sdk/lib/core/bool.dart

// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

part of dart.core;

/// The reserved words `true` and `false` denote objects that are the only two
/// instances of this class.
///
/// It is a compile-time error for a class to attempt to extend or implement
/// bool.
@pragma("vm:entry-point")
class bool {
  /// Returns the boolean value of the environment declaration [name].
  ///
  /// The boolean value of the declaration is `true` if the declared value is
  /// the string `"true"`, and `false` if the value is `"false"`.
  ///
  /// In all other cases, including when there is no declaration for `name`,
  /// the result is the [defaultValue].
  ///
  /// The result is the same as would be returned by:
  /// ```dart template:expression
  /// (const String.fromEnvironment(name) == "true")
  ///     ? true
  ///     : (const String.fromEnvironment(name) == "false")
  ///         ? false
  ///         : defaultValue
  /// ```
  /// Example:
  /// ```dart
  /// const loggingFlag = const bool.fromEnvironment("logging");
  /// ```
  /// If you want to use a different truth-string than `"true"`, you can use the
  /// [String.fromEnvironment] constructor directly:
  /// ```dart
  /// const isLoggingOn = (const String.fromEnvironment("logging") == "on");
  /// ```
  ///
  /// The string value, or lack of a value, associated with a [name]
  /// must be consistent across all calls to [String.fromEnvironment],
  /// [int.fromEnvironment], `bool.fromEnvironment` and [bool.hasEnvironment]
  /// in a single program.
  ///
  /// This constructor is only guaranteed to work when invoked as `const`.
  /// It may work as a non-constant invocation on some platforms which
  /// have access to compiler options at run-time, but most ahead-of-time
  /// compiled platforms will not have this information.
  // The .fromEnvironment() constructors are special in that we do not want
  // users to call them using "new". We prohibit that by giving them bodies
  // that throw, even though const constructors are not allowed to have bodies.
  // Disable those static errors.
  //ignore: const_constructor_with_body
  //ignore: const_factory
  external const factory bool.fromEnvironment(String name,
      {bool defaultValue = false});

  /// Whether there is an environment declaration [name].
  ///
  /// Returns true iff there is an environment declaration with the name [name]
  /// If there is then the value of that declaration can be accessed using
  /// `const String.fromEnvironment(name)`. Otherwise,
  /// `String.fromEnvironment(name, defaultValue: someString)`
  /// will evaluate to the given `defaultValue`.
  ///
  /// This constructor can be used to handle an absent declaration
  /// specifically, in ways that cannot be represented by providing
  /// a default value to the `C.fromEnvironment` constructor where `C`
  /// is one of [String], [int], or [bool].
  ///
  /// Example:
  /// ```dart
  /// const loggingIsDeclared = bool.hasEnvironment("logging");
  ///
  /// const String? logger = loggingIsDeclared
  ///     ? String.fromEnvironment("logging")
  ///     : null;
  /// ```
  ///
  /// The string value, or lack of a value, associated with a [name]
  /// must be consistent across all calls to [String.fromEnvironment],
  /// [int.fromEnvironment], [bool.fromEnvironment] and `bool.hasEnvironment`
  /// in a single program.
  ///
  /// This constructor is only guaranteed to work when invoked as `const`.
  /// It may work as a non-constant invocation on some platforms which
  /// have access to compiler options at run-time, but most ahead-of-time
  /// compiled platforms will not have this information.
  // The .hasEnvironment() constructor is special in that we do not want
  // users to call them using "new". We prohibit that by giving them bodies
  // that throw, even though const constructors are not allowed to have bodies.
  // Disable those static errors.
  //ignore: const_constructor_with_body
  //ignore: const_factory
  external const factory bool.hasEnvironment(String name);

  external int get hashCode;

  /// The logical conjunction ("and") of this and [other].
  ///
  /// Returns `true` if both this and [other] are `true`, and `false` otherwise.
  @Since("2.1")
  bool operator &(bool other) => other && this;

  /// The logical disjunction ("inclusive or") of this and [other].
  ///
  /// Returns `true` if either this or [other] is `true`, and `false` otherwise.
  @Since("2.1")
  bool operator |(bool other) => other || this;

  /// The logical exclusive disjunction ("exclusive or") of this and [other].
  ///
  /// Returns whether this and [other] are neither both `true` nor both `false`.
  @Since("2.1")
  bool operator ^(bool other) => !other == this;

  /// Returns either `"true"` for `true` and `"false"` for `false`.
  String toString() {
    return this ? "true" : "false";
  }
}