Re: [GHC] #13314: StandaloneDeriving and DeriveAnyClass don't work together

21 Feb 2017

      #13314: StandaloneDeriving and DeriveAnyClass don't work together
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        Reporter:  chris-martin      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by chris-martin:

Old description:
...
I'm starting with the `SPretty` example from the `DeriveAnyClass` section
in the GHC users guide:
{{{#!hs
#!/usr/bin/env stack
-- stack --resolver lts-8.0
{-# LANGUAGE DefaultSignatures, DeriveAnyClass, StandaloneDeriving #-}
import Prelude
import Numeric.Natural (Natural)
class SPretty a where
    sPpr :: a -> String
    default sPpr :: Show a => a -> String
    sPpr = show
}}}
I can write an empty instance for `Natural`:
{{{#!hs
instance SPretty Natural where
}}}
So then I would expect to be able to write an equivalent definition using
standalone deriving:
{{{#!hs
deriving instance SPretty Natural
}}}
But instead it fails with this error:
{{{
error:
    • Can't make a derived instance of ‘SPretty Natural’:
        The data constructors of ‘Natural’ are not all in scope
          so you cannot derive an instance for it
    • In the stand-alone deriving instance for ‘SPretty Natural’
}}}
It seems like this ought to work; I'm not sure why the constructors
should need to be in scope, given that the instance can be derived
trivially without defining any methods.
New description:

 I'm starting with the `SPretty` example from the `DeriveAnyClass` section
 in the GHC users guide:

 {{{#!hs
 #!/usr/bin/env stack
 -- stack --resolver lts-8.0
 {-# LANGUAGE DefaultSignatures, DeriveAnyClass, StandaloneDeriving #-}

 import Prelude
 import Numeric.Natural (Natural)

 class SPretty a where
     sPpr :: a -> String
     default sPpr :: Show a => a -> String
     sPpr = show
 }}}

 I can write an empty instance for `Natural`:

 {{{#!hs
 instance SPretty Natural where
 }}}

 So then I would expect to be able to write an equivalent definition using
 standalone-deriving and derive-and-class:

 {{{#!hs
 deriving instance SPretty Natural
 }}}

 But instead it fails with this error:

 {{{
 error:
     • Can't make a derived instance of ‘SPretty Natural’:
         The data constructors of ‘Natural’ are not all in scope
           so you cannot derive an instance for it
     • In the stand-alone deriving instance for ‘SPretty Natural’
 }}}

 It seems like this ought to work; I'm not sure why the constructors should
 need to be in scope, given that the instance can be derived trivially
 without defining any methods.

--

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13314#comment:2
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