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quiver/lib/src/core/optional.dart

-// Copyright 2013 Google Inc. 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.
-
-part of quiver.core;
-
-/**
- * A value that might be absent.
- *
- * Use Optional as an alternative to allowing fields, parameters or return
- * values to be null. It signals that a value is not required and provides
- * convenience methods for dealing with the absent case.
- */
-class Optional<T> {
-  final T _value;
-
-  /**
-   * Constructs an empty Optional.
-   */
-  const Optional.absent() : _value = null;
-
-  /**
-   * Constructs an Optional of the given [value].
-   *
-   * Throws [ArgumentError] if [value] is null.
-   */
-  Optional.of(T value) : this._value = value {
-    if (this._value == null) throw new ArgumentError('Must not be null.');
-  }
-
-  /**
-   * Constructs an Optional of the given [value].
-   *
-   * If [value] is null, returns [absent()].
-   */
-  const Optional.fromNullable(T value) : this._value = value;
-
-  /**
-   * Whether the Optional contains a value.
-   */
-  bool get isPresent => _value != null;
-
-  /**
-   * