StructuredSwiftRepresentation.swift

/*
 * Copyright 2023, gRPC Authors All rights reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
//===----------------------------------------------------------------------===//
//
// This source file is part of the SwiftOpenAPIGenerator open source project
//
// Copyright (c) 2023 Apple Inc. and the SwiftOpenAPIGenerator project authors
// Licensed under Apache License v2.0
//
// See LICENSE.txt for license information
// See CONTRIBUTORS.txt for the list of SwiftOpenAPIGenerator project authors
//
// SPDX-License-Identifier: Apache-2.0
//
//===----------------------------------------------------------------------===//

/// A description of an import declaration.
///
/// For example: `import Foo`.
package struct ImportDescription: Equatable, Codable, Sendable {
  /// The access level of the imported module.
  ///
  /// For example, the `public` in `public import Foo`.
  ///
  /// - Note: This is optional, as explicit access-level modifiers are not required on `import` statements.
  var accessLevel: AccessModifier? = nil

  /// The name of the imported module.
  ///
  /// For example, the `Foo` in `import Foo`.
  var moduleName: String

  /// An array of module types imported from the module, if applicable.
  ///
  /// For example, if there are type imports like `import Foo.Bar`, they would be listed here.
  var moduleTypes: [String]?

  /// The name of the private interface for an `@_spi` import.
  ///
  /// For example, if `spi` was "Secret" and the module name was "Foo" then the import
  /// would be `@_spi(Secret) import Foo`.
  var spi: String? = nil

  /// Requirements for the `@preconcurrency` attribute.
  var preconcurrency: PreconcurrencyRequirement = .never

  /// If the dependency is an item, the property's value is the item representation.
  /// If the dependency is a module, this property is nil.
  var item: Item? = nil

  /// Describes any requirement for the `@preconcurrency` attribute.
  enum PreconcurrencyRequirement: Equatable, Codable, Sendable {
    /// The attribute is always required.
    case always
    /// The attribute is not required.
    case never
    /// The attribute is required only on the named operating systems.
    case onOS([String])
  }

  /// Represents an item imported from a module.
  struct Item: Equatable, Codable, Sendable {
    /// The keyword that specifies the item's kind (e.g. `func`, `struct`).
    var kind: Kind

    /// The name of the imported item.
    var name: String

    init(kind: Kind, name: String) {
      self.kind = kind
      self.name = name
    }
  }

  enum Kind: String, Equatable, Codable, Sendable {
    case `typealias`
    case `struct`
    case `class`
    case `enum`
    case `protocol`
    case `let`
    case `var`
    case `func`
  }
}

/// A description of an access modifier.
///
/// For example: `public`.
package enum AccessModifier: String, Sendable, Equatable, Codable, CaseIterable {
  /// A declaration accessible outside of the module.
  case `public`

  /// A declaration accessible outside of the module but only inside the containing package or project.
  case `package`

  /// A declaration only accessible inside of the module.
  case `internal`

  /// A declaration only accessible inside the same Swift file.
  case `fileprivate`

  /// A declaration only accessible inside the same type or scope.
  case `private`
}

/// A description of a comment.
///
/// For example `/// Hello`.
enum Comment: Equatable, Codable, Sendable {

  /// An inline comment.
  ///
  /// For example: `// Great code below`.
  case inline(String)

  /// A documentation comment.
  ///
  /// For example: `/// Important type`.
  case doc(String)

  /// A mark comment.
  ///
  /// For example: `// MARK: - Public methods`, with the optional
  /// section break (`-`).
  case mark(String, sectionBreak: Bool)

  /// A comment that is already formatted,
  /// meaning that it already has the `///` and
  /// can contain multiple lines
  ///
  /// For example both the string and the comment
  /// can look like `/// Important type`.
  case preFormatted(String)
}

/// A description of a literal.
///
/// For example `"hello"` or `42`.
enum LiteralDescription: Equatable, Codable, Sendable {

  /// A string literal.
  ///
  /// For example `"hello"`.
  case string(String)

  /// An integer literal.
  ///
  /// For example `42`.
  case int(Int)

  /// A Boolean literal.
  ///
  /// For example `true`.
  case bool(Bool)

  /// The nil literal: `nil`.
  case `nil`

  /// An array literal.
  ///
  /// For example `["hello", 42]`.
  case array([Expression])

  /// A dictionary literal.
  ///
  /// For example: `["hello": "42"]`
  case dictionary([KeyValue])

  struct KeyValue: Codable, Equatable {
    var key: Expression
    var value: Expression
  }
}

/// A description of an identifier, such as a variable name.
///
/// For example, in `let foo = 42`, `foo` is an identifier.
enum IdentifierDescription: Equatable, Codable, Sendable {

  /// A pattern identifier.
  ///
  /// For example, `foo` in `let foo = 42`.
  case pattern(String)

  /// A type identifier.
  ///
  /// For example, `Swift.String` in `let foo: Swift.String = "hi"`.
  case type(ExistingTypeDescription)
}

/// A description of a member access expression.
///
/// For example `foo.bar`.
struct MemberAccessDescription: Equatable, Codable, Sendable {

  /// The expression of which a member `right` is accessed.
  ///
  /// For example, in `foo.bar`, `left` represents `foo`.
  var left: Expression?

  /// The member name to access.
  ///
  /// For example, in `foo.bar`, `right` is `bar`.
  var right: String
}

/// A description of a function argument.
///
/// For example in `foo(bar: 42)`, the function argument is `bar: 42`.
struct FunctionArgumentDescription: Equatable, Codable, Sendable {

  /// An optional label of the function argument.
  ///
  /// For example, in `foo(bar: 42)`, the `label` is `bar`.
  var label: String?

  /// The expression passed as the function argument value.
  ///
  /// For example, in `foo(bar: 42)`, `expression` represents `42`.
  var expression: Expression
}

/// A description of a function call.
///
/// For example `foo(bar: 42)`.
struct FunctionCallDescription: Equatable, Codable, Sendable {

  /// The expression that returns the function to be called.
  ///
  /// For example, in `foo(bar: 42)`, `calledExpression` represents `foo`.
  var calledExpression: Expression

  /// The arguments to be passed to the function.
  var arguments: [FunctionArgumentDescription]

  /// A trailing closure.
  var trailingClosure: ClosureInvocationDescription?

  /// Creates a new function call description.
  /// - Parameters:
  ///   - calledExpression: An expression that returns the function to be called.
  ///   - arguments: Arguments to be passed to the function.
  ///   - trailingClosure: A trailing closure.
  init(
    calledExpression: Expression,
    arguments: [FunctionArgumentDescription] = [],
    trailingClosure: ClosureInvocationDescription? = nil
  ) {
    self.calledExpression = calledExpression
    self.arguments = arguments
    self.trailingClosure = trailingClosure
  }

  /// Creates a new function call description.
  /// - Parameters:
  ///   - calledExpression: An expression that returns the function to be called.
  ///   - arguments: Arguments to be passed to the function.
  ///   - trailingClosure: A trailing closure.
  init(
    calledExpression: Expression,
    arguments: [Expression],
    trailingClosure: ClosureInvocationDescription? = nil
  ) {
    self.init(
      calledExpression: calledExpression,
      arguments: arguments.map { .init(label: nil, expression: $0) },
      trailingClosure: trailingClosure
    )
  }
}

/// A type of a variable binding: `let` or `var`.
enum BindingKind: Equatable, Codable, Sendable {

  /// A mutable variable.
  case `var`

  /// An immutable variable.
  case `let`
}

/// A description of a variable declaration.
///
/// For example `let foo = 42`.
struct VariableDescription: Equatable, Codable, Sendable {

  /// An access modifier.
  var accessModifier: AccessModifier?

