How to store a generic Measurement in Swift? - swift

Question

I'd like to have a variable which can represent a single Measurement but may have different unit types. For example, it could store a length or a mass. It seems so simple, but I can't figure it out.
Here's what I tried:
struct Data {
var weight: Measurement<UnitMass>
var length: Measurement<UnitLength>
var target = "Weight"
var valueOfTarget: Measurement<Unit> {
if target == "Weight" {
return weight
} else {
return length
}
}
}
var data = Data(weight: Measurement<UnitMass>(value: 10, unit: UnitMass.kilograms),
length: Measurement<UnitLength>(value: 10, unit: UnitLength.centimeters),
target: "Weight")
print(data.valueOfTarget)
I also tried using <Dimension> as suggested in another answer, but that had a similar error.
This results in a compiler error:
error: cannot convert return expression of type 'Measurement<UnitMass>' to return type 'Measurement<Unit>'
Am I'm missing something obvious or is this just not possible?

Answer 1

You could just create new generic return values. This seems to compile OK for me.
struct Data {
var weight: Measurement<UnitMass>
var length: Measurement<UnitLength>
var target = "Weight"
var valueOfTarget: Measurement<Unit> {
if target == "Weight" {
return Measurement<Unit>(value: weight.value, unit: weight.unit)
} else {
return Measurement<Unit>(value: length.value, unit: length.unit)
}
}
}
var data = Data(weight: Measurement<UnitMass>(value: 10, unit: UnitMass.kilograms),
length: Measurement<UnitLength>(value: 10, unit: UnitLength.centimeters),
target: "Weight")
print(data.valueOfTarget)

Answer 2

First, don't make your own struct named Data, because Foundation already has a type named Data that's pretty widely used.
Second, since (in a comment) you said “It's for weight loss, the user can choose a target as either a weight or a waist size”, it seems like you should probably model this using an enum rather than a struct:
enum WeightLossTarget {
case weight(Measurement<UnitMass>)
case waistSize(Measurement<UnitLength>)
}
Third, if you really need to use a struct, you can fall back to the Objective-C type NSMeasurement for non-generic type:
struct WeightLossTarget {
enum TargetType {
case weight
case waistSize
}
var weight: Measurement<UnitMass>
var waistSize: Measurement<UnitLength>
var target: TargetType
var valueOfTarget: NSMeasurement {
switch target {
case .weight: return weight as NSMeasurement
case .waistSize: return waistSize as NSMeasurement
}
}
}