[swift-build-dev] [Draft] Package Manager Manifest API Redesign

Rick Ballard rballard at apple.com
Fri Feb 24 18:35:23 CST 2017

Hi all,

Ankit, Daniel, Anders, Boris and I have a draft proposal in progress for a Package.swift manifest API redesign for the Package Manager. We'll welcome comments or discussion at this time. My hope is that we can get this polished up and ready for evolution within the next week or so, but we'll see how the conversation goes!

You can see the proposal in progress at https://github.com/aciidb0mb3r/swift-evolution/blob/manifest-api-redesign/proposals/xxxx-package-manager-manifest-api-redesign.md. I'm also including the current version inline in this email.


	- Rick

# Package Manager Manifest API Redesign

* Proposal: [SE-XXXX](xxxx-package-manager-manifest-api-redesign.md)
* Author: [Ankit Aggarwal](https://github.com/aciidb0mb3r)
* Review Manager: TBD
* Status: **Discussion**

## Introduction

This is a proposal for redesigning the `Package.swift` manifest APIs provided
by Swift Package Manager.  
This proposal only redesigns the existing public APIs and does not add any
new functionality; any API to be added for new functionality will happen in
separate proposals.

## Motivation

The `Package.swift` manifest APIs were designed prior to the [API Design
Guidelines] (https://swift.org/documentation/api-design-guidelines/), and their
design was not reviewed by the evolution process. Additionally, there are
several small areas which can be cleaned up to make the overall API more

We would like to redesign these APIs as necessary to provide clean,
conventions-compliant APIs that we can rely on in the future. Because we
anticipate that the user community for the Swift Package Manager will grow
considerably in Swift 4, we would like to make these changes now, before
more packages are created using the old API.

## Proposed solution

Note: Access modifier is omitted from the diffs and examples for brevity. The
access modifier is `public` for all APIs unless specified.

* Remove `successor()` and `predecessor()` from `Version`.

    These methods neither have well defined semantics nor are used a lot
    (internally or publicly). For e.g., the current implementation of
    `successor()` always just increases the patch version.

      <summary>View diff</summary>
    struct Version {
    -    func successor() -> Version

    -    func predecessor() -> Version

* Make all properties of `Package` and `Target` mutable.

    Currently, `Package` has three immutable and four mutable properties, and
    `Target` has one immutable and one mutable property. We propose to make all
    properties mutable to allow complex customization on the package object
    after initial declaration.

      <summary>View diff and example</summary>

    final class Target {
    -    let name: String
    +    var name: String

    final class Package {
    -    let name: String
    +    var name: String

    -    let pkgConfig: String?
    +    var pkgConfig: String?

    -    let providers: [SystemPackageProvider]?
    +    var providers: [SystemPackageProvider]?

    let package = Package(
        name: "FooPackage",
        targets: [
            Target(name: "Foo", dependencies: ["Bar"]),

    #if os(Linux)
    package.targets[0].dependencies = ["BarLinux"]

* Change `Target.Dependency` enum cases to lowerCamelCase.

    According to API design guidelines, everything other than types should be in lowerCamelCase.

      <summary>View diff and example</summary>

    enum Dependency {
    -    case Target(name: String)
    +    case target(name: String)

    -    case Product(name: String, package: String?)
    +    case product(name: String, package: String?)

    -    case ByName(name: String)
    +    case byName(name: String)

    let package = Package(
        name: "FooPackage",
        targets: [
                name: "Foo", 
                dependencies: [
    -                .Target(name: "Bar"),
    +                .target(name: "Bar"),

    -                .Product(name: "SwiftyJSON", package: "SwiftyJSON"),
    +                .product(name: "SwiftyJSON", package: "SwiftyJSON"),

* Add default parameter to the enum case `Target.Dependency.product`.

    The associated value `package` in the (enum) case `product`, is an optional
    `String`. It should have the default value `nil` so clients don't need to
    write it if they prefer using explicit enum cases but don't want to specify
    the package name i.e. it should be possible to write `.product(name:
    "Foo")` instead of `.product(name: "Foo", package: nil)`.

    is accepted, we can directly add a default value. Otherwise, we will use a
    static factory method to provide default value for `package`.

* Upgrade `SystemPackageProvider` enum to a struct.

    This enum allows SwiftPM System Packages to emit hints in case of build
    failures due to absence of a system package. Currently, only one system
    package per system packager can be specified. We propose to allow
    specifying multiple system packages by replacing the enum with this struct:

    public struct SystemPackageProvider {
        enum PackageManager {
            case apt
            case brew
        /// The system package manager.
        let packageManager: PackageManager 

        /// The array of system packages.
        let packages: [String]
        init(_ packageManager: PackageManager, packages: [String])

      <summary>View diff and example</summary>

    -enum SystemPackageProvider {
    -    case Brew(String)
    -    case Apt(String)

    +struct SystemPackageProvider {
    +    enum PackageManager {
    +        case apt
    +        case brew
    +    }
    +    /// The system package manager.
    +    let packageManager: PackageManager 
    +    /// The array of system packages.
    +    let packages: [String]
    +    init(_ packageManager: PackageManager, packages: [String])


    let package = Package(
        name: "Copenssl",
        pkgConfig: "openssl",
        providers: [
    -        .Brew("openssl"),
    +        SystemPackageProvider(.brew, packages: ["openssl"]),

    -        .Apt("openssl-dev"),
    +        SystemPackageProvider(.apt, packages: ["openssl", "libssl-dev"]),

* Remove implicit target dependency rule for test targets.

    There is an implicit test target dependency rule: a test target "FooTests"
    implicity depends on a target "Foo", if "Foo" exists and "FooTests" doesn't
    explicitly declare any dependency. We propose to remove this rule because:

    1. It is a non obvious "magic" rule that has to be learned.
    2. It is not possible for "FooTests" to remove dependency on "Foo" while
       having no other (target) dependency.
    3. It makes real dependencies less discoverable.
    4. It may cause issues when we get support for mechanically editing target

* Introduce an "identity rule" to determine if an API should use an initializer
  or a factory method:

    Under this rule, an entity having an identity, will use a type initializer
    and everything else will use factory methods. `Package`, `Target` and
    `Product` are identities. However, a product referenced in a target
    dependency is not an identity.

    This means the `Product` enum should be converted into an identity. We
    propose to introduce a `Product` class with two subclasses: `Executable`
    and `Library`.  These subclasses will be nested inside `Product` class
    instead of being a top level declaration in the module.  The major
    advantage of nesting is that we get a namespace for products and it is easy
    to find all the supported products when the product types grows to a large
    number. A downside of nesting is that the product initializers will have to
    used with the dot notation (e.g.: `.Executable(name: "tool", targets:
    ["tool"])`) which is a little awkward because we expect factory methods to
    use the dots.

    They will be defined as follow:

    /// Represents a product.
    class Product {
        /// The name of the product.
        let name: String

        /// The names of the targets in this product.
        let targets: [String]
        private init(name: String, targets: [String]) {
            self.name = name
            self.targets = targets
        /// Represents an executable product.
        final class Executable: Product {

            /// Creates an executable product with given name and targets.
            override init(name: String, targets: [String])
        /// Represents a library product.
        final class Library: Product {
            /// The type of library product.
            enum LibraryType: String {
                case `static`
                case `dynamic`
            /// The type of the library.
            /// If the type is unspecified, package manager will automatically choose a type.
            let type: LibraryType?
            /// Creates a library product.
            init(name: String, type: LibraryType? = nil, targets: [String])

      <summary>View example</summary>


    let package = Package(
        name: "Foo",
        target: [
            Target(name: "Foo", dependencies: ["Utility"]),
            Target(name: "tool", dependencies: ["Foo"]),
        products: [
            .Executable(name: "tool", targets: ["tool"]), 
            .Library(name: "Foo", targets: ["Foo"]), 
            .Library(name: "FooDy", type: .dynamic, targets: ["Foo"]), 

* Special syntax for version initializers.

