What is wrong with my enum decoding in Swift?

Question

I just tried this:

let test = "{ \"de\": \"Bisasam\", \"en\": \"Bulbasaur\" }"
let data = test.data(using: .utf8)!
do {
    let result = try JSONDecoder().decode([String: String].self, from: data)
    print("") // breakpoint here works
} catch {
    print("")
}

This works fine, however when I try to do this instead:

enum Lg: String, CaseIterable, Decodable {
    case deutsch = "de"
    case english = "en"
}

let test = "{ \"de\": \"Bisasam\", \"en\": \"Bulbasaur\" }"
let data = test.data(using: .utf8)!
do {
    let result = try JSONDecoder().decode([Lg: String].self, from: data)
    print("")
} catch {
    print("") // breakpoint here
}

With this error:

▿ DecodingError ▿ typeMismatch : 2 elements

.0 : Swift.Array ▿ .1 : Context

codingPath : 0 elements

debugDescription : "Expected to decode Array but found a dictionary instead."

underlyingError : nil

What am I doing wrong?

Thanks for your help

See Why does Int enum as dictionary key, produce different json string than Int as dictionary key? for relevant information about JSON and enum keys. — HangarRash
– HangarRash, Commented Nov 9, 2023 at 23:07
Here is a nice solution to your issue.... fivestars.blog/articles/codable-swift-dictionaries — burnsi
– burnsi, Commented Nov 10, 2023 at 0:17

matt · Accepted Answer · 2025-06-13 10:05:07Z

When you use a non-String non-Int type as the dictionary key, Dictionary's Decodable implementation expects an array of key value pairs, i.e.:

["de", "foo", "en", "bar"]

This decodes to [.deutsch: "foo", .english: "bar"].

JSON keys can only be strings after all, and the Decodable implementation doesn't know how to convert a random Decodable value into a string. It could have been made to check for RawRepresentable with RawValue == String, but it wasn't.

One thing that it does check for though, is CodingKeyRepresentable, if you just conform Lg to that, then you can decode a JSON dictionary correctly.

Conforming a type to CodingKeyRepresentable lets you opt in to encoding and decoding Dictionary values keyed by the conforming type to and from a keyed container, rather than encoding and decoding the dictionary as an unkeyed container of alternating key-value pairs.

extension Lg: CodingKeyRepresentable, CodingKey {
    init?<T>(codingKey: T) where T : CodingKey {
        self.init(rawValue: codingKey.stringValue)
    }
    
    var codingKey: CodingKey {
        self
    }
}

Note that I also conformed it to CodingKey to make it easier to write the codingKey property required by CodingKeyRepresentable.

Collectives™ on Stack Overflow

What is wrong with my enum decoding in Swift?

1 Answer 1

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related