How To Write a Swift Generic Function Based Only On Return Type

TL;DR	Annotate the return variable with a type, e.x. let foo:String? = Utility.nullableValueFromKey(“identifier", dictionary: jsonDictionary)

Versions:	OS X 10.10.4	Xcode 6.4	iOS SDK 8.4

When I started working with Objective-C coming from .NET, one of language features that I missed the most was Generics. They solve a whole class of problems that are tedious and/or require way more code than without generics. When Swift was announced with Generics...

But Generics can be hard, thinking in T for any giving problem can make you a little crazy, especially when the compiler keeps yelling at you.

In Swift 1.0, I used generic functions to help parsing JSON server responses, the functions worked, but they were less than ideal.

The typical problem I wanted to solve was getting a primitive type out of the response dictionary that could be null.

I ended up with this function to do the trick:

class func originalNullableValue<T>(valueType: T, key: String, dictionary: NSDictionary) -> T? {

    var value:T? = nil

    var valueTemp = dictionary[key] as AnyObject! as? T

    if valueTemp != nil {

        value = valueTemp!

    }

    return value

}

Ugly! Why did I end up with this? Either I wasn’t smart enough to figure this out or the 1.0 compiler wasn’t.

Getting either me or the compiler to figure out what type T was without passing an argument of that type into the method, was, well let’s just say it was the solution I found.

What I wanted was this:

class func nullableValueFromKey<T>(key: String, dictionary: NSDictionary) -> T? {

var value:T? = nil

    var valueTemp = dictionary[key] as AnyObject! as? T

    if valueTemp != nil {

        value = valueTemp!

    }

    return value

}

So I dusted off the original method and tried making it what I wanted with Xcode 6.4 & Swift 1.2.

Defining it works fine, but If you attempt to call it:

let foo = JSONUtility.nullableArrayFromKey("fooBar", json: Dictionary<String, AnyObject>())

The compiler returns this error:

I have no way to know/test if this is the same error that caused me trouble in Swift 1.0, but this time, either I or the compiler were smart enough to figure it out!

All you have to do is add the type to the variable declaration:

let bar:String? = JSONUtility.nullableValueFromKey("name", dictionary: Dictionary<String, AnyObject>())