What is this function parameter for?

Question

func load<T: Decodable>(_ filename: String, as type: T.Type = T.self) -> T {
    let data: Data

    guard let file = Bundle.main.url(forResource: filename, withExtension: nil)
        else {
            fatalError("Couldn't find \(filename) in main bundle.")
    }

    do {
        data = try Data(contentsOf: file)
    } catch {
        fatalError("Couldn't load \(filename) from main bundle:\n\(error)")
    }

    do {
        let decoder = JSONDecoder()
        return try decoder.decode(T.self, from: data)
    } catch {
        fatalError("Couldn't parse \(filename) as \(T.self):\n\(error)")
    }
}

load("jsonFilePath")

The function extracts the data form a JSON. What is that 'parameter' type: T.Type = T.self for? If I remove this parameter the code still works. So what do I need it for?

Not related to your question but you can use a single do catch — Leo Dabus, Jun 06 '19 at 21:46
Regarding your question, You don’t need as long as you explicitly specify the resulting type somewhere — Leo Dabus, Jun 06 '19 at 21:46
You need to assign the resulting object to a variable. Related https://stackoverflow.com/questions/47544675/flatten-any-array-swift/47544741?r=SearchResults&s=1%7C43.7365#47544741 — Leo Dabus, Jun 06 '19 at 22:14

Kamran · Accepted Answer · 2019-06-06T23:23:07.327

5

This parameter is just a helper to use three kind of declarations as below,

1) let model: Model = self.load("Countries")
2) let model = self.load("Countries", as: Model.self)
3) let model = self.load("Countries") as Model

You can remove it from the method signature if you want to use the first kind of declaration.

edited Jun 06 '19 at 23:23

answered Jun 06 '19 at 22:32

Kamran

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3) let model = self.load("Countries") as Model – Leo Dabus Jun 06 '19 at 23:21
I am trying to understand where T.self comes in. Is that somehow automatically assumed based on the type declaration in an assignment? – Chris Jul 08 '20 at 17:32
@Chris Yes, Swift auto inference will figure out the actual type. – Kamran Jul 09 '20 at 04:44

What is this function parameter for?

1 Answers1