Swift - Take Nil as Argument in Generic Function with Optional Argument

user_3398839 0
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I am trying to create a generic function that can take an optional argument. Here's what I have so far:

func somethingGeneric<T>(input: T?) {
    if (input != nil) {
        print(input!);
    }
}

somethingGeneric("Hello, World!") // Hello, World!
somethingGeneric(nil) // Errors!

It works with a String as shown, but not with nil. Using it with nil gives the following two errors:

error: cannot invoke 'somethingGeneric' with an argument list of type '(_?)'
note: expected an argument list of type '(T?)'

What am I doing wrong and how should I correctly declare/use this function? Also, I want to keep the usage of the function as simple as possible (I don't want do something like nil as String?).

function-declaration swift null function generics
7 Answers

Here is the solution I came up with that compiles on Swift 5, as many of the solutions here did not compile for me. It might be considered hacky as I use a stored variable to help things along. I was unable to come up with a Swift 5 version of the nil parameters that resolve to type T.

class MyClass {
    func somethingGeneric<T>(input: T?) {
        if let input = input {
            print(input)
        }
    }

    func somethingGeneric() {
        somethingGeneric(Object.Nil)
    }

}

final class Object {
    static var Nil: Object? //this should never be set
}
user_8165460

Actually there is a way to do this, inspired by Alamofire's internal code.

You do not have to install Alamofire to use this solution.

Usage

Your problematic method definition

func someMethod<SomeGenericOptionalCodableType: Codable>(with someParam: SomeGenericOptionalCodableType? = nil) {
        // your awesome code goes here
}

What works ✅

// invoke `someMethod` correctly 
let someGoodParam1 = Alamofire.Empty.value
someMethod(with: someGoodParam1)

I think it is possible to use Alamofire.Empty.value as a default value in someMethod definition as a parameter.

What does not work ❌

// invoke `someMethod` incorrectly
let someBadParam1: Codable? = nil
let someBadParam2 = nil
someMethod(with: someBadParam1)
someMethod(with: someBadParam2)

Solution definition (source)

/// Type representing an empty value. Use `Empty.value` to get the static instance.
public struct Empty: Codable {
    /// Static `Empty` instance used for all `Empty` responses.
    public static let value = Empty()
}
user_108300910
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