[swift-evolution] implementing real (floating point) number comparison tolerance as a compiler directive.

[Ted F.A. van Gaalen]

Hi Joe,

to just fuzzy compare 2 floating point numbers,
the solution you describe uses
- 2 functions, with therein 6 calls to frexp, abs, max and scalb...
isn’t that overkill ? 

I still think a compiler directive embedded in sources at desired locations as e.g.

@setFLoatingPointTolerance: 0.0001 
    . 
   if a == b 
   ...
@setFloatingPointToleranceOff
   …
@setFLoatingPointTolerance: 0.04 
    . 
   if temperature == roomTemperature 
   ...
@setFloatingPointToleranceOff

So every floating point compare in source between these directives will be 
will be compiled differently.
Leave it to the compiler, so no special functions/ parameters are needed,
and would be far more efficient I think.

Kind Regards
TedvG



> On 02.03.2016, at 22:28, Joe Groff <jgroff at apple.com> wrote:
> 
>> 
>> On Mar 2, 2016, at 1:23 PM, Tino Heth via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> I hope that I'll finally have the time to write a pitch for "inheritance for structs" this weekend — this looks like another good use case for a "newtype" feature:
>> 
>> struct Length: Double {
>> 	static let tolerance: Double = 0.001
>> }
>> 
>> override func ==(a: Length, b: Length) -> Bool {
>> 	return (a - b).abs() < Length.tolerance
>> }
>> 
>> Of course, this example leaves many questions, but I hope the principle is clear.
> 
> This is also a place where function partial application might be interesting:
> 
> func equalityWithTolerance(tolerance: Double) -> (Double, Double) -> Bool {
>   return {
>     var exp0 = 0, exp1 = 0
>     frexp($0, &exp0)
>     frexp($1, &exp1)
>     return abs($0 - $1) < scalb(tolerance, max(exp0, exp1))
>   }
> }
> 
> If you could locally bind operators, you could then do this:
> 
> func compareLengths(x: Double, y: Double) -> Bool {
>   let (==) = equalityWithTolerance(0x1p-44)
>   return x == y // Uses local (==)
> }
> 
> -Joe

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