[swift-evolution] Implicit truncation

Haravikk swift-evolution at haravikk.me
Mon May 22 17:21:00 CDT 2017

> On 22 May 2017, at 21:16, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> On Mon, May 22, 2017 at 10:39 Haravikk <swift-evolution at haravikk.me <mailto:swift-evolution at haravikk.me>> wrote:
>> On 22 May 2017, at 15:51, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
>> If we're to speak of intuition for new developers who've never used a programming language, who are falling back to what they know about mathematics, then quite literally a decimal point _is_ about division by ten.
> I don't think this necessarily follows; the issue here is that the constructor isn't explicit enough that it is simply lopping off the fractional part. My own experience of maths as taught in school, to go from a decimal to an integer I would expect to round,
> You would also expect that 3 / 4 in integer math gives you 1. With integer division, however, 3 / 4 == 0. By definition the decimal point separates an integer from a fractional part, so the behaviors are inextricably linked. To test this out in practice, I asked the first person with no programming experience I just encountered today.
> I said: "Let me teach you one fact about integers in a programming language. When two integers are divided, the integer result has the fractional part discarded; for example, 3/4 computes to 0. What would you expect to be the result of converting 0.75 to an integer?"
> He answered immediately: "I would have expected that 3/4 gives you 1, but since 3/4 gives you 0, I'd expect 0.75 to convert to 0."

These are two different case; Int(3) / Int(4) is a division of integer values with an integer result, there's no intermediate floating point value that needs to be coerced back into an Int. The issue here is converse of a Float/Double to an Integer, it's a different operation.

3.0 / 4.0 = 0.75 is a property of Float
3 / 4 = 0 is a property of Int

What's under discussion here is conversion between the two, they're not really comparable cases.

> Discarding the fractional part of a floating point value is a bit pattern operation only in the sense that any operation on any data is a bit pattern operation. It is clearly not, however, an operation truncating a bit pattern.
> as the conversion is simplistically taking the significand, dropping it into an Int then shifting by the exponent.
> That's not correct. If you shift the significand or the significand bit pattern of pi by its exponent, you don't get 3.

I think you're misunderstanding me. If you think of it in base 10 terms 1.2345 is equivalent to 12345 x 10-4; when you convert that to an Int it effectively becomes 12345 shifted four places to the right, leaving you with 1. In that sense it's a truncation of the bit-pattern as you're chopping part of it off, or at the very least are manipulating it.

Regardless it's also very literally a truncation since you're specifically truncating any fraction part, it's simply the most correct term to use; frankly I find restricting that to bit-pattern truncation to be entirely arbitrary and unhelpful. The types involved should make it clear whether the value is being made narrower or not. Int64 -> Int32 is a form of truncation, but so to is Float -> Int; in both cases the target can't represent all values of the source, so something will be lost.

>> 	func init(rounding:Float, _ strategy: FloatingPointRoundingRule) { … }
>> Again, here, as an addition to the API, this fails the six criteria of Ben Cohen, as it is strictly duplicative of `T(value.rounded(strategy))`.
> Maybe, but init(rounding:) is explicit that something is being done to the value, at which point there's no obvious harm in clarifying what (or allowing full freedom). While avoiding redundancy is good as a general rule, it doesn't mean there can't be any at all if there's some benefit to it; in this case clarity of exactly what kind of rounding is taking place to the Float/Double value.
> The bar for adding new API to the standard library is *far* higher than "some benefit"; `Int(value.rounded(.up))` is the approved spelling for which you are proposing a second spelling that does the same thing.

The main benefit is that the constructor I proposed would actually require the developer to do this, what you're showing is entirely optional; i.e- any value can be passed without consideration of the rounding that is occurring, or that it may not be as desired. With a label the constructor at least would remind the developer that rounding is occurring (i.e- the value may not be as passed). Going further and requiring them to provide a rounding strategy would also force them to consider what method of rounding should actually be used, eliminating any confusion entirely. What you're demonstrating there does not provide any of these protections against mistakes, as you can omit the rounding operation without any warning, and end up with a value you didn't expect.

A secondary benefit is that any rounding that does take place can do so within the integer type itself, potentially eliminating a Float to Float rounding followed by truncation; i.e- since rounding towards zero is the same as truncation it can optimise away entirely.
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