[nhc-bugs] More on integer division

[Dylan Thurston]

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On Fri, Jun 28, 2002 at 03:54:50PM -0400, Dylan Thurston wrote:
> On Fri, Jun 28, 2002 at 10:24:13AM +0100, Malcolm Wallace wrote:
> > Yes,    -5`div`2  =3D=3D  -(5`div`2)  =3D=3D  -2
> > but   (-5)`div`2  =3D=3D  -3
> >=20
> > Ghc 5.02.2 has the infix priority wrong, and interprets the former as t=
he latter.
> > But more bizarrely, ghc 5.02.2 gets this very wrong:
> >=20
> >     Prelude> (-1796254192) `div` 357566600
> >     5
>=20
> Thanks for the clear summary.

After a looking a little more, there seem to be other problems
(including errors in my proposed solution).  I don't know where the
code for quotRem is, but it is also buggy.  For instance,

Prelude> 9 `quotRem` (-5)
(-1,4)

(The correct answer is (-1,-4).)  I'm frankly astonished: has noone
used these functions with negative arguments before?

This happens with both ghci and compiled programs, both -fvia-C and
-fasm.

Hugs (see below) and nhc make the same error in this case.

There seems to be too strong a reliance on C's operators '/' and '%',
which give these results (with gcc on x86).  This may be a bug in
gcc.  I'm using gcc 2.95.4; I'm not sure which version compiled the Debian
packages I'm using.  According to
http://home.tiscalinet.ch/t_wolf/tw/c/c9x_changes.html#Semantics, the
C9x standard specifies that division should truncate towards 0; I
believe that earlier it was undefined.

I'm shocked that non of the three Haskell implementations had a test
suite that caught this problem.

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||   || ||  || ||  || ||__      Hugs 98: Based on the Haskell 98 standard
||___|| ||__|| ||__||  __||     Copyright (c) 1994-2001
||---||         ___||           World Wide Web: http://haskell.org/hugs
||   ||                         Report bugs to: hugs-bugs@haskell.org
||   || Version: February 2001  _________________________________________

Haskell 98 mode: Restart with command line option -98 to enable extensions

Reading file "/usr/share/hugs98/lib/Prelude.hs":

Hugs session for:
/usr/share/hugs98/lib/Prelude.hs
Type :? for help
Prelude> 9 `quotRem` (-5)
(-1,4)
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