  /// A Boolean value that indicates whether the variable is static.
  var isStatic: Bool = false

  /// The variable binding kind.
  var kind: BindingKind

  /// The name of the variable.
  ///
  /// For example, in `let foo = 42`, `left` is `foo`.
  var left: Expression

  /// The type of the variable.
  ///
  /// For example, in `let foo: Int = 42`, `type` is `Int`.
  var type: ExistingTypeDescription?

  /// The expression to be assigned to the variable.
  ///
  /// For example, in `let foo = 42`, `right` represents `42`.
  var right: Expression? = nil

  /// Body code for the getter.
  ///
  /// For example, in `var foo: Int { 42 }`, `body` represents `{ 42 }`.
  var getter: [CodeBlock]? = nil

  /// Effects for the getter.
  ///
  /// For example, in `var foo: Int { get throws { 42 } }`, effects are `[.throws]`.
  var getterEffects: [FunctionKeyword] = []

  /// Body code for the setter.
  ///
  /// For example, in `var foo: Int { set { _foo = newValue } }`, `body`
  /// represents `{ _foo = newValue }`.
  var setter: [CodeBlock]? = nil

  /// Body code for the `_modify` accessor.
  ///
  /// For example, in `var foo: Int { _modify { yield &_foo } }`, `body`
  /// represents `{ yield &_foo }`.
  var modify: [CodeBlock]? = nil
}

/// A requirement of a where clause.
enum WhereClauseRequirement: Equatable, Codable, Sendable {

  /// A conformance requirement.
  ///
  /// For example, in `extension Array where Element: Foo {`, the first tuple value is `Element` and the second `Foo`.
  case conformance(String, String)
}

/// A description of a where clause.
///
/// For example: `extension Array where Element: Foo {`.
struct WhereClause: Equatable, Codable, Sendable {

  /// One or more requirements to be added after the `where` keyword.
  var requirements: [WhereClauseRequirement]
}

/// A description of an extension declaration.
///
/// For example `extension Foo {`.
struct ExtensionDescription: Equatable, Codable, Sendable {

  /// An access modifier.
  var accessModifier: AccessModifier? = nil

  /// The name of the extended type.
  ///
  /// For example, in `extension Foo {`, `onType` is `Foo`.
  var onType: String

  /// Additional type names that the extension conforms to.
  ///
  /// For example: `["Sendable", "Codable"]`.
  var conformances: [String] = []

  /// A where clause constraining the extension declaration.
  var whereClause: WhereClause? = nil

  /// The declarations that the extension adds on the extended type.
  var declarations: [Declaration]
}

/// A description of a struct declaration.
///
/// For example `struct Foo {`.
struct StructDescription: Equatable, Codable, Sendable {

  /// An access modifier.
  var accessModifier: AccessModifier? = nil

  /// The name of the struct.
  ///
  /// For example, in `struct Foo {`, `name` is `Foo`.
  var name: String

  /// The generic types of the struct.
  var generics: [ExistingTypeDescription] = []

  /// The type names that the struct conforms to.
  ///
  /// For example: `["Sendable", "Codable"]`.
  var conformances: [String] = []

  /// A where clause constraining the struct declaration.
  var whereClause: WhereClause? = nil

  /// The declarations that make up the main struct body.
  var members: [Declaration] = []
}

/// A description of an enum declaration.
///
/// For example `enum Bar {`.
struct EnumDescription: Equatable, Codable, Sendable {

  /// A Boolean value that indicates whether the enum has a `@frozen`
  /// attribute.
  var isFrozen: Bool = false

  /// A Boolean value that indicates whether the enum has the `indirect`
  /// keyword.
  var isIndirect: Bool = false

  /// An access modifier.
  var accessModifier: AccessModifier? = nil

  /// The name of the enum.
  ///
  /// For example, in `enum Bar {`, `name` is `Bar`.
  var name: String

  /// The type names that the enum conforms to.
  ///
  /// For example: `["Sendable", "Codable"]`.
  var conformances: [String] = []

  /// The declarations that make up the enum body.
  var members: [Declaration] = []
}

/// A description of a type reference.
indirect enum ExistingTypeDescription: Equatable, Codable, Sendable {

  /// A type with the `any` keyword in front of it.
  ///
  /// For example, `any Foo`.
  case any(ExistingTypeDescription)

  /// An optional type.
  ///
  /// For example, `Foo?`.
  case optional(ExistingTypeDescription)

  /// A wrapper type generic over a list of wrapped types.
  ///
  /// For example, `Wrapper<Wrapped>`.
  case generic(wrapper: ExistingTypeDescription, wrapped: [ExistingTypeDescription])

  /// A type reference represented by the components.
  ///
  /// For example, `MyModule.Foo`.
  case member([String])

  /// An array with an element type.
  ///
  /// For example, `[Foo]`.
  case array(ExistingTypeDescription)

  /// A dictionary where the key is `Swift.String` and the value is
  /// the provided type.
  ///
  /// For example, `[String: Foo]`.
  case dictionaryValue(ExistingTypeDescription)

  /// A type with the `some` keyword in front of it.
  ///
  /// For example, `some Foo`.
  case some(ExistingTypeDescription)

  /// A closure signature as a type.
  ///
  /// For example: `(String) async throws -> Int`.
  case closure(ClosureSignatureDescription)

  /// A wrapper type generic over a list of wrapped types.
  ///
  /// For example, `Wrapper<Wrapped>`.
  static func generic(
    wrapper: ExistingTypeDescription,
    wrapped: ExistingTypeDescription...
  ) -> Self {
    return .generic(wrapper: wrapper, wrapped: Array(wrapped))
  }
}

/// A description of a typealias declaration.
///
/// For example `typealias Foo = Int`.
struct TypealiasDescription: Equatable, Codable, Sendable {

  /// An access modifier.
  var accessModifier: AccessModifier?

