[GHC] #11509: Incorrect error message in StandaloneDeriving: "The data constructors of <typeclass> are not all in scope"

29 Jan 2016

      #11509: Incorrect error message in StandaloneDeriving: "The data constructors of
<typeclass> are not all in scope"
-------------------------------------+-------------------------------------
           Reporter:  edsko          |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1-rc1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Consider

 {{{#!hs
 {-# OPTIONS_GHC -Wall #-}
 {-# LANGUAGE ConstraintKinds            #-}
 {-# LANGUAGE FlexibleInstances          #-}
 {-# LANGUAGE GADTs                      #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE KindSignatures             #-}
 {-# LANGUAGE ScopedTypeVariables        #-}
 {-# LANGUAGE StandaloneDeriving         #-}
 {-# LANGUAGE StaticPointers             #-}
 {-# LANGUAGE UndecidableInstances       #-}
 {-# LANGUAGE UndecidableSuperClasses    #-}
 module Bug where

 import Data.Kind
 import Data.Typeable
 import GHC.StaticPtr

 {-------------------------------------------------------------------------------
   Standard Cloud-Haskell-like infrastructure

   See <https://ghc.haskell.org/trac/ghc/wiki/TypeableT> for a dicussion of
 'SC'.
 -------------------------------------------------------------------------------}

 class Serializable a -- empty class, just for demonstration purposes

 instance Serializable a => Serializable [a]

 data Static :: * -> * where
   StaticPtr :: StaticPtr a -> Static a
   StaticApp :: Static (a -> b) -> Static a -> Static b

 staticApp :: StaticPtr (a -> b) -> Static a -> Static b
 staticApp = StaticApp .  StaticPtr

 data Dict :: Constraint -> * where
   Dict :: c => Dict c

 class c => SC c where
   dict :: Static (Dict c)

 instance (Typeable a, SC (Serializable a)) => SC (Serializable [a]) where
   dict = aux `staticApp` dict
     where
       aux :: StaticPtr (Dict (Serializable a) -> Dict (Serializable [a]))
       aux = static (\Dict -> Dict)

 {-------------------------------------------------------------------------------
   Demonstrate the bug
 -------------------------------------------------------------------------------}

 newtype MyList a = MyList [a]

 deriving instance (Typeable a, SC (Serializable a)) => SC (Serializable
 (MyList a))
 }}}

 This gives the following type error:

 {{{
 Bug1.hs:40:1: error:
     • Can't make a derived instance of ‘SC (Serializable (MyList a))’:
         The data constructors of ‘Serializable’ are not all in scope
           so you cannot derive an instance for it
     • In the stand-alone deriving instance for
         ‘SC (Serializable a) => SC (Serializable (MyList a))’
 }}}

 This of course doesn't make much sense: `Serializable` is a type class,
 not a datatype, and doesn't have data constructors.

 I wasn't sure if this deriving clause was going to work at all, or whether
 I would expect it to. Since `MyList` is a newtype wrapper around `[a]`,
 and we have the requisite instance

 {{{#!hs
 instance (Typeable a, SC (Serializable a)) => SC (Serializable [a])
 }}}

 I was kind of hoping that `GeneralizedNewtypeDeriving` would work its
 magic. However, even if it cannot, the error message should probably
 change.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11509>
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