🤝 swift-enum-properties

Struct and enum data access in harmony.

Motivation

In Swift, struct data access is far more ergonomic than enum data access by default.

A struct field can be accessed in less than a single line using expressive dot-syntax:

user.name

An enum's associated value requires as many as seven lines to bring it into the current scope:

let optionalValue: String?
if case let .success(value) = result {
  optionalValue = value
} else {
  optionalValue = nil
}
optionalValue

That's a lot of boilerplate getting in the way of what we care about: getting at the value of a success.

This difference is also noticeable when working with higher-order functions like map and compactMap.

An array of struct values can be transformed succinctly in a single expression:

users.map { $0.name }

But transforming an array of enum values requires a version of the following incantation:

results.compactMap { result -> String? in
  guard case let .success(value) = result else { return nil }
  return value
}

The imperative nature of unwrapping an associated value spills over multiple lines, which requires us to give Swift an explicit return type, name our closure argument, and provide two explicit returns.

Solution

We can recover all of the ergonomics of struct data access for enums by defining "enum properties": computed properties that optionally return a value when the case matches:

extension Result {
  var success: Success? {
    guard case let .success(value) = self else { return nil }
    return value
  }
  
  var failure: Failure? {
    guard case let .failure(value) = self else { return nil }
    return value
  }
}

This is work we are used to doing in an ad hoc way throughout our code bases, but we can centralize it in a computed property and are free to access underlying data in a succinct fashion:

// Optionally-chain into a successful result.
result.success?.count

// Collect a bunch of successful values.
results.compactMap { $0.success }

By defining a computed property, we bridge another gap: our enums now have key paths!

\Result<String, Error>.success
// KeyPath<Result<String, Error>, String?>

Despite these benefits, defining enum properties from scratch is a tall ask. Instead, enter generate-enum-properties.

Usage

usage: generate-enum-properties [--help|-h] [--dry-run|-n] [<file>...]

    -h, --help
        Print this message.

    -n, --dry-run
        Don't update files in place. Print to stdout instead.

    --version
        Print the version.

Once installed, you can invoke generate-enum-properties from the command line and feed it any number of Swift source files:

# Insert enum properties into every enum declaration. 
$ generate-enum-properties **/*.swift

It will automatically generate and inline enum properties for every enum with associated values. Please note that it updates source files in place. Use version control to avoid accidental insertions! You can use the --dry-run flag to preview the updated source.

$ generate-enum-properties --dry-run **/*.swift

Without the --dry-run flag, the following source file as input:

enum Validated<Valid, Invalid> {
  case valid(Valid)
  case invalid(Invalid)
}

Will have its contents replaced with the following output:

enum Validated<Valid, Invalid> {
  case valid(Valid)
  case invalid(Invalid)

  var valid: Valid? {
    get {
      guard case let .valid(value) = self else { return nil }
      return value
    }
    set {
      guard case .valid = self, let newValue = newValue else { return }
      self = .valid(newValue)
    }
  }
}

Note that both a setter and getter are generated, which means you can also optionally dive into enum data and update a part of it.

validatedUser.valid?.name = "Blob"

Running generate-enum-properties is idempotent: it will only insert properties that aren't already defined in the enum declaration. One caveat:

⚠️ If you have defined an enum property of the same name defined in an extension, it will collide with the one generated by generate-enum-properties.

Now you may be wondering: why not generate extensions that can be hidden away in another file? Unfortunately, this is problematic for enums that depend on types that need to be imported and types that are nested. By inlining enum properties, we can ensure that every associated value's type is in scope.

Installation

Homebrew

You can install generate-enum-properties using our custom tap:

$ brew install pointfreeco/swift/generate-enum-properties
$ generate-enum-properties

SwiftPM

As a dependency

If you want to use generate-enum-properties in a project that uses SwiftPM, it's as simple as adding a dependencies clause to your Package.swift:

dependencies: [
  .package(url: "https://github.com/pointfreeco/swift-enum-properties.git", from: "0.1.0")
]

And invoking swift run from the command line:

$ swift run generate-enum-properties

As a CLI

If you want to run generate-enum-properties using SwiftPM, it's as simple as cloning the repository and invoking swift run:

$ git clone https://github.com/pointfreeco/swift-enum-properties.git
$ cd swift-enum-properties
$ swift run generate-enum-properties

Make

If you want to build and install generate-enum-properties yourself:

$ git clone https://github.com/pointfreeco/swift-enum-properties.git
$ cd swift-enum-properties
$ make install

Mint

If you want to install with Mint:

$ mint install pointfreeco/swift-enum-properties

Interested in learning more?

These concepts (and more) are explored thoroughly in Point-Free, a video series exploring functional programming and Swift hosted by Brandon Williams and Stephen Celis.

The design of this library was explored in the following Point-Free episodes:

• Episode 52: Enum Properties
• Episode 53: Swift Syntax Enum Properties
• Episode 54: Advanced Swift Syntax Enum Properties
• Episode 55: Swift Syntax Command Line Tool 🆓

License

All modules are released under the MIT license. See LICENSE for details.