriveTypeable fails to generate instances -------------------------------------+------------------------------------- Reporter: hvr | Owner: dreixel Type: bug | Status: new Priority: high | Milestone: 7.8.4 Component: Compiler | Version: 7.8.3 Resolution: | Keywords: Operating System: | Architecture: Unknown/Multiple Unknown/Multiple | Difficulty: Unknown Type of failure: | Blocked By: None/Unknown | Related Tickets: Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by hvr): Replying to [comment:2 rwbarton]:
See ticket:8950#comment:3.
I don't see what there is to gain by backporting a fix though, even if you could detect GHC 7.8.4, it's still more effort to do so than to just add `Typeable` to the deriving list. And it could break old code that uses `AutoDeriveTypeable`, by causing it to export more instances than it did under 7.8.3.
How would this break old code (unless orphan instances are involved) by exporting //more// instances? However, at the very least this should be documented (unless it's easy enough to fix) as a known-bug in 7.8.4's manual (should 7.8.4 be ever be released). Or GHC 7.8.4 could even warn when -XAutoDeriveTypeable is used to raise awareness, that it doesn't work as advertised in GHC 7.8, so people don't start using it next year with GHC 7.10 (where it works as intended), and then assume it worked in GHC 7.8 as well. Alas, missing instances like `Typeable` go easily undetected in libraries. And fwiw, you can discriminate GHC 7.8.4 quite easily via Cabal's `if impl(ghc>=7.8.4) ...` conditionals... -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9575#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler