#7401: Can't derive instance for Eq when datatype has no constructor, while it is trivial do do so. -------------------------------------+------------------------------------- Reporter: jpbernardy | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.1 Resolution: | Keywords: deriving, | newcomer Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D978 Wiki Page: | -------------------------------------+------------------------------------- Description changed by osa1: @@ -1,34 +1,10 @@ - mpickering reminded me this ticket so before I leave the ownership of this - ticket let me write down the current status: - - Basically there are a couple of things we can do and we just need someone - to - decide which one. - - 1. Implement {{{==}}} as {{{_ == _ = True}}}. Pro: this is consistent with - {{{Void}}}'s {{{Eq}}} instance. Con: I think this lets some bugs to sneak - in. I think it doesn't make sense to compute things based on equality of - empty types, so at this point you've probably made a mistake and it's - better to just bail out. - - 2. Implement {{{==}}} as `x == y = case x of {}`. Pro: it fixes the - problem with (1) and in a good way - you've probably used something like - {{{error "..."}}} for the empty type terms, so you get your error message. - Con: this inconsistent with {{{Void}}}. - - 3. Leave this as is - no {{{deriving (Eq)}}}. This way we make the user - decide which behaviour to have. Pro: You can have whatever you want. Con: - Inconvenient. - - These were also discussed at some point but IIRC most people agree that - these - are bad ideas: - - 4. {{{x == y = error "(==) on empty type"}}}: If it's going to fail it's - better to fail like (2). - - 5. {{{_ == _ = False}}}: Denotationally / type-theoretically bottoms are - equal or something like that? So it's better to implement (1) instead. - - 6. Implement (2), change {{{Void}}}s instance too: Can potentially break - code. + On a "phantom datatype" D, one gets the message: + {{{ + Can't make a derived instance of `Eq D': + `D' must have at least one data constructor + }}} + However there is a trivial, correct instance of Eq D: + {{{ + instance Eq D where + (==) = undefined + }}} New description: On a "phantom datatype" D, one gets the message: {{{ Can't make a derived instance of `Eq D': `D' must have at least one data constructor }}} However there is a trivial, correct instance of Eq D: {{{ instance Eq D where (==) = undefined }}} -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7401#comment:47 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler