Swift Gyb

The gyb.py plugin for SwiftPM project.

Usage

GYB is a lightweight templating system that allows you to use Python code for variable substitution and flow control:

• The sequence %{ code } evaluates a block of Python code
• The sequence % code: ... % end manages control flow
• The sequence ${ code } substitutes the result of an expression

A good example of GYB can be found in Codable.swift.gyb. At the top of the file, the base Codable types are assigned to an instance variable:

%{
codable_types = ['Bool', 'String', 'Double', 'Float',
                 'Int', 'Int8', 'Int16', 'Int32', 'Int64',
                 'UInt', 'UInt8', 'UInt16', 'UInt32', 'UInt64']
}%

Later on, in the implementation of SingleValueEncodingContainer, these types are iterated over to generate the methods declarations for the protocol’s requirements:

% for type in codable_types:
  mutating func encode(_ value: ${type}) throws
% end

Evaluating the GYB template results in the following declarations:

mutating func encode(_ value: Bool) throws
mutating func encode(_ value: String) throws
mutating func encode(_ value: Double) throws
mutating func encode(_ value: Float) throws
mutating func encode(_ value: Int) throws
mutating func encode(_ value: Int8) throws
mutating func encode(_ value: Int16) throws
mutating func encode(_ value: Int32) throws
mutating func encode(_ value: Int64) throws
mutating func encode(_ value: UInt) throws
mutating func encode(_ value: UInt8) throws
mutating func encode(_ value: UInt16) throws
mutating func encode(_ value: UInt32) throws
mutating func encode(_ value: UInt64) throws

This pattern is used throughout the file to generate similarly formulaic declarations for methods like encode(_:forKey:), decode(_:forKey:), and decodeIfPresent(_:forKey:). In total, GYB reduces the amount of boilerplate code by a few thousand LOC:

$ wc -l Codable.swift.gyb
2183 Codable.swift.gyb
$ wc -l Codable.swift
5790 Codable.swift

Ref: Swift GYB - NSHipster

Adding swift-gyb as a Dependency

To use the swift-gyb plugin in a SwiftPM project, add it to the dependencies for your package and your .swift.gyb files contained target:

let package = Package(
  // name, platforms, products, etc.
  dependencies: [
    // other dependencies
    .package(url: "https://github.com/swift-library/swift-gyb", from: "0.0.1"),
  ],
  targets: [
    .executableTarget(
      name: "<command-line-tool>",
      dependencies: [
        // other dependencies
      ],
      plugins: [
        .plugin(name: "GybPlugin", package: "swift-gyb"),
      ]
    ),
    // other targets
  ]
)

Supported Versions

The most recent versions of swift-gyb support Swift 5.8 and newer. The minimum Swift version supported by swift-gyb releases are detailed below:

swift-gyb	Minimum Swift Version
0.0.1	5.8