Re: popCount is a method of Bits, not a FiniteBits

A few points: 1. The Bits instance for Integer essentially already behaves as if they are the 2-adic numbers. For example, if you use testBit you can discover that (-2 :: Integer) is treated as if it were an infinite sequence of 1's followed by a single 0. 2. However, because testBit takes an Int index, instances of Bits actually can't have an infinite number of 1 bits---at least not in practice---since you cannot observe anything past the (maxBound :: Int)th bit. 3. Currently, popCount on negative Integer values satisfies popCount (-x) = -(popCount x) which does not seem very well motivated and does not match the way negative values are presented via testBit. I am strongly -1 on moving popCount to FiniteBits. popCount on positive Integers is useful and well-defined. I am mildly +1 on Zemlya's proposal to change the behavior of popCount for negative Integer values, though I am not sure it is really worth the trouble. -Brent On Sat, Nov 30, 2019 at 10:10 AM Zemyla wrote:

popCount is a perfectly sensible method for Natural, and it could theoretically become one for Integer as well if we say that, whenever there's an infinite number of 1s and a finite number of 0s, then the result is -(1 + count of 0s), as though it were maxBound :: Word bits in size and merely converted to an Int (a sensible assumption, considering memory limits). The results for types where there can be both infinite 0s and 1s should still be an error.

On Sat, Nov 30, 2019, 09:21 Oleg Grenrus wrote:

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Although, popCount for Integer/Natural kind of makes sense, as they aren't infinite list of [Bit]s, but smarter structure.

On 30.11.2019 17.17, Oleg Grenrus wrote:

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I propose to change it to be member of FiniteBits

I recall, there was a proposal to remove bitSize from Bits, so it's an opportunity to introduce another small, yet breaking change at the same time.

Discussion time 2 week.

- Oleg

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