  /// The name of the typealias.
  ///
  /// For example, in `typealias Foo = Int`, `name` is `Foo`.
  var name: String

  /// The existing type that serves as the underlying type of the alias.
  ///
  /// For example, in `typealias Foo = Int`, `existingType` is `Int`.
  var existingType: ExistingTypeDescription
}

/// A description of a protocol declaration.
///
/// For example `protocol Foo {`.
struct ProtocolDescription: Equatable, Codable, Sendable {

  /// An access modifier.
  var accessModifier: AccessModifier? = nil

  /// The name of the protocol.
  ///
  /// For example, in `protocol Foo {`, `name` is `Foo`.
  var name: String

  /// The type names that the protocol conforms to.
  ///
  /// For example: `["Sendable", "Codable"]`.
  var conformances: [String] = []

  /// The function and property declarations that make up the protocol
  /// requirements.
  var members: [Declaration] = []
}

/// A description of a function parameter declaration.
///
/// For example, in `func foo(bar baz: String = "hi")`, the parameter
/// description represents `bar baz: String = "hi"`
struct ParameterDescription: Equatable, Codable, Sendable {

  /// An external parameter label.
  ///
  /// For example, in `bar baz: String = "hi"`, `label` is `bar`.
  var label: String? = nil

  /// An internal parameter name.
  ///
  /// For example, in `bar baz: String = "hi"`, `name` is `baz`.
  var name: String? = nil

  /// The type name of the parameter.
  ///
  /// For example, in `bar baz: String = "hi"`, `type` is `String`.
  var type: ExistingTypeDescription? = nil

  /// A default value of the parameter.
  ///
  /// For example, in `bar baz: String = "hi"`, `defaultValue`
  /// represents `"hi"`.
  var defaultValue: Expression? = nil

  /// An inout parameter type.
  ///
  /// For example, `bar baz: inout String`.
  var `inout`: Bool = false
}

/// A function kind: `func` or `init`.
enum FunctionKind: Equatable, Codable, Sendable {

  /// An initializer.
  ///
  /// For example: `init()`, or `init?()` when `failable` is `true`.
  case initializer(failable: Bool)

  /// A function or a method. Can be static.
  ///
  /// For example `foo()`, where `name` is `foo`.
  case function(
    name: String,
    isStatic: Bool
  )
}

/// A function keyword, such as `async` and `throws`.
enum FunctionKeyword: Equatable, Codable, Sendable {

  /// An asynchronous function.
  case `async`

  /// A function that can throw an error.
  case `throws`

  /// A function that can rethrow an error.
  case `rethrows`
}

/// A description of a function signature.
///
/// For example: `func foo(bar: String) async throws -> Int`.
struct FunctionSignatureDescription: Equatable, Codable, Sendable {

  /// An access modifier.
  var accessModifier: AccessModifier? = nil

  /// The kind of the function.
  var kind: FunctionKind

  /// The generic types of the function.
  var generics: [ExistingTypeDescription] = []

  /// The parameters of the function.
  var parameters: [ParameterDescription] = []

  /// The keywords of the function, such as `async` and `throws.`
  var keywords: [FunctionKeyword] = []

  /// The return type name of the function, such as `Int`.
  var returnType: Expression? = nil

  /// The where clause for a generic function.
  var whereClause: WhereClause?
}

/// A description of a function definition.
///
/// For example: `func foo() { }`.
struct FunctionDescription: Equatable, Codable, Sendable {

  /// The signature of the function.
  var signature: FunctionSignatureDescription

  /// The body definition of the function.
  ///
  /// If nil, does not generate `{` and `}` at all for the body scope.
  var body: [CodeBlock]? = nil

  /// Creates a new function description.
  /// - Parameters:
  ///   - signature: The signature of the function.
  ///   - body: The body definition of the function.
  init(signature: FunctionSignatureDescription, body: [CodeBlock]? = nil) {
    self.signature = signature
    self.body = body
  }

  /// Creates a new function description.
  /// - Parameters:
  ///   - accessModifier: An access modifier.
  ///   - kind: The kind of the function.
  ///   - parameters: The parameters of the function.
  ///   - keywords: The keywords of the function, such as `async`.
  ///   - returnType: The return type name of the function, such as `Int`.
  ///   - body: The body definition of the function.
  init(
    accessModifier: AccessModifier? = nil,
    kind: FunctionKind,
    generics: [ExistingTypeDescription] = [],
    parameters: [ParameterDescription] = [],
    keywords: [FunctionKeyword] = [],
    returnType: Expression? = nil,
    whereClause: WhereClause? = nil,
    body: [CodeBlock]? = nil
  ) {
    self.signature = .init(
      accessModifier: accessModifier,
      kind: kind,
      generics: generics,
      parameters: parameters,
      keywords: keywords,
      returnType: returnType,
      whereClause: whereClause
    )
    self.body = body
  }

  /// Creates a new function description.
  /// - Parameters:
  ///   - accessModifier: An access modifier.
  ///   - kind: The kind of the function.
  ///   - parameters: The parameters of the function.
  ///   - keywords: The keywords of the function, such as `async`.
  ///   - returnType: The return type name of the function, such as `Int`.
  ///   - body: The body definition of the function.
  init(
    accessModifier: AccessModifier? = nil,
    kind: FunctionKind,
    generics: [ExistingTypeDescription] = [],
    parameters: [ParameterDescription] = [],
    keywords: [FunctionKeyword] = [],
    returnType: Expression? = nil,
    whereClause: WhereClause? = nil,
    body: [Expression]
  ) {
    self.init(
      accessModifier: accessModifier,
      kind: kind,
      generics: generics,
      parameters: parameters,
      keywords: keywords,
      returnType: returnType,
      whereClause: whereClause,
      body: body.map { .expression($0) }
    )
  }
}

/// A description of a closure signature.
///
/// For example: `(String) async throws -> Int`.
struct ClosureSignatureDescription: Equatable, Codable, Sendable {
  /// The parameters of the function.
  var parameters: [ParameterDescription] = []

  /// The keywords of the function, such as `async` and `throws.`
  var keywords: [FunctionKeyword] = []

  /// The return type name of the function, such as `Int`.
  var returnType: Expression? = nil

  /// The ``@Sendable`` attribute.
  var sendable: Bool = false

  /// The ``@escaping`` attribute.
  var escaping: Bool = false
}

/// A description of the associated value of an enum case.
///
/// For example, in `case foo(bar: String)`, the associated value
/// represents `bar: String`.
struct EnumCaseAssociatedValueDescription: Equatable, Codable, Sendable {

  /// A variable label.
  ///
  /// For example, in `bar: String`, `label` is `bar`.
  var label: String?

  /// A variable type name.
  ///
  /// For example, in `bar: String`, `type` is `String`.
  var type: ExistingTypeDescription
}

/// An enum case kind.
///
/// For example: `case foo` versus `case foo(String)`, and so on.
enum EnumCaseKind: Equatable, Codable, Sendable {

  /// A case with only a name.
  ///
  /// For example: `case foo`.
  case nameOnly

  /// A case with a name and a raw value.
  ///
  /// For example: `case foo = "Foo"`.
  case nameWithRawValue(LiteralDescription)

  /// A case with a name and associated values.
  ///
  /// For example: `case foo(String)`.
  case nameWithAssociatedValues([EnumCaseAssociatedValueDescription])
}

/// A description of an enum case.
///
/// For example: `case foo(String)`.
struct EnumCaseDescription: Equatable, Codable, Sendable {

  /// The name of the enum case.
  ///
  /// For example, in `case foo`, `name` is `foo`.
  var name: String

  /// The kind of the enum case.
  var kind: EnumCaseKind = .nameOnly
}

/// A declaration of a Swift entity.
indirect enum Declaration: Equatable, Codable, Sendable {

  /// A declaration that adds a comment on top of the provided declaration.
  case commentable(Comment?, Declaration)

  /// A declaration that adds a comment on top of the provided declaration.
  case deprecated(DeprecationDescription, Declaration)

  /// A variable declaration.
  case variable(VariableDescription)

  /// An extension declaration.
  case `extension`(ExtensionDescription)

  /// A struct declaration.
  case `struct`(StructDescription)

  /// An enum declaration.
  case `enum`(EnumDescription)

  /// A typealias declaration.
  case `typealias`(TypealiasDescription)

  /// A protocol declaration.
  case `protocol`(ProtocolDescription)

  /// A function declaration.
  case function(FunctionDescription)

  /// An enum case declaration.
  case enumCase(EnumCaseDescription)
}

/// A description of a deprecation notice.
///
/// For example: `@available(*, deprecated, message: "This is going away", renamed: "other(param:)")`
struct DeprecationDescription: Equatable, Codable, Sendable {

  /// A message used by the deprecation attribute.
  var message: String?

  /// A new name of the symbol, allowing the user to get a fix-it.
  var renamed: String?
}

/// A description of an availability guard.
///
/// For example: `@available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *)`
struct AvailabilityDescription: Equatable, Codable, Sendable {
  /// The array of OSes and versions which are specified in the availability guard.
  var osVersions: [OSVersion]
  init(osVersions: [OSVersion]) {
    self.osVersions = osVersions
  }

  /// An OS and its version.
  struct OSVersion: Equatable, Codable, Sendable {
    var os: OS
    var version: String
    init(os: OS, version: String) {
      self.os = os
      self.version = version
    }
  }

  /// One of the possible OSes.
  // swift-format-ignore: DontRepeatTypeInStaticProperties
  struct OS: Equatable, Codable, Sendable {
    var name: String

    init(name: String) {
      self.name = name
    }

    static let macOS = Self(name: "macOS")
    static let iOS = Self(name: "iOS")
    static let watchOS = Self(name: "watchOS")
    static let tvOS = Self(name: "tvOS")
    static let visionOS = Self(name: "visionOS")
  }
}

/// A description of an assignment expression.
///
/// For example: `foo = 42`.
struct AssignmentDescription: Equatable, Codable, Sendable {

  /// The left-hand side expression, the variable to assign to.
  ///
  /// For example, in `foo = 42`, `left` is `foo`.
  var left: Expression

  /// The right-hand side expression, the value to assign.
  ///
  /// For example, in `foo = 42`, `right` is `42`.
  var right: Expression
}

/// A switch case kind, either a `case` or a `default`.
enum SwitchCaseKind: Equatable, Codable, Sendable {

  /// A case.
  ///
  /// For example: `case let foo(bar):`.
  case `case`(Expression, [String])

  /// A case with multiple comma-separated expressions.
  ///
  /// For example: `case "foo", "bar":`.
  case multiCase([Expression])

  /// A default. Spelled as `default:`.
  case `default`
}

/// A description of a switch case definition.
///
/// For example: `case foo: print("foo")`.
struct SwitchCaseDescription: Equatable, Codable, Sendable {

  /// The kind of the switch case.
  var kind: SwitchCaseKind

  /// The body of the switch case.
  ///
  /// For example, in `case foo: print("foo")`, `body`
  /// represents `print("foo")`.
  var body: [CodeBlock]
}

/// A description of a switch statement expression.
///
/// For example: `switch foo {`.
struct SwitchDescription: Equatable, Codable, Sendable {

  /// The expression evaluated by the switch statement.
  ///
  /// For example, in `switch foo {`, `switchedExpression` is `foo`.
  var switchedExpression: Expression

  /// The cases defined in the switch statement.
  var cases: [SwitchCaseDescription]
}

/// A description of an if branch and the corresponding code block.
///
/// For example: in `if foo { bar }`, the condition pair represents
/// `foo` + `bar`.
struct IfBranch: Equatable, Codable, Sendable {

  /// The expressions evaluated by the if statement and their corresponding
  /// body blocks. If more than one is provided, an `else if` branch is added.
  ///
  /// For example, in `if foo { bar }`, `condition` is `foo`.
  var condition: Expression

  /// The body executed if the `condition` evaluates to true.
  ///
  /// For example, in `if foo { bar }`, `body` is `bar`.
  var body: [CodeBlock]
}

/// A description of an if[[/elseif]/else] statement expression.
///
/// For example: `if foo { } else if bar { } else { }`.
struct IfStatementDescription: Equatable, Codable, Sendable {

  /// The primary `if` branch.
  var ifBranch: IfBranch

  /// Additional `else if` branches.
  var elseIfBranches: [IfBranch]

  /// The body of an else block.
  ///
  /// No `else` statement is added when `elseBody` is nil.
  var elseBody: [CodeBlock]?
}

/// A description of a do statement.
///
/// For example: `do { try foo() } catch { return bar }`.
struct DoStatementDescription: Equatable, Codable, Sendable {

  /// The code blocks in the `do` statement body.
  ///
  /// For example, in `do { try foo() } catch { return bar }`,
  /// `doBody` is `try foo()`.
  var doStatement: [CodeBlock]

  /// The code blocks in the `catch` statement.
  ///
  /// If nil, no `catch` statement is added.
  ///
  /// For example, in `do { try foo() } catch { return bar }`,
  /// `catchBody` is `return bar`.
  var catchBody: [CodeBlock]?
}

/// A description of a value binding used in enums with associated values.
///
/// For example: `let foo(bar)`.
struct ValueBindingDescription: Equatable, Codable, Sendable {

  /// The binding kind: `let` or `var`.
  var kind: BindingKind

  /// The bound values in a function call expression syntax.
  ///
  /// For example, in `let foo(bar)`, `value` represents `foo(bar)`.
  var value: FunctionCallDescription
}

/// A kind of a keyword.
enum KeywordKind: Equatable, Codable, Sendable {

  /// The return keyword.
  case `return`

  /// The try keyword.
  case `try`(hasPostfixQuestionMark: Bool)

  /// The await keyword.
  case `await`

  /// The throw keyword.
  case `throw`

  /// The yield keyword.
  case `yield`
}

/// A description of an expression that places a keyword before an expression.
struct UnaryKeywordDescription: Equatable, Codable, Sendable {

  /// The keyword to place before the expression.
  ///
  /// For example, in `return foo`, `kind` represents `return`.
  var kind: KeywordKind

  /// The expression prefixed by the keyword.
  ///
  /// For example, in `return foo`, `expression` represents `foo`.
  var expression: Expression? = nil
}

/// A description of a closure invocation.
///
/// For example: `{ foo in return foo + "bar" }`.
struct ClosureInvocationDescription: Equatable, Codable, Sendable {

  /// The names of the arguments taken by the closure.
  ///
  /// For example, in `{ foo in return foo + "bar" }`, `argumentNames`
  /// is `["foo"]`.
  var argumentNames: [String] = []

  /// The code blocks of the closure body.
  ///
  /// For example, in `{ foo in return foo + "bar" }`, `body`
  /// represents `return foo + "bar"`.
  var body: [CodeBlock]? = nil
}

/// A binary operator.
///
/// For example: `+=` in `a += b`.
enum BinaryOperator: String, Equatable, Codable, Sendable {

  /// The += operator, adds and then assigns another value.
  case plusEquals = "+="

  /// The == operator, checks equality between two values.
  case equals = "=="

  /// The ... operator, creates an end-inclusive range between two numbers.
  case rangeInclusive = "..."

