[GHC] #9600: Bad error message: Applicative is not a derivable class

[GHC]

#9600: Bad error message: Applicative is not a derivable class
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       Reporter:  nh2                |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:  7.10.1
      Component:  Compiler           |                 Version:  7.8.3
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Easy (less than 1  |         Type of failure:
  hour)                              |  None/Unknown
     Blocked By:                     |               Test Case:
Related Tickets:                     |                Blocking:
                                     |  Differential Revisions:
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 You have `GeneralizedNewtypeDeriving` on and use `deriving Applicative`
 with something that you cannot derive Applicative on, GHC correctly says
 `cannot eta-reduce the representation type enough`.

 If you don't have `GeneralizedNewtypeDeriving` on yet and deriving
 Applicative will work, GHC says something like "perhaps you want to enable
 GeneralizedNewtypeDeriving"?

 However, if you don't have `GeneralizedNewtypeDeriving` AND if GHC figures
 out that if you enable it, the next message will be `cannot eta-reduce the
 representation type enough`, then GHC will state the most unhelpful
 `Applicative is not a derivable class`, which suggests to me that '''in
 general''', Applicative is not derivable (which is wrong).

 rwbarton's suggestion: Suggest enabling `GeneralizedNewtypeDeriving` in
 any case and then let the user see why it cannot be derived.

 Some relevant #ghc chat:


 {{{
 rwbarton, that's GND again
 rwbarton: I a ~ K a where K = ErrorT ImmError (ReaderT CliOptions (ReaderT
 Config IO)) is an instance of Applicative
 rwbarton: so it can coerce the instance for K to an instance for I
 nh2:      so is it the error message that is the confusing part here? It
 suggests that "In general, Applicative is not derivable"
 nh2:      I mean it as: Shouldn't it give an error message like "can't
 derive an Applicative instance in this case because ..."?
 rwbarton: well maybe it could add something like "Did you mean to use
 GeneralizedNewtypeDeriving?"
 rwbarton: oh I see what you mean, even if you turn that extension on it
 gives the same error message
 nh2:      yes, because we already have such a message in some cases
 nh2:      no, in this case I had really forgotten the extension. If I add
 it, I get the "cannot eta-reduce the representation type enough" error,
 but I haven't figured out why I get it in this case yet
 rwbarton: I wrote the condition above basically, "I a ~ K a where K = ...
 is an instance of Applicative" where ~ is the coercion/isomorphism
 introduced by the newtype
 rwbarton: if you can't write down K then you can't use GND because where
 would it get the instance?
 rwbarton: writing down K is eta reduction
 nh2:      I'm wondering whether it doesn't suggest adding the extension
 because it already figured out I will not be able to eta-reduce?
 rwbarton: oh yeah, it seems so
 rwbarton: that seems a bit silly
 nh2:      in this case, I think we should pimp the error message a bit
 nh2:      (this is the thing I want to derive bwt: https://github.com
 /ganeti-github-testing/ganeti-
 test-1/blob/master/src/Ganeti/BasicTypes.hs#L147)
 nh2:      what do you think would be the best error message to show in
 such a case?
 nh2:      just suggest the GNTD suggestion regardless and let the user see
 afterwards that it cannot work?
 rwbarton: I would just have it always suggest GND ... yeah
 nh2:      ok I file a ticket for that
 rwbarton: seems like more suggestions > fewer suggestions
 rwbarton: even if this is a weird second-order case where there are two
 errors
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9600>
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