    A simplified summary of what is commonly supported in other package managers:

    | Package Manager | x-ranges      | tilde (`~` or `~>`)     | caret (`^`)   |
    | npm             | Supported     | Allows patch-level changes if a minor version is specified on the comparator. Allows minor-level changes if not.  | patch and minor updates |
    | Cargo           | Supported     | Same as above           | Same as above |
    | CocoaPods       | Not supported | Same as above           | Not supported |
    | Carthage        | Not supported | patch and minor updates | Not supported |

    Some general observations:

    * Every package manager we looked at for this supports the tilde `~` operator in some form.
    * The widely accepted suggestion on how to constrain your versions is "use
      `~>`, it does the right thing".
    * It's not clear to us why this has so much traction as "the right thing", as it can
      prevent upgrades that should be compatible (one minor version to next minor version).
    * Most users may not really understand `~`, and just use it per recommendations.
      See e.g. how Google created a [6-minute instructional video](https://www.youtube.com/watch?v=x4ARXyovvPc)
      about this operator for CocoaPods.
    * A lot of people even explicitly set a single exact version simply because
      they don't know better. This leads to "dependency hell" (unresolvable dependencies
      due to conflicting requirements for a package in the dependency graph).
    * The Swift Package Manager will probably have many novice users, because it
      comes built-in to Swift.
    * We think caret `^` has the right behaviour most of the time. That is, you
      should be able to specify a minimum version, and you should be willing to let
      your package use anything after that up to the next major version. This policy
      works if packages correctly follow semantic versioning, and it prevents "dependency
      hell" by expressing permissive constraints.
    * We also think caret `^` is syntactically non-obvious, and we'd prefer a syntax
      that doesn't require reading a manual for novices to understand, even if that
      means we break with the syntactic convention established by the other package managers which
      support caret `^`.
    * We'd like a convenient syntax for caret `^`, but to still support the use
      case that tilde `~` is used for; but tilde `~` (or a single exact version) should
      be less convenient than caret `^`, to encourge permissive dependency constraints.

    What we propose:

    * We will introduce a factory method which takes a lower bound version and
      forms a range that goes upto the next major version (i.e. caret).

      // 1.0.0 ..< 2.0.0
      .package(url: "/SwiftyJSON", from: "1.0.0"),

      // 1.2.0 ..< 2.0.0
      .package(url: "/SwiftyJSON", from: "1.2.0"),

      // 1.5.8 ..< 2.0.0
      .package(url: "/SwiftyJSON", from: "1.5.8"),

    * We will introduce a factory method which takes `VersionSetSpecifier`, to
      conveniently specify common ranges.

      `VersionSetSpecifier` is an enum defined as follows:

      enum VersionSetSpecifier {
          case exact(Version)
          case range(Range<Version>)

          /// Creates a specifier for an exact version.
          static func only(_ version: Version) -> VersionSetSpecifier

          /// Creates a specified for a range starting at the given lower bound
          /// and going upto next major version.
          static func uptoNextMajor(_ version: Version) -> VersionSetSpecifier

          /// Creates a specified for a range starting at the given lower bound
          /// and going upto next minor version.
          static func uptoNextMinor(_ version: Version) -> VersionSetSpecifier


      // 1.5.8 ..< 2.0.0
      .package(url: "/SwiftyJSON", .uptoNextMajor("1.5.8")),

      // 1.5.8 ..< 1.6.0
      .package(url: "/SwiftyJSON", .uptoNextMinor("1.5.8")),

      // 1.5.8
      .package(url: "/SwiftyJSON", .only("1.5.8")),

    * This will also give us ability to add more complex features in future:

      > Note that we're not actually proposing these as part of this proposal.

      .package(url: "/SwiftyJSON", .uptoNextMajor("1.5.8").excluding("1.6.4")),

      .package(url: "/SwiftyJSON", .only("1.5.8", "1.6.3")),


    * We will introduce a factory method which takes `Range<Version>`, to specify
      arbitrary open range.

      // Constraint to an arbitrary open range.
      .package(url: "/SwiftyJSON", "1.2.3"..<"1.2.6"),

    * We will introduce a factory method which takes `ClosedRange<Version>`, to specify
      arbitrary closed range.

