I agree with all of your points. And about this first.. Yes, I think this
variant is better:
2) Works quite well on assigning values, and works as expected with a lot
of control of allowed operations, etc. However can make things verbose as
you also mentioned when needing to define many different types, but that's
a limitation of Swift not supporting this "Typed Numerics" natively rather
than your code :)
3) Requires an extra byte to store the enumeration value internally, so
whereas Double is 8 bytes, Distance would be 9 bytes. Sometimes it might
be
desirable to store the original type the value was assigned with, for
example .km, and only convert to appropriate type when reading, whereas
other times it's best to convert immediately to the preferred type on
assignment which would be .m (or meters) in this case, perhaps use the
"default" keyword next to the type?. Also compiler support would be
advantageous here also, so that constant expressions can be evaluated at
compile time with no overhead, so that if I put... let dist : Distance =
125.km <http://125.km>, dist is converted at compile time to 125000.m.
4) Agree that this "feature" should rather be a type alias, as we dealing
with types rather than a structure.
5) If Swift could support the shortened typed numeric syntax natively, and
with the efficiency that can only happen in the compiler itself so that it
works as fast and efficiently as normal untyped numbers that would be
awesome :)
6) I think a feature like this could make Swift even more useful in
scientific and other critical applications where they deal with many
different types of numbers, such as velocity, acceleration, gravity,
power,
watts, distance, etc. etc.
Also normal everyday code, for example,
func circle(radius: Double) ... //Is that radius in pixels, meters,
millimetres or something else? But if radius was defined as our fancy
Distance (assuming we add pixels (px) too :), then they could call it
with:
circle(radius: 150.px) //or
circle(radius: 100.mm <http://100.mm>) //or
circle(radius: 10.cm <http://10.cm>)
//etc.
This would help make the code clearer, and lead to better quality code and
hopefully less bugs.
Regards,
Nur
On Mon, Aug 22, 2016 at 6:07 PM, Vladimir.S <svabox@gmail.com >> <mailto:svabox@gmail.com>> wrote:
The first question is what the meaning of , for example, a
multiplication of Distance ? I.e.
let x1 : Distance = 10.m
let x2 : Distance = 10.km <http://10.km>
let x3 = x1 * x2 // ???
I.e. as soon as your Distance is Double, you allows all kind of
floating point operations on instances of this type. And some such
operations has no meaning for Distance. So, in your proposal, you need
to somehow control allowed operations.
And I believe you can get what you want right now with enums(If I'm
not
missing something):
enum Distance {
case km(Double)
case m(Double)
case mm(Double)
func inKilometers() -> Double {
return inMeters() / 1000.0
}
func inMeters() -> Double {
switch self {
case .km(let value) : return value * 1000.0
case .m(let value) : return value
case .mm(let value) : return value / 1000.0
}
}
func inMillimeters() -> Double {
return inMeters() * 1000.0
}
static func +(lhs: Distance, rhs: Distance) -> Distance {
return .m(lhs.inMeters() + rhs.inMeters())
}
static func -(lhs: Distance, rhs: Distance) -> Distance {
return .m(lhs.inMeters() - rhs.inMeters())
}
// implement needed operations here..
}
extension Double {
var km : Distance { return Distance.km(self) }
var m : Distance { return Distance.m(self) }
var mm : Distance { return Distance.mm(self) }
}
let dist : Distance = 10.km <http://10.km> + 5.m + 5.mm <http://5.mm>
print(dist.inMeters()) // 10005.005
print(dist.inKilometers()) // 10.005005
More verbose, but more control. From other point of view, currently
you'll need a lot of code for each such type(like Distance).
But I do think that such ability to create custom domain-specific
types
based on standard value types is a useful feature which can improve a
quality of code and can reduce the number of bugs.
The syntax to declare such type should be simple and clear, to be able
to declare a number of such types without a lot of boilerplate code.
Something like this:
typealias Distance : Double {
var km : Distance { return self * 1000.0 }
var m : Distance { return self }
var mm : Distance { return self / 1000.0 }
inherit [+,-,/]
}
IMO such type is more a type alias than new structure
On 22.08.2016 17 <tel:22.08.2016%2017>:54, Nur Ismail via >> swift-evolution wrote:
Hi,
I'm new to the list, but have an idea for Typed Numerics.
Basically numeric values (such as Double, Int, etc.) that are
strongly
typed to a specific use case, for example Distance, Weight, etc.
and cannot
be intermixed with untyped values.
So if I have (the syntax is made up, but perhaps something like
this):
=====
//Distance
struct fixedtype Distance : Double {
var km: …
var m: …
typealias meters: m
var feet: ...
...
}
//Weight
struct fixedtype Weight : Double {
var kg: …
var g: …
typealias grams : g
var pound: ...
}
…
var weight : Weight = 5.kg <http://5.kg> <http://5.kg/> + 5.g +
7.m
………………………………………..^ Compiler Error: Can’t add Distance to Weight...
var distance: Distance = 7.km <http://7.km> <http://7.km/> +
12.5.m
+ 5.0 + 3
………………………………………………...^ Compiler Error: can’t add untyped number to
Distance...
===
The main restriction this syntax should do is disallow
intermixing of
numeric types (even if they all descend from Double, Int, etc.)
and not
allow adding untyped numerics (i.e. those without a type suffix),
unless
explicitly asked for in the code.
Any of these can be converted to it's raw untyped value, for
example:
=====
let number : Double = distance.rawValue + 5.0 //This is allowed
distance += number.m //number is converted to m (meters)
=====
>From the Swift 3 Language guide, we are for example given the
following
example:
=====
extension Double {
var km: Double { return self * 1_000.0 }
var m: Double { return self }
var cm: Double { return self / 100.0 }
var mm: Double { return self / 1_000.0 }
var ft: Double { return self / 3.28084 }
}
let aMarathon = 42.km <http://42.km> <http://42.km/> + 195.m
print("A marathon is \(aMarathon) meters long")
// Prints "A marathon is 42195.0 meters long"
=====
This is quite nice to suffix a conversion method after the value,
but if I
had another extension that converts the values to pounds and
kilograms,
then one can illegally do this:
let aValue = 42.km <http://42.km> <http://42.km/> + 195.m +
17.pounds + 5.0
and then the code would still compile and run, but not work as
intended.
Extra reading, and inspiration for a feature like the above:
Mars Probe Lost Due to Simple Math Error
Mars Probe Lost Due to Simple Math Error
<http://articles.latimes.com/1999/oct/01/news/mn-17288>
<Mars Probe Lost Due to Simple Math Error
<http://articles.latimes.com/1999/oct/01/news/mn-17288>>
"NASA lost its $125-million Mars Climate Orbiter because
spacecraft
engineers failed to convert from English to metric measurements
when
exchanging vital data before the craft was launched, space agency
officials
said Thursday.
A navigation team at the Jet Propulsion Laboratory used the metric
system
of millimeters and meters in its calculations, while Lockheed
Martin
Astronautics in Denver, which designed and built the spacecraft,
provided
crucial acceleration data in the English system of inches, feet
and
pounds.
As a result, JPL engineers mistook acceleration readings measured
in
English units of pound-seconds for a metric measure of force
called
newton-seconds."
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