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iOS Error Handling in Swift Error Handling Handling Errors

Is it me or is the unit test? Using `keys.contains`

I'm trying to determine whether my code isn't working the way I expect it to and my understanding is flawed or the unit test/compiler is too inflexible to account for my solution.

I wanted to try to solve this challenge using the keys.contains method rather than data["someKey"] != nil since that's what the directions suggest.

Here's the code I wrote:

enum ParserError: Error {
    case emptyDictionary
    case invalidKey
}

struct Parser {
    var data: [String : String?]?

    func parse(data dict: [String : String?]) throws {
        guard let data = data else {
            throw ParserError.emptyDictionary
        }

        guard data.keys.contains("someKey") else {
            throw ParserError.invalidKey
        }
    }
}

let data: [String : String?]? = ["someKey": nil]
let parser = Parser(data: data)

// NOTE: I'm not including the following in the code challenge, just checking it in Xcode to validate the same output with valid data.
let validData: [String : String?]? = ["someKey": "I'm valid"]
let validParser = Parser(data: validData)

This returns the following if I fully expand it:

["someKey": nil]
    (key "someKey", nil)
        key "someKey"
        nil
["someKey": "I'm valid"]
    (key "someKey", value "I'm valid")
        key "someKey"
        value "I'm valid"

Unfortunately, I get a compiler error from the Treehouse compiler and when I try to preview it it shakes and gives me no feedback.

After searching around I found this in one of the answers:

struct Parser {
    var data: [String : String?]?

    func parse() throws {
        guard data != nil else {
            throw ParserError.emptyDictionary
        }

        guard data?.keys.contains("someKey") == true else {
            throw ParserError.invalidKey
        }
    }
}

This returns:

["someKey": nil]
    (key "someKey", nil)
        key "someKey"
        nil
["someKey": "I'm valid"]
    (key "someKey", value "I'm valid")
        key "someKey"
        value "I'm valid"

Unless I'm missing something, the output of the function is identical and presumably should pass Treehouse's unit tests — especially since my code is closer to the style @pasan seems to favor in the course — so I'm surprised I'm not able to pass the challenge.

I have to admit, I'm still having a hard time fully wrapping my head around optionals.

Ah, it definitely is different because with my method I have to pass it data and unwrap it but in the second solution I just call the method

Mine:

let data: [String : String?]? = ["someKey": nil]
let parser = Parser(data: data)

do {
    try parser.parse(data: data!)
} catch {
    print("Invalid")
}

vs

let data: [String : String?]? = ["someKey": nil]
let parser = Parser(data: data)

do {
    try parser.parse()
} catch {
    print("Invalid")
}

However, I don't understand why, is this a scope problem?