[GHC] #7931: Deriving Read of an empty datatype crashes

24 May 2013

      #7931: Deriving Read of an empty datatype crashes
-----------------------------+----------------------------------------------
Reporter:  monoidal          |          Owner:                  
    Type:  bug               |         Status:  new             
Priority:  normal            |      Component:  Compiler        
 Version:  7.6.3             |       Keywords:                  
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
 Failure:  None/Unknown      |      Blockedby:                  
Blocking:                    |        Related:                  
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 Standalone deriving Read of an empty datatype crashes:

 {{{
 {-# LANGUAGE StandaloneDeriving #-}
 data A
 deriving instance Read A
 }}}

 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.7.20130515 for x86_64-unknown-linux):
         Prelude.foldr1: empty list

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 (Oddly, the "panic" message is given only by ghc, not ghci, but this is
 minor.)

 While the Read instance seems to be of little use, it can be used for
 reading types such as `Either Void Int`.

 Patch attached.

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7931
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