[swift-evolution] Implicit truncation

Xiaodi Wu xiaodi.wu at gmail.com
Mon May 22 09:51:59 CDT 2017

On Mon, May 22, 2017 at 9:30 AM, Haravikk <swift-evolution at haravikk.me>

> On 22 May 2017, at 15:09, Xiaodi Wu via swift-evolution <
> swift-evolution at swift.org> wrote:
> Let's be clear: it _is_ rounded, just toward zero. It is consistent with
> the behavior of integer division. I would guess that your students also
> repeatedly struggle with the result that `2 / 3 == 0`? If so, they have not
> been taught some important fundamentals of integer arithmetic. If not, then
> it should be natural that `Int(2 / 3 as Double == Int(2 / 3 as UInt)`.
> Just to add my thoughts; but while I agree that it's important developers
> learn the intricacies of Int vs. Float, I don't think this is quite the
> same issue as divisions.
> If you're asking for an Int then you should have some idea that you're
> asking for a whole number only, and so that follows on to division as Ints
> simply cannot handle fractions.
> However, for conversion from Float there's an expectation that some
> rounding must occur, the problem is that it's not as expected; while you or
> I would expect it to be this way with familiarity of other languages, for
> someone new to the language this isn't always going to be the case. While
> it's reasonable to expect new developers to know what an Int is, I think
> it's unreasonable for them also to remember what the default rounding
> strategy of an implicitly rounding constructor is.

It's _not_ an arbitrarily chosen default rounding strategy to be
remembered. I mean, if you want to memorize it and move on, then of course
that's fine. But at heart it goes to what a decimal point is. Let's go back
to grade school. What does it mean to write "1.9"? Well, it means "one
whole thing and nine parts out of ten of a thing," or in other words, "1 +
9/10" or "19 / 10".  Now, what happens in Swift?

let x = Int(19 / 10) // x == 1
let y = Int(1 + 9 / 10) // y == 1
let z = Int(1.9) // z == 1

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

For this reason I tend to agree with the principle that the Int(_:Float)
> constructor should probably be labelled more intuitively, personally I'd
> like to see:
> func init(truncating:Float) { … }

Again, this particular naming suggestion has been discussed as part of the
review of integer protocols and not adopted. The rationale was that the
term "truncating" is intended to be left for bit patterns only. The term in
Swift is exclusively "rounded toward zero."

> 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))`.

Here the init(truncating:) constructor is just a convenience form of
> init(rounding:) with a strategy of .towardZero, which I believe is
> consistent with the current behaviour. It's also easily swapped in anywhere
> that init(_:Float) etc. are currently used.
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