JSON to Swift

Conformance to Codable is the whole of JSON decoding in Swift — no annotations, no library, and no generated decoder, provided the property names line up with the keys. This produces the structs, let properties throughout, and a CodingKeys enum for the cases where the names do not line up.

Swift's mapping is the tightest of the nine targets, because the language has an optional type and a synthesised initialiser and needs neither a builder nor an annotation to use them:

• IntJSON integers, not Int64. Int is 64-bit on every platform Swift ships on, and the shorter spelling is what Swift code actually uses.
• DoubleAnything with a fractional part. Codable will happily decode a JSON integer into a Double, so a field that is sometimes 1 and sometimes 1.5 is safe.
• String?Optional or nullable. Codable treats an absent key and an explicit null identically for an optional property, which is either exactly what you want or a distinction you will need to make by hand.
• AnyCodable?A field with no observable type. Not a standard library type — see below.
• letEvery property. A decoded response is a value, and the memberwise initialiser comes free.

AnyCodable is not in the standard library. What do I do with it?

Right — it is a placeholder for a field that was null or empty in every record, and there is nothing in Foundation that means "any JSON value". Either replace it with the concrete type once you have a populated sample, or add one of the small AnyCodable implementations from the community. It is left in deliberately: emitting String? would be a guess presented as a fact.

Why do only some structs have a CodingKeys enum?

It appears only when at least one key differs from its property name. Where they match, Codable synthesises the mapping and the enum is dead weight — and the enum is all-or-nothing, so one mismatched key means listing every case.

Why a struct rather than a class?

A decoded response is a value: two responses with the same fields are the same response, and nothing should be mutating one through a second reference. Structs give you that, plus the memberwise initialiser. Reach for a class only when you genuinely need identity or inheritance.

Will this handle a snake_case API without CodingKeys?

It will, and then you do not need the enum at all: set decoder.keyDecodingStrategy = .convertFromSnakeCase and Codable does the renaming. Worth knowing before pasting a CodingKeys block with thirty cases in it — though the enum is more explicit, and it survives a key that the strategy converts wrongly.

How are optional fields decided?

Every record in an array is inspected, not just the first. A field that appears in some records and not others is typed as optional, and a value that is sometimes null widens accordingly. Generators that read only the first element get this wrong on exactly the payloads where it matters.

What happens to a key that is a reserved word?

It is renamed to something legal in the target language, and where the language supports it, an annotation records the original JSON key so serialisation still round-trips.