[swift-evolution] floating point numbers implicit conversion

Stephen Canon scanon at apple.com
Mon Jun 19 12:58:28 CDT 2017

> On Jun 19, 2017, at 11:46 AM, Ted F.A. van Gaalen via swift-evolution <swift-evolution at swift.org> wrote:
> var result: Float = 0.0
> result = float * integer * uint8 +  double   
> // here, all operands should be implicitly promoted to Double before the complete expression evaluation.

You would have this produce different results than:

	let temp = float * integer * uint8
	result = temp + double

That would be extremely surprising to many unsuspecting users.

Don’t get me wrong; I *really want* implicit promotions (I proposed one scheme for them  way back when Swift was first unveiled publicly). But there’s a lot more subtlety around them than it seems (for example the C and C++ implicit promotion rules can easily be described on a half-sheet of paper, but are the source of *innumerable* bugs). I would rather have no implicit promotions than half-baked implicit promotions.

– Steve
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