[GHC] #9035: ghci sometimes displays Word32 as Word64

24 Apr 2014

      #9035: ghci sometimes displays Word32 as Word64
--------------------------+------------------------------------------------
       Reporter:          |             Owner:
  MikeIzbicki             |            Status:  new
           Type:  bug     |         Milestone:
       Priority:  normal  |           Version:  7.8.2
      Component:  GHCi    |  Operating System:  Linux
       Keywords:          |   Type of failure:  Incorrect result at runtime
   Architecture:  x86_64  |         Test Case:
  (amd64)                 |          Blocking:
     Difficulty:          |
  Unknown                 |
     Blocked By:          |
Related Tickets:          |
--------------------------+------------------------------------------------
 Given this code:

 {{{
 module Main
     where

 import Data.Word
 import Unsafe.Coerce
 import System.IO

 nanFloat :: Float
 nanFloat = unsafeCoerce (maxBound :: Word32)

 float2word32 :: Float -> Word32
 float2word32 = unsafeCoerce

 nanDouble :: Double
 nanDouble = unsafeCoerce (maxBound :: Word64)

 double2word64 :: Double -> Word64
 double2word64 = unsafeCoerce

 main = do
     putStrLn $ "nanFloat  = " ++ show (float2word32 nanFloat)
     putStrLn $ "nanFloat  = " ++ show (float2word32 $ nanFloat + 1)
     putStrLn $ "nanDouble = " ++ show (double2word64 nanDouble)
     putStrLn $ "nanDouble = " ++ show (double2word64 $ nanDouble + 1)
 }}}

 If we compile with GHC and run, we correctly output:

 {{{
 nanFloat  = 4294967295
 nanFloat  = 4294967295
 nanDouble = 18446744073709551615
 nanDouble = 18446744073709551615
 }}}

 But if we instead load in ghci, we get the following output:

 {{{
 nanFloat  = 4294967295
 nanFloat  = 140247862083583
 nanDouble = 18446744073709551615
 nanDouble = 18446744073709551615
 }}}

 For some reason, ghci is displaying (nanFloat+1) as having significantly
 more digits than can possibly stored in a Word32 value.

 Test system: Intel Core 2 Duo running Debian with GHC 7.8.2

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9035
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler