#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 jpbernardy): Ok, I am just pointing out that the implementation is weird from my point of view. I don't want to make a big fuss about this, but I feel that it's something that should be considered. I suppose in a few years someone may raise the issue again :) Regarding blame: The point is that if you get a value of an empty type in your context, it is the caller which is to blame. The proper way to report this blame at runtime is force the caller to produce the value, by forcing it. If you raise an exception yourself you get the blame instead, and so it is harder to track which code is really wrong. I think that a good entry point to the concept of blame is "Well-typed programs can't be blamed" http://homepages.inf.ed.ac.uk/wadler/topics/blame.html -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7401#comment:33 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler