Re: [Haskell-cafe] abs minBound < (0 :: Int) && negate minBound == (minBound :: Int)

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:gc...

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 #include 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.

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