Re: Equality predicates, signed zeroes, R5RS and R6RS
> From:Mark H Weaver > To:guile-devel@gnu.org > Cc:Andy Wingo ; Taylor R Campbell > Sent:Monday, January 31, 2011 11:53 AM > Subject:Equality predicates, signed zeroes, R5RS and R6RS > > An issue has come to my attention that deserves wider discussion. > > Since at least Guile 1.8, (= 0.0 -0.0) has returned #t > but (eqv? 0.0 -0.0) has returned #f, and this is still the case. > > PLT Scheme agrees with us that (eqv? 0.0 -0.0) is #f, but MIT/GNU Scheme, > SCM, Chicken, and Gauche all return #t in this case. As an aside, as of a couple of months ago, it was possible to create the following seven versions of zero in Guile using the default reader. 0, ±0.0, ±0.0±0.0i Note that there is no way to make negative zero integer (-0). And there are no zero rationals: ±0/2, for example. Each scheme has its quirks. MzScheme 4.1 would let you create complex numbers with integer representation real and float representation imaginary parts: 0+0.0i, for example. The integer parts can only be positive zero and the float parts can be ±0.0. But as a physicist, I declare that this is all madness. -Mike
Re: Equality predicates, signed zeroes, R5RS and R6RS
Mike Gran writes: > As an aside, as of a couple of months ago, it was possible to create > the following seven versions of zero in Guile using the default reader. > > 0, ±0.0, ±0.0±0.0i Really? I don't understand how this could have happened. mem2complex uses scm_make_rectangular to create complex numbers, and scm_make_rectangular calls scm_c_make_rectangular, which creates a real if the imaginary part is zero. I looked at the history, and I don't see any relevant changes to this behavior in the last several years at least, unless the default reader formerly used something other than mem2complex, and created the numbers without scm_c_make_rectangular. In any case, I will soon submit patches to allow us to create all seven of these zeroes again, as required by R6RS. > But as a physicist, I declare that this is all madness. I agree that it doesn't sit entirely well with me either. I would like for computer languages to be mathematically rigorous in their behavior. However, having looked closely at the rationale behind the signed zeroes, I can see why the designers of IEEE 754 mandated them. They are important when working with functions involving branch cuts, as the cuts are approached from either side and underflow may change a small number into a zero. Making robust numerical programs is unfortunately a messy business. Best, Mark
Re: Equality predicates, signed zeroes, R5RS and R6RS
> From:Mark H Weaver > To:Mike Gran > Cc:"guile-devel@gnu.org" > Sent:Monday, January 31, 2011 2:42 PM > Subject:Re: Equality predicates, signed zeroes, R5RS and R6RS > > Mike Gran writes: > > As an aside, as of a couple of months ago, it was possible to create > > the following seven versions of zero in Guile using the default reader. > > > > 0, ±0.0, ±0.0±0.0i > > Really? I don't understand how this could have happened. mem2complex > uses scm_make_rectangular to create complex numbers, and > scm_make_rectangular calls scm_c_make_rectangular, which creates a real > if the imaginary part is zero. Sorry, I was completely wrong with that. I was looking at the output for few different Schemes, and I got confused. As you say, Guile 2.0 only gets you, 0 and ±0.0. Sorry for the noise. -Mike
Re: Equality predicates, signed zeroes, R5RS and R6RS
On Mon 31 Jan 2011 20:53, Mark H Weaver writes: > Given that everyone agrees that `eqv?' must distinguish 0 from 0.0, it > is already not useful as a numerical `='. Any program that uses it this > way is asking for trouble. Therefore, I don't have qualms about keeping > our existing behavior, namely that (eqv? 0.0 -0.0) returns #f. > > What do you think? This sounds OK to me. I don't have a horse in the race, but the argument that (eqv? 0 0.0) => #f is convincing, especially given the 0.0-or-0 games that many procedures are allowed to play. Andy -- http://wingolog.org/