  /// The || operator, used between two Boolean values.
  case booleanOr = "||"
}

/// A description of a binary operation expression.
///
/// For example: `foo += 1`.
struct BinaryOperationDescription: Equatable, Codable, Sendable {

  /// The left-hand side expression of the operation.
  ///
  /// For example, in `foo += 1`, `left` is `foo`.
  var left: Expression

  /// The binary operator tying the two expressions together.
  ///
  /// For example, in `foo += 1`, `operation` represents `+=`.
  var operation: BinaryOperator

  /// The right-hand side expression of the operation.
  ///
  /// For example, in `foo += 1`, `right` is `1`.
  var right: Expression
}

/// A description of an inout expression, which provides a read-write
/// reference to a variable.
///
/// For example, `&foo` passes a reference to the `foo` variable.
struct InOutDescription: Equatable, Codable, Sendable {

  /// The referenced expression.
  ///
  /// For example, in `&foo`, `referencedExpr` is `foo`.
  var referencedExpr: Expression
}

/// A description of an optional chaining expression.
///
/// For example, in `foo?`, `referencedExpr` is `foo`.
struct OptionalChainingDescription: Equatable, Codable, Sendable {

  /// The referenced expression.
  ///
  /// For example, in `foo?`, `referencedExpr` is `foo`.
  var referencedExpr: Expression
}

/// A description of a tuple.
///
/// For example: `(foo, bar)`.
struct TupleDescription: Equatable, Codable, Sendable {

  /// The member expressions.
  ///
  /// For example, in `(foo, bar)`, `members` is `[foo, bar]`.
  var members: [Expression]
}

/// A Swift expression.
indirect enum Expression: Equatable, Codable, Sendable {

  /// A literal.
  ///
  /// For example `"hello"` or `42`.
  case literal(LiteralDescription)

  /// An identifier, such as a variable name.
  ///
  /// For example, in `let foo = 42`, `foo` is an identifier.
  case identifier(IdentifierDescription)

  /// A member access expression.
  ///
  /// For example: `foo.bar`.
  case memberAccess(MemberAccessDescription)

  /// A function call.
  ///
  /// For example: `foo(bar: 42)`.
  case functionCall(FunctionCallDescription)

  /// An assignment expression.
  ///
  /// For example `foo = 42`.
  case assignment(AssignmentDescription)

  /// A switch statement expression.
  ///
  /// For example: `switch foo {`.
  case `switch`(SwitchDescription)

  /// An if statement, with optional else if's and an else statement attached.
  ///
  /// For example: `if foo { bar } else if baz { boo } else { bam }`.
  case ifStatement(IfStatementDescription)

  /// A do statement.
  ///
  /// For example: `do { try foo() } catch { return bar }`.
  case doStatement(DoStatementDescription)

  /// A value binding used in enums with associated values.
  ///
  /// For example: `let foo(bar)`.
  case valueBinding(ValueBindingDescription)

  /// An expression that places a keyword before an expression.
  case unaryKeyword(UnaryKeywordDescription)

  /// A closure invocation.
  ///
  /// For example: `{ foo in return foo + "bar" }`.
  case closureInvocation(ClosureInvocationDescription)

  /// A binary operation expression.
  ///
  /// For example: `foo += 1`.
  case binaryOperation(BinaryOperationDescription)

  /// An inout expression, which provides a reference to a variable.
  ///
  /// For example, `&foo` passes a reference to the `foo` variable.
  case inOut(InOutDescription)

  /// An optional chaining expression.
  ///
  /// For example, in `foo?`, `referencedExpr` is `foo`.
  case optionalChaining(OptionalChainingDescription)

  /// A tuple expression.
  ///
  /// For example: `(foo, bar)`.
  case tuple(TupleDescription)
}

/// A code block item, either a declaration or an expression.
enum CodeBlockItem: Equatable, Codable, Sendable {

  /// A declaration, such as of a new type or function.
  case declaration(Declaration)

  /// An expression, such as a call of a declared function.
  case expression(Expression)
}

/// A code block, with an optional comment.
struct CodeBlock: Equatable, Codable, Sendable {

  /// The comment to prepend to the code block item.
  var comment: Comment?

  /// The code block item that appears below the comment.
  var item: CodeBlockItem?
}

/// A description of a Swift file.
struct FileDescription: Equatable, Codable, Sendable {

  /// A comment placed at the top of the file.
  var topComment: Comment?

  /// Import statements placed below the top comment, but before the code
  /// blocks.
  var imports: [ImportDescription]?

  /// The code blocks that represent the main contents of the file.
  var codeBlocks: [CodeBlock]
}

/// A description of a named Swift file.
struct NamedFileDescription: Equatable, Codable, Sendable {

  /// A file name, including the file extension.
  ///
  /// For example: `Foo.swift`.
  var name: String

  /// The contents of the file.
  var contents: FileDescription
}

/// A file with contents made up of structured Swift code.
struct StructuredSwiftRepresentation: Equatable, Codable, Sendable {

  /// The contents of the file.
  var file: NamedFileDescription
}

// MARK: - Conveniences

extension Declaration {

  /// A variable declaration.
  ///
  /// For example: `let foo = 42`.
  /// - Parameters:
  ///   - accessModifier: An access modifier.
  ///   - isStatic: A Boolean value that indicates whether the variable
  ///   is static.
  ///   - kind: The variable binding kind.
  ///   - left: The name of the variable.
  ///   - type: The type of the variable.
  ///   - right: The expression to be assigned to the variable.
  ///   - getter: Body code for the getter of the variable.
  ///   - getterEffects: Effects of the getter.
  ///   - setter: Body code for the setter of the variable.
  ///   - modify: Body code for the `_modify` accessor.
  /// - Returns: Variable declaration.
  static func variable(
    accessModifier: AccessModifier? = nil,
    isStatic: Bool = false,
    kind: BindingKind,
    left: String,
    type: ExistingTypeDescription? = nil,
    right: Expression? = nil,
    getter: [CodeBlock]? = nil,
    getterEffects: [FunctionKeyword] = [],
    setter: [CodeBlock]? = nil,
    modify: [CodeBlock]? = nil

  ) -> Self {
    .variable(
      accessModifier: accessModifier,
      isStatic: isStatic,
      kind: kind,
      left: .identifierPattern(left),
      type: type,
      right: right,
      getter: getter,
      getterEffects: getterEffects,
      setter: setter,
      modify: modify
    )
  }

  /// A variable declaration.
  ///
  /// For example: `let foo = 42`.
  /// - Parameters:
  ///   - accessModifier: An access modifier.
  ///   - isStatic: A Boolean value that indicates whether the variable
  ///   is static.
  ///   - kind: The variable binding kind.
  ///   - left: The name of the variable.
  ///   - type: The type of the variable.
  ///   - right: The expression to be assigned to the variable.
  ///   - getter: Body code for the getter of the variable.
  ///   - getterEffects: Effects of the getter.
  ///   - setter: Body code for the setter of the variable.
  ///   - modify: Body code for the `_modify` accessor.
  /// - Returns: Variable declaration.
  static func variable(
    accessModifier: AccessModifier? = nil,
    isStatic: Bool = false,
    kind: BindingKind,
    left: Expression,
    type: ExistingTypeDescription? = nil,
    right: Expression? = nil,
    getter: [CodeBlock]? = nil,
    getterEffects: [FunctionKeyword] = [],
    setter: [CodeBlock]? = nil,
    modify: [CodeBlock]? = nil

  ) -> Self {
    .variable(
      .init(
        accessModifier: accessModifier,
        isStatic: isStatic,
        kind: kind,
        left: left,
        type: type,
        right: right,
        getter: getter,
        getterEffects: getterEffects,
        setter: setter,
        modify: modify
      )
    )
  }

  /// A description of an enum case.
  ///
  /// For example: `case foo(String)`.
  /// - Parameters:
  ///   - name: The name of the enum case.
  ///   - kind: The kind of the enum case.
  /// - Returns: An enum case declaration.
  static func enumCase(name: String, kind: EnumCaseKind = .nameOnly) -> Self {
    .enumCase(.init(name: name, kind: kind))
  }

  /// A description of a typealias declaration.
  ///
  /// For example `typealias Foo = Int`.
  /// - Parameters:
  ///   - accessModifier: An access modifier.
  ///   - name: The name of the typealias.
  ///   - existingType: The existing type that serves as the
  ///   underlying type of the alias.
  /// - Returns: A typealias declaration.
  static func `typealias`(
    accessModifier: AccessModifier? = nil,
    name: String,
    existingType: ExistingTypeDescription
  )
    -> Self
  { .typealias(.init(accessModifier: accessModifier, name: name, existingType: existingType)) }

  /// A description of a function definition.
  ///
  /// For example: `func foo() { }`.
  /// - Parameters:
  ///   - accessModifier: An access modifier.
  ///   - kind: The kind of the function.
  ///   - parameters: The parameters of the function.
  ///   - keywords: The keywords of the function, such as `async` and
  ///   `throws.`
  ///   - returnType: The return type name of the function, such as `Int`.
  ///   - body: The body definition of the function.
  /// - Returns: A function declaration.
  static func function(
    accessModifier: AccessModifier? = nil,
    kind: FunctionKind,
    generics: [ExistingTypeDescription] = [],
    parameters: [ParameterDescription],
    keywords: [FunctionKeyword] = [],
    returnType: Expression? = nil,
    whereClause: WhereClause?,
    body: [CodeBlock]? = nil
  ) -> Self {
    .function(
      .init(
        accessModifier: accessModifier,
        kind: kind,
        generics: generics,
        parameters: parameters,
        keywords: keywords,
        returnType: returnType,
        whereClause: whereClause,
        body: body
      )
    )
  }

  /// A description of a function definition.
  ///
  /// For example: `func foo() { }`.
  /// - Parameters:
  ///   - signature: The signature of the function.
  ///   - body: The body definition of the function.
  /// - Returns: A function declaration.
  static func function(signature: FunctionSignatureDescription, body: [CodeBlock]? = nil) -> Self {
    .function(.init(signature: signature, body: body))
  }

  /// A description of an enum declaration.
  ///
  /// For example `enum Bar {`.
  /// - Parameters:
  ///   - isFrozen: A Boolean value that indicates whether the enum has
  ///   a `@frozen` attribute.
  ///   - accessModifier: An access modifier.
  ///   - name: The name of the enum.
  ///   - conformances: The type names that the enum conforms to.
  ///   - members: The declarations that make up the enum body.
  /// - Returns: An enum declaration.
  static func `enum`(
    isFrozen: Bool = false,
    accessModifier: AccessModifier? = nil,
    name: String,
    conformances: [String] = [],
    members: [Declaration] = []
  ) -> Self {
    .enum(
      .init(
        isFrozen: isFrozen,
        accessModifier: accessModifier,
        name: name,
        conformances: conformances,
        members: members
      )
    )
  }

  /// A description of an extension declaration.
  ///
  /// For example `extension Foo {`.
  /// - Parameters:
  ///   - accessModifier: An access modifier.
  ///   - onType: The name of the extended type.
  ///   - conformances: Additional type names that the extension conforms to.
  ///   - whereClause: A where clause constraining the extension declaration.
  ///   - declarations: The declarations that the extension adds on the
  ///   extended type.
  /// - Returns: An extension declaration.
  static func `extension`(
    accessModifier: AccessModifier? = nil,
    onType: String,
    conformances: [String] = [],
    whereClause: WhereClause? = nil,
    declarations: [Declaration]
  ) -> Self {
    .extension(
      .init(
        accessModifier: accessModifier,
        onType: onType,
        conformances: conformances,
        whereClause: whereClause,
        declarations: declarations
      )
    )
  }
}

extension FunctionKind {
  /// Returns a non-failable initializer, for example `init()`.
  static var initializer: Self { .initializer(failable: false) }

  /// Returns a non-static function kind.
  static func function(name: String) -> Self {
    .function(name: name, isStatic: false)
  }
}

extension CodeBlock {

  /// Returns a new declaration code block wrapping the provided declaration.
  /// - Parameter declaration: The declaration to wrap.
  /// - Returns: A new `CodeBlock` instance containing the provided declaration.
  static func declaration(_ declaration: Declaration) -> Self {
    CodeBlock(item: .declaration(declaration))
  }

  /// Returns a new expression code block wrapping the provided expression.
  /// - Parameter expression: The expression to wrap.
  /// - Returns: A new `CodeBlock` instance containing the provided declaration.
  static func expression(_ expression: Expression) -> Self {
    CodeBlock(item: .expression(expression))
  }
}

extension Expression {

  /// A string literal.
  ///
  /// For example: `"hello"`.
  /// - Parameter value: The string value of the literal.
  /// - Returns: A new `Expression` representing a string  literal.
  static func literal(_ value: String) -> Self { .literal(.string(value)) }

  /// An integer literal.
  ///
  /// For example `42`.
  /// - Parameter value: The integer value of the literal.
  /// - Returns: A new `Expression` representing an integer literal.
  static func literal(_ value: Int) -> Self { .literal(.int(value)) }

  /// Returns a new expression that accesses the member on the current
  /// expression.
  /// - Parameter member: The name of the member to access on the expression.
  /// - Returns: A new expression representing member access.
  func dot(_ member: String) -> Expression { .memberAccess(.init(left: self, right: member)) }

  /// Returns a new expression that calls the current expression as a function
  /// with the specified arguments.
  /// - Parameter arguments: The arguments used to call the expression.
  /// - Returns: A new expression representing a function call.
  func call(_ arguments: [FunctionArgumentDescription]) -> Expression {
    .functionCall(.init(calledExpression: self, arguments: arguments))
  }

  /// Returns a new member access expression without a receiver,
  /// starting with dot.
  ///
  /// For example: `.foo`, where `member` is `foo`.
  /// - Parameter member: The name of the member to access.
  /// - Returns: A new expression representing member access with a dot prefix.
  static func dot(_ member: String) -> Self { Self.memberAccess(.init(right: member)) }

