On 19/08/2013, at 3:38 AM, Nicolas Frisby wrote:
> The docs at
>
>
> http://www.haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:gcd
>
> give a NB mentioning that (abs minBound == minBound) is possible for
> fixed-width types.
At least three ways to represent negative integers in binary have been used:
- twos-complement (asymmetric, -INT_MIN == INT_MIN, abs can have a negative
result)
- ones-complement (symmetric, -INT_MIN == INT_MAX, abs is always
non-negative)
- sign-and-magnitude (symmetric, -INT_MIN == INT_MAX, abs is always
non-negative)
Having used a B6700 as an undergraduate, I still think sign-and-magnitude is the
only really safe-and-simple scheme. However, twos-complement has conquered.
The argument for twos-complement, which always puzzled me, is that the other
systems have two ways to represent zero. I never found this to be a problem,
not even for bitwise operations, on the B6700. I *did* find "abs x < 0"
succeeding to be a pain in the posterior. (The B6700 had two different tests:
'are these two numbers equal' and 'are these two bit patterns equal'.)
> Two questions:
>
> 1) This behavior surprised me. Does it surprise enough people to include a
> warning in the Haddock for abs and negate?
We cannot expect everyone who uses Haskell to be familiar with
the eccentricities of popular hardware. I think it's worth mentioning.
> 2) Is this a common behavior in other languages?
Yes.
> My tinkering with gcc suggests it does not support the value -2^63, but
> instead bottoms out at (-2^63+1).
Your tinkering was insufficient.
f% cat 2c.c
#include <stdio.h>
#include <limits.h>
int main(void) {
printf("%d\n", INT_MIN + INT_MAX);
return 0;
}
f% gcc 2c.c
f% a.out
-1
Oh wait. You said "63", not "31".
Change the key line to
printf("%lld\n", LLONG_MIN + LLONG_MAX);
LLONG_MIN is going to be -(2**63) on any SPARC, x86, x86-64,
Power(PC), Alpha, ARM, MIPS, or z/Series machine, and a host
of others.
>
> Thanks.
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