[swift-users] Detect if a generic type is numeric

[davelist at mac.com]

> On Oct 1, 2017, at 2:32 PM, Glenn L. Austin <glenn at austinsoft.com> wrote:
> 
>> 
>> On Oct 1, 2017, at 8:56 AM, Dave Reed via swift-users <swift-users at swift.org> wrote:
>> 
>> 
>>> On Sep 21, 2017, at 3:58 PM, V T via swift-users <swift-users at swift.org> wrote:
>>> 
>>> Hi there!
>>> 
>>> Is there a best way to check if a given type conforms to numeric protocol (Integer or FP) at runtime?
>>> 
>>> func checkNumeric<T>(_ value: T) {
>>> 	/* return true if vaiue is Integer or FP */
>>> 	/* this will not compile: */
>>> 	if value is Numeric {
>>> 		
>>> 	}
>>> }
>>> 
>>> Best regards!
>>> 
>>> VT
>>> 
>> 
>> I think the way to do it is to try casting as the type, but you can't use "as? Numeric" as you get:
>> 
>> error: protocol 'Numeric' can only be used as a generic constraint because it has Self or associated type requirements
>> 
>> but you could check for each specific numeric type such as:
>> 
>> func checkNumeric<T>(_ value: T) {
>>    if (value as? Int != nil) || (value as? Float != nil) {
>>        print("numeric")
>>    } else {
>>        print("not numeric")
>>    }
>> }
>> 
>> checkNumeric(3)
>> checkNumeric(3.0)
>> checkNumeric("3")
> 
> You can also use the 'is' operator, as in 'value is Int || value is Float || value is Double'
> 
> -- 
> Glenn L. Austin, Computer Wizard, AustinSoft.com


Ah, I had forgotten "is" works in Swift

Just be careful as:

func checkNumeric<T>(_ value: T) {
    if (value is Int) || (value is Float) {
        print("numeric")
    } else {
        print("not numeric")
    }
}

checkNumeric(3)
checkNumeric(3.0)
checkNumeric("3")

outputs:

numeric
not numeric
not numeric

Since the literal 3.0 is a Double so you'd have to catch every floating point type (including CGFloat, I suspect etc.) vs. just trying to cast to a Float (although I guess casting to a Float could have unexpected results also if someone made an extension that allows a type to be cast as a Float).

Dave