Re: [GHC] #14047: "Illegal instance for type synonym" while deriving Typeable1 for data type

28 Jul 2017

      #14047: "Illegal instance for type synonym" while deriving Typeable1 for data type
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        Reporter:  alexbiehl     |                Owner:  (none)
            Type:  bug           |               Status:  new
        Priority:  normal        |            Milestone:
       Component:  Compiler      |              Version:  8.2.1
      Resolution:                |             Keywords:
Operating System:  Windows       |         Architecture:  x86_64 (amd64)
 Type of failure:  None/Unknown  |            Test Case:
      Blocked By:                |             Blocking:
 Related Tickets:                |  Differential Rev(s):
       Wiki Page:                |
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Description changed by alexbiehl:

Old description:
...
This occured to me in `network-ip` package today:
{{{#!hs
{-# LANGUAGE StandaloneDeriving #-}
module Repro where
import Data.Typeable (Typeable1)
data InetAddr a = InetAddr
deriving instance Typeable1 InetAddr
}}}
Results in GHC complaining with:
{{{#!hs
repro.hs:7:19: error:
    * Illegal instance for a type synonym
      A class instance must be for a class
    * In the stand-alone deriving instance for `Typeable1 InetAddr'
  |
7 | deriving instance Typeable1 InetAddr
  |                   ^^^^^^^^^^^^^^^^^^
}}}
This is due to `Typeable1` defined as type synonym for `Typeable` (see
here http://hackage.haskell.org/package/base-4.10.0.0/docs/Data-
Typeable.html#t:Typeable1).
I can see it is deprecated but I would still expect it to work correctly
still.
New description:

 This occured to me in `network-ip` package today:

 {{{#!hs
 {-# LANGUAGE StandaloneDeriving #-}
 module Repro where

 import Data.Typeable (Typeable1)

 data InetAddr a = InetAddr
 deriving instance Typeable1 InetAddr
 }}}

 Results in GHC complaining with:

 {{{#!hs
 repro.hs:7:19: error:
     * Illegal instance for a type synonym
       A class instance must be for a class
     * In the stand-alone deriving instance for `Typeable1 InetAddr'
   |
 7 | deriving instance Typeable1 InetAddr
   |                   ^^^^^^^^^^^^^^^^^^
 }}}

 This is due to `Typeable1` defined as type synonym for `Typeable` (see
 here http://hackage.haskell.org/package/base-4.10.0.0/docs/Data-
 Typeable.html#t:Typeable1).

 I see it is deprecated but I would still expect it to work correctly
 still.

--

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14047#comment:1
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