#9757: Warn about derivable instances -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: feature | Status: closed request | Milestone: Priority: normal | Version: 7.9 Component: Compiler | Keywords: Resolution: wontfix | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: | Related Tickets: None/Unknown | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by goldfire): I'm not sure if this works with the way HLint is organized, but there is a decent way of detecting placing where GND can be used: take the Core produced from the instance definition of a newtype `N` (wrapping representation type `T`), strip out all coercions, and compare it against the Core for `T`'s instance. If each method in the instance for `N` is either a call to the method for `T`'s instance ''or'' symbol-for-symbol identical to `T`s instance, then GND is suitable. I think this algorithm would catch the common cases easily enough. But, like I said, I don't know how well looking at Core code matches with the way HLint normally goes about its business. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9757#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler