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[SR-7498] Decoder needs containerKeyedWithUnknownKeys #50041

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mattneub opened this issue Apr 22, 2018 · 2 comments
Open

[SR-7498] Decoder needs containerKeyedWithUnknownKeys #50041

mattneub opened this issue Apr 22, 2018 · 2 comments
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Codable Area → standard library: `Codable` and co. improvement standard library Area: Standard library umbrella

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@mattneub
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Previous ID SR-7498
Radar None
Original Reporter @mattneub
Type Improvement
Additional Detail from JIRA
Votes 0
Component/s Standard Library
Labels Improvement, Codable
Assignee None
Priority Medium

md5: 7ce81accf23b6883abf11011686d9146

Issue Description:

Currently, if the keys to my JSON dictionary are unknown in advance, I have to write a "mop-up" CodingKey adopter struct, like this:

        struct Top : Decodable {
            init(from decoder: Decoder) throws {
                struct CK : CodingKey {
                    var stringValue: String
                    init?(stringValue: String) {
                        self.stringValue = stringValue
                    }
                    var intValue: Int?
                    init?(intValue: Int) {
                        return nil
                    }
                }
                let con = try! decoder.container(keyedBy: CK.self)
                for key in con.allKeys {
                    // find out what the keys are, decode the values, deal with it
                }
            }
        }

This is boilerplate so it shouldn't be necessary. Decoder should provide a method e.g. `containerKeyedWithUnknownKeys` that does the work for me.
@hamishknight
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Collaborator

Personally I think this would be best solved by adding an AnyCodingKey type to the standard library, which would look something like this (I was going to make a Swift Evolution pitch but never got round to it):

struct AnyCodingKey : CodingKey {
  var stringValue: String
  var intValue: Int?

  init(_ codingKey: CodingKey) {
    self.stringValue = codingKey.stringValue
    self.intValue = codingKey.intValue
  }

  init(stringValue: String) {
    self.stringValue = stringValue
    self.intValue = nil
  }

  init(intValue: Int) {
    self.stringValue = String(intValue)
    self.intValue = intValue
  }
}

Then you'd be able to ask for containers keyed by AnyCodingKey.self. Actually the original motivation for me wanting this was when using the .custom key strategy for JSONEncoder/JSONDecoder – it requires a closure that returns a CodingKey, so you have to manually build your own CodingKey wrapper, which feels pretty sub-optimal.

Adding AnyCodingKey would certainly reduce boilerplate in both cases.

@mattneub
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Brilliant point, thanks @hamishknight

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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Codable Area → standard library: `Codable` and co. improvement standard library Area: Standard library umbrella
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@mattneub @hamishknight