DefaultCodable

DefaultCodable is a Swift package that provides a macro for automatically generating Codable conformance for types with default values. The @DefaultCodable macro expands to create the required CodingKeys enumeration and an initializer that decodes properties while falling back to their declared defaults when keys are missing.

Features

• Automatically synthesizes a CodingKeys enum for all stored properties.
• Generates an init(from:) that decodes each property using decodeIfPresent, returning to the property's default value when a key is not present.
• Basic type inference from literal initializers for properties that do not specify an explicit type.

Installation

Add the package to the dependencies of your Package.swift file:

.package(url: "https://github.com/Kihron/DefaultCodable.git", branch: "main")

and include "DefaultCodable" as a dependency for any target that should use the macro.

Usage

Import the library and apply @DefaultCodable to your Codable struct. All stored properties must provide a default value.

import DefaultCodable

@DefaultCodable
struct Foo: Codable {
    var x = 42
    var y = "hello"
}

During macro expansion the type becomes:

struct Foo: Codable {
    var x = 42
    var y = "hello"

    enum CodingKeys: String, CodingKey {
        case x
        case y
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        self.x = try container.decodeIfPresent(Int.self, forKey: .x) ?? 42
        self.y = try container.decodeIfPresent(String.self, forKey: .y) ?? "hello"
    }
}

Any missing values during decoding fall back to the defaults provided in the property declarations.

Building and Testing

Use swift build to compile the package and swift test to run the unit tests. These commands require fetching the SwiftSyntax dependency from the network.

swift build
swift test

Repository Layout

• Sources/DefaultCodable – the public macro definition.
• Sources/DefaultCodableMacros – implementation of the macro using SwiftSyntax.
• Sources/DefaultCodableClient – small executable target that demonstrates using the package.
• Tests/DefaultCodableTests – tests validating macro expansion behavior.

Contributing

Issues and pull requests are welcome. Feel free to open a discussion for feature requests or questions.