[cl-debian] Bug#433592: clisp: Float/rational comparison sometimes underflows; should never

[J.P. Larocque]

Package: clisp
Version: 1:2.41-1
Severity: normal

Hi,

I'm trying to compare a floating-point number and a rational.
According to the rule of float and rational contagion, described in
CLHS 12.1.4.1, and X3J13 issue
CONTAGION-ON-NUMERICAL-COMPARISONS:TRANSITIVE, such a comparison
should always return the mathematically correct result, as if the
floating-point number were first converted to a rational with
RATIONAL.  Instead, CLISP signals an underflow condition:

$ clisp -q -x '(< (expt 10 -100) least-positive-short-float)'
*** - floating point underflow

The Implementation Notes for GNU CLISP, sections 12.2.3.1 and
12.2.3.2, don't seem to say anything about comparisons, but mention
two variables which control ANSI-compliance.  CLISP behaves the same
when invoked with "-ansi", to set these and any similar variables to
T.

$ clisp -ansi -q -x '(< (expt 10 -100) least-positive-short-float)'
*** - floating point underflow

This X3J13 issue is listed as supported in the Implementation Notes for GNU CLISP:
http://clisp.cons.org/impnotes.html#issues

CLISP behaves as expected if the floating-point number is filtered
through RATIONAL.  (Note that CLHS 12.1.4.1 explicitly states "When
rationals and floats are compared by a numerical function, the
function rational is effectively called to convert the float to a
rational and then an exact comparison is performed.")

$ clisp -q -x '(< (expt 10 -100) (rational least-positive-short-float))'
T
$ clisp -q -x '(< (expt 10 -10) (rational least-positive-short-float))'
NIL

I'm fairly certain CLISP is in error.

Thanks,

-- System Information:
Debian Release: 4.0
  APT prefers stable
  APT policy: (500, 'stable')
Architecture: amd64 (x86_64)
Shell:  /bin/sh linked to /bin/bash
Kernel: Linux 2.6.18-12-custom-xen-ws-1
Locale: LANG=C, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8)

Versions of packages clisp depends on:
ii  common-lisp-controller      6.9          This is a Common Lisp source and c
ii  libc6                       2.3.6.ds1-13 GNU C Library: Shared libraries
ii  libice6                     1:1.0.1-2    X11 Inter-Client Exchange library
ii  libncurses5                 5.5-5        Shared libraries for terminal hand
ii  libreadline5                5.2-2        GNU readline and history libraries
ii  libsm6                      1:1.0.1-3    X11 Session Management library
ii  libx11-6                    2:1.0.3-7    X11 client-side library
ii  libxext6                    1:1.0.1-2    X11 miscellaneous extension librar
ii  libxpm4                     1:3.5.5-2    X11 pixmap library

clisp recommends no packages.

-- no debconf information

-- 
J.P. Larocque: <piranha at thoughtcrime.us>, <piranha at ely.ath.cx>