[GHC] #7401: Can't derive instance for Eq when datatype has no constructor, while it is trivial do do so.
GHC
ghc-devs at haskell.org
Wed Jun 17 11:34:32 UTC 2015
#7401: Can't derive instance for Eq when datatype has no constructor, while it is
trivial do do so.
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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
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Comment (by jpbernardy):
simonpj: I'm not sure we're talking about the same thing. The whole point
of my request was that, for any class C a where all methods have an `a`
argument, it is trivial to implement a meaningful instance C Foo for any
empty type Foo.
In fact, I am guessing that the instance generator for Eq and Ord generate
case statements. So, one could just let the normal instance generator run,
to get function definitions with
empty case statements.
I see now that the instance for Read is fine (because failure is the only
option), but that is more than I was asking.
--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7401#comment:30>
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