Re: [GHC] #13759: Strange error message for deriving Data

30 May 2017

      #13759: Strange error message for deriving Data
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        Reporter:  alanz             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12245            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 alanz helped me figure out what was going on. You won't hit the bug with
 the original code, but you will with this slightly tweaked code:

 {{{#!hs
 {-# LANGUAGE DeriveDataTypeable #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE StandaloneDeriving #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE FlexibleInstances #-}

 import Data.Data

 data Pass = Parsed | Renamed | Typechecked
          deriving (Data)

 data GHC (c :: Pass)

 deriving instance Data GhcPs
 deriving instance Data GhcRn
 deriving instance Data GhcTc

 -- Type synonyms as a shorthand for tagging
 type GhcPs  = GHC 'Parsed
 type GhcRn  = GHC 'Renamed
 type GhcTc  = GHC 'Typechecked
 }}}

 Crucially, we're deriving instances for the type synonyms.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13759#comment:2
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