How to use Swift generics to pass a type and return an object as that type?

Question

I'm writing a function that, given a type, simply unarchives an object from the disk and returns it as that type. This is what it looks like:

public class func objectWithFileName<T>(fileName: String, inFolder folder: NSSearchPathDirectory) -> T? {
    // Build an NSURL var called fileURL based on the fileName and folder.
    // ...

    // If any object was found
    if let object: AnyObject = NSKeyedUnarchiver.unarchiveObjectWithFile(fileURL.path!) {
        // Attempt to return the object as an object of type T.
        if let typedObject = object as? T {
            return typedObject
        }
    }
    return nil
}

The planned way of consuming the function is this, and it doesn't work (I'm getting "Cannot specialize a non-generic definition"):

if let user = MQFileManager.objectWithFileName<User>("User", inFolder: .DocumentDirectory) {

}

How can I pull this off correctly?

Answer 1

If you specify the type of the variable the return value is assigned to, type inference can do the rest. So if you have a User type, simply invoke the function as follows:

if let user: User = MQFileManager.objectWithFileName("User", inFolder: .DocumentDirectory) {

}

Since the user variable is of User type, the compiler can infer the T generic to be of User type