#7401: Can't derive instance for Eq when datatype has no constructor, while it is trivial do do so. -------------------------------------+------------------------------------- Reporter: jpbernardy | Owner: osa1 Type: feature request | Status: new Priority: normal | Milestone: 7.12.1 Component: Compiler | Version: 7.6.1 Resolution: | Keywords: deriving, Operating System: Unknown/Multiple | newcomer Type of failure: GHC rejects | Architecture: valid program | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: Phab:D978 -------------------------------------+------------------------------------- Comment (by simonpj): Responding to Phab here, because the design discussion is less likely to get lost here. 1. Thank you for the user manual update. Can you add a paragraph or two explaining the design choice? After all, if I did `deriving( Eq )` on an empty data type, or standalone deriving, I would expect a static error; the generated code is never useful. But on the contrary, we now propose to give a dynamic error, postponing a crash until runtime. Why is this good? Explain either in the user manual or on a wiki page that discusses. comment:6 seems relevant, but is cryptic. 2. I'm certain that standalone deriving and `deriving(...)` on a data type declaration should behave the same. Imagine trying to explain the difference in the user manual, if they differed! Can you make it so? 3. For Ix, Enum, Bounded, the [https://www.haskell.org/onlinereport/haskell2010/haskellch19.html#x27-227000... 2010 report] uses words like "derived instance declarations for the class Ix are only possible for enumerations (i.e. datatypes having only nullary constructors) and single-constructor datatypes". It seems odd to make them fail with a static error if Eq etc fail with a dynamic error. We can interpret the words "having only nullary constructors" as true for an empty data type; after all all the constructors it has are nullary. So let's make `isEnumeration` true for empty types too. (Make a `Note` to explain; and the user manual.) Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7401#comment:25 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler