[GHC] #11535: Derived instance of Read/Show causes runtime exception for infix Unicode value constructors

3 Feb 2016


      #11535: Derived instance of Read/Show causes runtime exception for infix Unicode
value constructors
-------------------------------------+-------------------------------------
           Reporter:  nushio         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #1544, #11529
Differential Rev(s):                 |         Wiki Page:
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 When I run the following program `test-show.hs` on ghc 7.10.3:

 {{{#!hs
 data Y = Д星 String  String  deriving (Eq, Ord, Show, Read)

 data X = String :☆ String  deriving (Eq, Ord, Show, Read)

 main :: IO ()
 main = do
   let y = Д星 "" ""
       d = 0
   print $ map fromEnum $ show y
   print $ (y, "") `elem` (readsPrec d (showsPrec d y ""))
   print $ (read $ show y :: Y)

   let x = "" :☆ ""
       d = 0
   print $ map fromEnum $ show x
   print $ (x, "") `elem` (readsPrec d (showsPrec d x ""))
   print $ (read $ show x :: X)
 }}}

 I got the following results:

 {{{
 $ stack ghc -- --version
 The Glorious Glasgow Haskell Compilation System, version 7.10.3
 $ stack runhaskell test-show.hs
 [1044,26143,32,34,34,32,34,34]
 True
 Д星 "" ""
 [34,34,32,58,9734,32,34,34]
 False
 test-show.hs: Prelude.read: no parse
 }}}

 This behavior does not conform to Section 11.4 of Haskell 2010 Language
 Specification:
 https://www.haskell.org/onlinereport/haskell2010/haskellch11.html#x18-186000...
 :
...
It should be the case that
    (x,"") is an element of (readsPrec d (showsPrec d x ""))
--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11535
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