On Friday 26 August 2011, 19:24:37, Andrew Coppin wrote:
On 26/08/2011 02:40 AM, Daniel Peebles wrote:
And as Daniel mentioned earlier, it's not at all obvious what we mean by "bits used" when it comes to negative numbers.
I guess part of the problem is that the documentation asserts that bitSize will never depend on its argument. (So would will write things like "bitSize undefined :: ThreadID" or similar.)
I don't think that's a problem, it's natural for what bitSize does. And it would be bad if bitSize did something different for Integer than for Int, Word, ...
I can think of several possible results one might want from a bit size query:
Yup, though for some there are better names than bitSize would be.
1. The number of bits of precision which will be kept for values of this type. (For Word16, this is 16. For Integer, this is [almost] infinity.)
Not "almost infinity", what your RAM or Int allow, whichever cops out first, or "enough, unless you [try to] do really extreme stuff".
2. The amount of RAM that this value is using up. (But that would surely be measured in bytes, not bits. And processor registors make the picture more complicated.)
3. The bit count to the most significant bit, ignoring sign.
4. The bit count to the sign bit.
Currently, bitSize implements #1. I'm not especially interested in #2. I would usually want #3 or #4.
I'd usually be more interested in #2 than in #4.
Consider the case of 123 (decimal). The 2s complement representation of +123 is
...0000000000000001111011
The 2s complement representation of -123 is
...1111111111111110000101
For query #3, I would expect both +123 and -123 to yield 7.
One could make a case for the answer 3 for -123, I wouldn't know what to expect without it being stated in the docs.
For query #4, I would expect both to yield 8. (Since if you truncate both of those strings to 8 bits, then the positive value starts with 0, and the negative one start with 1.)
#4 would then generally be #3 + 1 for signed types, I think, so not very interesting, but for unsigned types?
Then of course, there's the difference between "count of the bits" and "bit index", which one might expect to be zero-based. (So that the Nth bit represents 2^N.)
Yes, but that shouldn't be a problem with good names. So, which of them are useful and important enough to propose for inclusion?