      // Constraint to an arbitrary closed range.
      .package(url: "/SwiftyJSON", "1.2.3"..."1.2.8"),

    * We will remove all of the current factory methods:

      // Constraint to a major version.
      .Package(url: "/SwiftyJSON", majorVersion: 1),

      // Constraint to a major and minor version.
      .Package(url: "/SwiftyJSON", majorVersion: 1, minor: 2),

      // Constraint to an exact version.
      .Package(url: "/SwiftyJSON", "1.2.3"),

      // Constraint to an arbitrary range.
      .Package(url: "/SwiftyJSON", versions: "1.2.3"..<"1.2.6"),

      // Constraint to an arbitrary closed range.
      .Package(url: "/SwiftyJSON", versions: "1.2.3"..."1.2.8"),

* Adjust order of parameters on `Package` class:

    We propose to reorder the parameters of `Package` class to: `name`,
    `pkgConfig`, `products`, `dependencies`, `targets`, `compatibleSwiftVersions`.

    The rationale behind this reorder is that the most interesting parts of a
    package are its product and dependencies, so they should be at the top.
    Targets are usually important during development of the package.  Placing
    them at the end keeps it easier for the developer to jump to end of the
    file to access them. Note that the compatibleSwiftVersions property will likely
    be removed once we support Build Settings, but that will be discussed in a separate proposal.

      <summary>View example</summary>


    let package = Package(
        name: "Paper",
        products: [
            .Executable(name: "tool", targets: ["tool"]),
            .Libary(name: "Paper", type: .static, targets: ["Paper"]),
            .Libary(name: "PaperDy", type: .dynamic, targets: ["Paper"]),
        dependencies: [
            .package(url: "http://github.com/SwiftyJSON", from: "1.2.3"),
            .package(url: "../CHTTPParser", .uptoNextMinor("2.2.0")),
            .package(url: "http://some/other/lib", .only("1.2.3")),
        targets: [
                name: "tool",
                dependencies: [
                name: "Paper",
                dependencies: [
                    .target(name: "Utility"),
                    .product(name: "CHTTPParser"),

* Eliminate exclude in future (via custom layouts feature).

    We expect to remove the `exclude` property after we get support for custom
    layouts. The exact details will be in the proposal of that feature.

## Impact on existing code

The above changes will be implemented only in the new Package Description v4
library. The v4 runtime library will release with Swift 4 and packages will be
able to opt-in into it as described by

There will be no automatic migration feature for updating the manifests from v3
to v4. To indicate the replacements of old APIs, we will annotate them using
the `@unavailable` attribute where possible. Unfortunately, this will not cover
all the changes for e.g. rename of the target dependency enum cases.

All new packages created with `swift package init` command in Swift 4 tools
will by default to use the v4 manifest. It will be possible to switch to v3
manifest version by changing the tools version using `swift package
tools-version --set 3.1`.  However, the manifest will needed to be adjusted to
use the older APIs manually.

Unless declared in the manifest, existing packages automatically default
to the Swift 3 minimum tools version; since the Swift 4 tools will also include
the v3 manifest API, they will build as expected.

A package which needs to support both Swift 3 and Swift 4 tools will need to
stay on the v3 manifest API and support the Swift 3 language version for its
sources, using the API described in the proposal

An existing package which wants to use the new v4 manifest APIs will need to bump its
minimum tools version to 4.0 or later using the command `$ swift package tools-version
--set-current`, and then modify the manifest file with the changes described in
this proposal.

## Alternatives considered

* Add variadic overloads.

    Adding variadic overload allows omitting parenthesis which leads to less
    cognitive load on eyes, especially when there is only one value which needs
    to be specified. For e.g.:

        Target(name: "Foo", dependencies: "Bar")

    might looked better than:

        Target(name: "Foo", dependencies: ["Bar"])

    However, plurals words like `dependencies` and `targets` imply a collection
    which implies brackets. It also makes the grammar wrong. Therefore, we
    reject this option.
* Version exclusion.
    It is not uncommon to have a specific package version break something, and
    it is undesirable to "fix" this by adjusting the range to exclude it
    because this overly constrains the graph and can prevent picking up the
    version with the fix.

    This is desirable but it should be proposed separately.

* Inline package declaration.

    We should probably support declaring a package dependency anywhere we
    support spelling a package name. It is very common to only have one target
    require a dependency, and annoying to have to specify the name twice.

    This is desirable but it should be proposed separately.

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