[Hugs] #37: Equal Ints are not equal

[Neil Mitchell]

Hi,

I experimented with this a bit, in particular the following function
generates output:

check unused = head [(int,x,x2) | x <- [-10000..10000], let int = tn
x, let x2 = ti int, x2 /= x]

(the unused is just to stop hugs saving the CAF)

This generates some output, even though it doesn't under GHC. The
output I got first was:

(-9646 in your data structure,-9646,1300), so it appears that ti is
the function that is going wrong, and in unpredictable ways - for
example doing this again gives a different error on a different
number.

The ti function looks incredibly simple, and I can only guess that
there is some memory corruption going on? Something like that, as the
program does different things on different executions.

Unfortunately I couldn't find any repeatable specific test case, but
the above check function is pretty much guaranteed to get it wrong
every time. This is using May 2006.

Thanks

Neil


On 9/22/06, Hugs <trac at galois.com> wrote:
> #37: Equal Ints are not equal
> ----------------------+-----------------------------------------------------
>   Reporter:  guest    |       Owner:  nobody
>       Type:  defect   |      Status:  new
>   Priority:  blocker  |   Milestone:
>  Component:  hugs     |     Version:  current
> Resolution:           |    Keywords:
> ----------------------+-----------------------------------------------------
> Comment (by guest):
>
>  {{{
>  Hi,
>
>  a few days ago I wrote a program which handles my own integers. Then I
>  check my own implementation against the normal Int implementation in Hugs.
>  Thereby I wonder about inequality of Ints which are definitely equal.
>
>  This is a test program I wrote after I recognized that hugs
>  has probably a bug in the inequality check of Ints.
>
>
>    data MInt = Zero | Succ MInt | Pred MInt deriving Show
>
>    tn :: Int -> MInt
>    tn x | x<0 = Pred (tn (x+1))
>    tn 0     =  Zero
>    tn (n+1) = Succ (tn n)
>
>    ti :: MInt -> Int
>    ti Zero = 0
>    ti (Succ x) = 1+(ti x)
>    ti (Pred x) = (ti x) -1
>
>    testi :: (MInt -> MInt -> MInt) -> (Int -> Int -> Int) -> Int -> Int ->
>  Bool
>    testi f g x y = (ti (f (tn x) (tn y))) /= (g x y)
>
>    myMul x y = tn ((ti x) * (ti y))
>
>    test = [(x,y,ti (myMul (tn x) (tn y)),x * y)|
>              x<-[-100..100],
>              y<-([-100..(-1)]++[1..100]),(testi myMul (*)  x y) ]
>
>  the list "test" should be empty in any case
>  but unfortunately hugs returns this (copy of the console):
>
>    __   __ __  __  ____   ___
>  _________________________________________
>    ||   || ||  || ||  || ||__      Hugs 98: Based on the Haskell 98
>  standard
>    ||___|| ||__|| ||__||  __||     Copyright (c) 1994-2003
>    ||---||         ___||           World Wide Web: http://haskell.org/hugs
>    ||   ||                         Report bugs to: hugs-bugs at haskell.org
>    ||   || Version: November 2003
>  _________________________________________
>
>    Haskell 98 mode: Restart with command line option -98 to enable
>  extensions
>
>    Type :? for help
>    Main> test
>    [(-100,80,-8000,-8000),(-100,99,-9900,-9900),
>    (-99,58,-5742,-5742),(-99,83,-8217,-8217),
>    (-98,71,-6958,-6958),(-98,86,-8428,-8428),
>    (-97,46,-4462,-4462),(-97,87,-8439,-8439),
>    (-96,76,-7296,-7296),(-95,88,-8360,-8360),(-94,73,-6862,-6862)
>    ERROR - Control stack overflow
>    Main>
>
>  I have checked this program with Hugs on different plattforms:
>  Gentoo Linux x86 64-Bit and Suse Linux 32-Bit. On both plattforms
>  the same bug occurs with different Ints for each run (i.e. the list
>  contains different tuples). I have also checked my program with
>  other Haskell implementations and there it returns an empty list
>  as expected.
>
>  Best regards,
>  Stephan Swiderski (swiderski at informatik.rwth-aachen.de)
>
>  P.S.: We have tested Sep2006 with similar results:
>  __   __ __  __  ____   ___      _________________________________________
>  ||   || ||  || ||  || ||__      Hugs 98: Based on the Haskell 98 standard
>  ||___|| ||__|| ||__||  __||     Copyright (c) 1994-2005
>  ||---||         ___||           World Wide Web: http://haskell.org/hugs
>  ||   ||                         Bugs: http://hackage.haskell.org/trac/hugs
>  ||   || Version: September 2006 _________________________________________
>
>  Haskell 98 mode: Restart with command line option -98 to enable extensions
>
>  Type :? for help
>  Main> :e
>  Main> test
>  [(-100,90,-9000,-9000),(-98,40,-3920,-3920),(-97,63,-6111,-6111),(-97,69,-6693,-
>  6693)
>  ERROR - Control stack overflow
>
>  }}}
>
> --
> Ticket URL: <http://hackage.haskell.org/trac/hugs/ticket/37>
> Hugs <http://www.haskell.org/hugs/>
> Hugs 98, an interpreter for Haskell
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>