Swift GYB

The term “boilerplate” goes back to the early days of print media. Small regional newspapers had column inches to fill, but typically lacked the writing staff to make this happen, so many of them turned to large print syndicates for a steady flow of content that could be added verbatim into the back pages of their dailies. These stories would often be provided on pre-set plates, which resembled the rolled sheets of steel used to make boilers, hence the name.

Through a process of metonymy, the content itself came to be known as “boilerplate”, and the concept was appropriated to encompass standardized, formulaic text in contracts, form letters, and, most relevant to this week’s article on NSHipster, code.

Not all code can be glamorous. In fact, a lot of the low-level infrastructure that makes everything work is a slog of boilerplate.

This is true of the Swift standard library, which includes families of types like signed integers (Int8, Int16, Int32, Int64) whose implementation varies only in the size of the respective type.

Copy-pasting code may work as a one-off solution (assuming you manage to get it right the first time), but it’s not sustainable. Each time you want to make changes to these derived implementations, you risk introducing slight inconsistencies that cause the implementations to diverge over time — not unlike the random mutations responsible for the variation of life on Earth.

Languages have various techniques to cope with this, from C++ templates and Lisp macros to eval and C preprocessor statements.

Swift doesn’t have a macro system, and because the standard library is itself written in Swift, it can’t take advantage of C++ metaprogramming capabilities. Instead, the Swift maintainers use a Python script called gyb.py to generate source code using a small set of template tags.

How GYB Works

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

All other text is passed through unchanged.

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

Installing GYB

GYB isn’t part of the standard Xcode toolchain, so you won’t find it with xcrun. Instead, you can download it using Homebrew:

$ brew install nshipster/formulae/gyb

Alternatively, you can download the source code and use the chmod command to make gyb executable (the default installation of Python on macOS should be able to run gyb):

$ wget https://github.com/apple/swift/raw/main/utils/gyb
$ wget https://github.com/apple/swift/raw/main/utils/gyb.py
$ chmod +x gyb

If you go this route, be sure to move these somewhere that can be accessed from your Xcode project, but keep them separate from your source files (for example, a Vendor directory at your project root).

Using GYB in Xcode

In Xcode, click on the blue project file icon in the navigator, select the active target in your project, and navigate to the “Build Phases” panel. At the top, you’ll see a + symbol that you can click to add a new build phase. Select “Add New Run Script Phase”, and enter the following into the source editor:

find . -name '*.gyb' |                                               \
    while read file; do                                              \
        ./path/to/gyb --line-directive '' -o "${file%.gyb}" "$file"; \

Now when you build your project any file with the .swift.gyb file extension is evaluated by GYB, which outputs a .swift file that’s compiled along with the rest of the code in the project.

When to Use GYB

As with any tool, knowing when to use it is just as important as knowing how. Here are some examples of when you might open your toolbox and reach for GYB.

Generating Formulaic Code

Are you copy-pasting the same code for elements in a set or items in a sequence? A for-in loop with variable substitution might be the solution.

As seen in the example with Codable from before, you can declare a collection at the top of your GYB template file and then iterate over that collection for type, property, or method declarations:

    abilities = ['strength', 'dexterity', 'constitution',
                 'intelligence', 'wisdom', 'charisma']
class Character {
  var name: String

% for ability in abilities:
  var ${ability}: Int
% end

Evaluating this with GYB produces the following Swift code:

class Character {
  var name: String

  var strength: Int
  var dexterity: Int
  var constitution: Int
  var intelligence: Int
  var wisdom: Int
  var charisma: Int

A lot of repetition in code is a smell and may indicate that there’s a better way to accomplish your task. Built-in language feature like protocol extensions and generics can eliminate a lot of code duplication, so be on the lookout to use these instead of brute-forcing with GYB.

Generating Code Derived from Data

Are you writing code based on a data source? Try incorporating GYB into your development!

GYB files can import Python packages like json, xml, and csv, so you can parse pretty much any kind of file you might encounter:

%{ import csv }
% with open('path/to/file.csv') as file:
    % for row in csv.DictReader(file):

If you want to see this in action, check out Currencies.swift.gyb which generates Swift enumerations for each of the currencies defined by the ISO 4217 specification.

Code generation makes it trivial to keep your code in sync with the relevant standards. Simply update the data file and re-run GYB.

Swift has done a lot to cut down on boilerplate recently with the addition of compiler synthesis of Encodable and Decodable in 4.0, Equatable and Hashable in 4.1, and CaseIterable in 4.2. We hope that this momentum is carried in future updates to the language.

In the meantime, for everything else, GYB is a useful tool for code generation.

“Don’t Repeat Yourself” may be a virtue in programming, but sometimes you have to say things a few times to make things work. When you do, you’ll be thankful to have a tool like GYB to say it for you.


Questions? Corrections? Issues and pull requests are always welcome.

This article uses Swift version 4.2. Find status information for all articles on the status page.

Written by Mattt

Mattt (@mattt) is a writer and developer in Portland, Oregon.

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