id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc
37	Equal Ints are not equal	guest	ross	"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@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 platforms  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@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
}}}"	defect	assigned	blocker		hugs	current