  /// Returns a new identifier expression for the provided pattern, such
  /// as a variable or function name.
  /// - Parameter name: The name of the identifier.
  /// - Returns: A new expression representing an identifier with
  ///   the specified name.
  static func identifierPattern(_ name: String) -> Self { .identifier(.pattern(name)) }

  /// Returns a new identifier expression for the provided type name.
  /// - Parameter type: The description of the type.
  /// - Returns: A new expression representing an identifier with
  ///   the specified name.
  static func identifierType(_ type: ExistingTypeDescription) -> Self { .identifier(.type(type)) }

  /// Returns a new identifier expression for the provided type name.
  /// - Parameter type: The name of the type.
  /// - Returns: A new expression representing an identifier with
  ///   the specified name.
  static func identifierType(_ type: TypeName) -> Self { .identifier(.type(.init(type))) }

  /// Returns a new identifier expression for the provided type name.
  /// - Parameter type: The usage of the type.
  /// - Returns: A new expression representing an identifier with
  ///   the specified name.
  static func identifierType(_ type: TypeUsage) -> Self { .identifier(.type(.init(type))) }

  /// Returns a new switch statement expression.
  /// - Parameters:
  ///   - switchedExpression: The expression evaluated by the switch
  ///    statement.
  ///   - cases: The cases defined in the switch statement.
  /// - Returns: A new expression representing a switch statement with the specified switched expression and cases
  static func `switch`(switchedExpression: Expression, cases: [SwitchCaseDescription]) -> Self {
    .`switch`(.init(switchedExpression: switchedExpression, cases: cases))
  }

  /// Returns an if statement, with optional else if's and an else
  /// statement attached.
  /// - Parameters:
  ///   - ifBranch: The primary `if` branch.
  ///   - elseIfBranches: Additional `else if` branches.
  ///   - elseBody: The body of an else block.
  /// - Returns: A new expression representing an `if` statement with the specified branches and blocks.
  static func ifStatement(
    ifBranch: IfBranch,
    elseIfBranches: [IfBranch] = [],
    elseBody: [CodeBlock]? = nil
  ) -> Self {
    .ifStatement(.init(ifBranch: ifBranch, elseIfBranches: elseIfBranches, elseBody: elseBody))
  }

  /// Returns a new function call expression.
  ///
  /// For example `foo(bar: 42)`.
  /// - Parameters:
  ///   - calledExpression: The expression to be called as a function.
  ///   - arguments: The arguments to be passed to the function call.
  ///   - trailingClosure: A trailing closure.
  /// - Returns: A new expression representing a function call with the specified called expression and arguments.
  static func functionCall(
    calledExpression: Expression,
    arguments: [FunctionArgumentDescription] = [],
    trailingClosure: ClosureInvocationDescription? = nil
  ) -> Self {
    .functionCall(
      .init(
        calledExpression: calledExpression,
        arguments: arguments,
        trailingClosure: trailingClosure
      )
    )
  }

  /// Returns a new function call expression.
  ///
  /// For example: `foo(bar: 42)`.
  /// - Parameters:
  ///   - calledExpression: The expression called as a function.
  ///   - arguments: The arguments passed to the function call.
  ///   - trailingClosure: A trailing closure.
  /// - Returns: A new expression representing a function call with the specified called expression and arguments.
  static func functionCall(
    calledExpression: Expression,
    arguments: [Expression],
    trailingClosure: ClosureInvocationDescription? = nil
  ) -> Self {
    .functionCall(
      .init(
        calledExpression: calledExpression,
        arguments: arguments.map { .init(label: nil, expression: $0) },
        trailingClosure: trailingClosure
      )
    )
  }

  /// Returns a new expression that places a keyword before an expression.
  /// - Parameters:
  ///   - kind: The keyword to place before the expression.
  ///   - expression: The expression prefixed by the keyword.
  /// - Returns: A new expression with the specified keyword placed before the expression.
  static func unaryKeyword(kind: KeywordKind, expression: Expression? = nil) -> Self {
    .unaryKeyword(.init(kind: kind, expression: expression))
  }

  /// Returns a new expression that puts the return keyword before
  /// an expression.
  /// - Parameter expression: The expression to prepend.
  /// - Returns: A new expression with the `return` keyword placed before the expression.
  static func `return`(_ expression: Expression? = nil) -> Self {
    .unaryKeyword(kind: .return, expression: expression)
  }

  /// Returns a new expression that puts the try keyword before
  /// an expression.
  /// - Parameter expression: The expression to prepend.
  /// - Returns: A new expression with the `try` keyword placed before the expression.
  static func `try`(_ expression: Expression) -> Self {
    .unaryKeyword(kind: .try, expression: expression)
  }

  /// Returns a new expression that puts the try? keyword before
  /// an expression.
  /// - Parameter expression: The expression to prepend.
  /// - Returns: A new expression with the `try?` keyword placed before the expression.
  static func optionalTry(_ expression: Expression) -> Self {
    .unaryKeyword(kind: .try(hasPostfixQuestionMark: true), expression: expression)
  }

  /// Returns a new expression that puts the await keyword before
  /// an expression.
  /// - Parameter expression: The expression to prepend.
  /// - Returns: A new expression with the `await` keyword placed before the expression.
  static func `await`(_ expression: Expression) -> Self {
    .unaryKeyword(kind: .await, expression: expression)
  }

  /// Returns a new expression that puts the yield keyword before
  /// an expression.
  /// - Parameter expression: The expression to prepend.
  /// - Returns: A new expression with the `yield` keyword placed before the expression.
  static func `yield`(_ expression: Expression) -> Self {
    .unaryKeyword(kind: .yield, expression: expression)
  }

  /// Returns a new expression that puts the provided code blocks into
  /// a do/catch block.
  /// - Parameter:
  ///   - doStatement: The code blocks in the `do` statement body.
  ///   - catchBody: The code blocks in the `catch` statement.
  /// - Returns: The expression.
  static func `do`(_ doStatement: [CodeBlock], catchBody: [CodeBlock]? = nil) -> Self {
    .doStatement(.init(doStatement: doStatement, catchBody: catchBody))
  }

  /// Returns a new value binding used in enums with associated values.
  ///
  /// For example: `let foo(bar)`.
  /// - Parameters:
  ///   - kind: The binding kind: `let` or `var`.
  ///   - value: The bound values in a function call expression syntax.
  /// - Returns: A new expression representing the value binding.
  static func valueBinding(kind: BindingKind, value: FunctionCallDescription) -> Self {
    .valueBinding(.init(kind: kind, value: value))
  }

  /// Returns a new closure invocation expression.
  ///
  /// For example: such as `{ foo in return foo + "bar" }`.
  /// - Parameters:
  ///   - argumentNames: The names of the arguments taken by the closure.
  ///   - body: The code blocks of the closure body.
  /// - Returns: A new expression representing the closure invocation
  static func `closureInvocation`(argumentNames: [String] = [], body: [CodeBlock]? = nil) -> Self {
    .closureInvocation(.init(argumentNames: argumentNames, body: body))
  }

  /// Creates a new binary operation expression.
  ///
  /// For example: `foo += 1`.
  /// - Parameters:
  ///   - left: The left-hand side expression of the operation.
  ///   - operation: The binary operator tying the two expressions together.
  ///   - right: The right-hand side expression of the operation.
  /// - Returns: A new expression representing the binary operation.
  static func `binaryOperation`(
    left: Expression,
    operation: BinaryOperator,
    right: Expression
  ) -> Self {
    .binaryOperation(.init(left: left, operation: operation, right: right))
  }

  /// Creates a new inout expression, which provides a read-write
  /// reference to a variable.
  ///
  /// For example, `&foo` passes a reference to the `foo` variable.
  /// - Parameter referencedExpr: The referenced expression.
  /// - Returns: A new expression representing the inout expression.
  static func inOut(_ referencedExpr: Expression) -> Self {
    .inOut(.init(referencedExpr: referencedExpr))
  }

  /// Creates a new assignment expression.
  ///
  /// For example: `foo = 42`.
  /// - Parameters:
  ///   - left: The left-hand side expression, the variable to assign to.
  ///   - right: The right-hand side expression, the value to assign.
  /// - Returns: Assignment expression.
  static func assignment(left: Expression, right: Expression) -> Self {
    .assignment(.init(left: left, right: right))
  }

  /// Returns a new optional chaining expression wrapping the current
  /// expression.
  ///
  /// For example, for the current expression `foo`, returns `foo?`.
  /// - Returns: A new expression representing the optional chaining operation.
  func optionallyChained() -> Self { .optionalChaining(.init(referencedExpr: self)) }

  /// Returns a new tuple expression.
  ///
  /// For example, in `(foo, bar)`, `members` is `[foo, bar]`.
  /// - Parameter expressions: The member expressions.
  /// - Returns: A tuple expression.
  static func tuple(_ expressions: [Expression]) -> Self { .tuple(.init(members: expressions)) }
}

extension MemberAccessDescription {
  /// Creates a new member access expression without a receiver, starting
  /// with dot.
  ///
  /// For example, `.foo`, where `member` is `foo`.
  /// - Parameter member: The name of the member to access.
  /// - Returns: A new member access expression.
  static func dot(_ member: String) -> Self { .init(right: member) }
}

extension LiteralDescription: ExpressibleByStringLiteral, ExpressibleByNilLiteral,
  ExpressibleByArrayLiteral
{
  init(arrayLiteral elements: Expression...) { self = .array(elements) }

  init(stringLiteral value: String) { self = .string(value) }

  init(nilLiteral: ()) { self = .nil }
}

extension VariableDescription {

  /// Returns a new mutable variable declaration.
  ///
  /// For example `var foo = 42`.
  /// - Parameter name: The name of the variable.
  /// - Returns: A new mutable variable declaration.
  static func `var`(_ name: String) -> Self {
    Self.init(kind: .var, left: .identifierPattern(name))
  }

  /// Returns a new immutable variable declaration.
  ///
  /// For example `let foo = 42`.
  /// - Parameter name: The name of the variable.
  /// - Returns: A new immutable variable declaration.
  static func `let`(_ name: String) -> Self {
    Self.init(kind: .let, left: .identifierPattern(name))
  }
}

extension Expression {

  /// Creates a new assignment description where the called expression is
  /// assigned the value of the specified expression.
  /// - Parameter rhs: The right-hand side of the assignment expression.
  /// - Returns: An assignment description representing the assignment.
  func equals(_ rhs: Expression) -> AssignmentDescription { .init(left: self, right: rhs) }
}

extension FunctionSignatureDescription {
  /// Returns a new function signature description that has the access
  /// modifier updated to the specified one.
  /// - Parameter accessModifier: The access modifier to use.
  /// - Returns: A function signature description with the specified access modifier.
  func withAccessModifier(_ accessModifier: AccessModifier?) -> Self {
    var value = self
    value.accessModifier = accessModifier
    return value
  }
}

extension SwitchCaseKind {
  /// Returns a new switch case kind with no argument names, only the
  /// specified expression as the name.
  /// - Parameter expression: The expression for the switch case label.
  /// - Returns: A switch case kind with the specified expression as the label.
  static func `case`(_ expression: Expression) -> Self { .case(expression, []) }
}

extension KeywordKind {

  /// Returns the try keyword without the postfix question mark.
  static var `try`: Self { .try(hasPostfixQuestionMark: false) }
}

extension Declaration {
  /// Returns a new deprecated variant of the declaration if `shouldDeprecate` is true.
  func deprecate(if shouldDeprecate: Bool) -> Self {
    if shouldDeprecate { return .deprecated(.init(), self) }
    return self
  }

  /// Returns the declaration one level deeper, nested inside the commentable
  /// declaration, if present.
  var strippingTopComment: Self {
    guard case let .commentable(_, underlyingDecl) = self else { return self }
    return underlyingDecl
  }
}

extension Declaration {

  /// An access modifier.
  var accessModifier: AccessModifier? {
    get {
      switch self {
      case .commentable(_, let declaration): return declaration.accessModifier
      case .deprecated(_, let declaration): return declaration.accessModifier
      case .variable(let variableDescription): return variableDescription.accessModifier
      case .extension(let extensionDescription): return extensionDescription.accessModifier
      case .struct(let structDescription): return structDescription.accessModifier
      case .enum(let enumDescription): return enumDescription.accessModifier
      case .typealias(let typealiasDescription): return typealiasDescription.accessModifier
      case .protocol(let protocolDescription): return protocolDescription.accessModifier
      case .function(let functionDescription): return functionDescription.signature.accessModifier
      case .enumCase: return nil
      }
    }
    set {
      switch self {
      case .commentable(let comment, var declaration):
        declaration.accessModifier = newValue
        self = .commentable(comment, declaration)
      case .deprecated(let deprecationDescription, var declaration):
        declaration.accessModifier = newValue
        self = .deprecated(deprecationDescription, declaration)
      case .variable(var variableDescription):
        variableDescription.accessModifier = newValue
        self = .variable(variableDescription)
      case .extension(var extensionDescription):
        extensionDescription.accessModifier = newValue
        self = .extension(extensionDescription)
      case .struct(var structDescription):
        structDescription.accessModifier = newValue
        self = .struct(structDescription)
      case .enum(var enumDescription):
        enumDescription.accessModifier = newValue
        self = .enum(enumDescription)
      case .typealias(var typealiasDescription):
        typealiasDescription.accessModifier = newValue
        self = .typealias(typealiasDescription)
      case .protocol(var protocolDescription):
        protocolDescription.accessModifier = newValue
        self = .protocol(protocolDescription)
      case .function(var functionDescription):
        functionDescription.signature.accessModifier = newValue
        self = .function(functionDescription)
      case .enumCase: break
      }
    }
  }
}

extension ExistingTypeDescription {

  /// Creates a member type description with the provided single component.
  /// - Parameter singleComponent: A single component of the name.
  /// - Returns: The new type description.
  static func member(_ singleComponent: String) -> Self { .member([singleComponent]) }
}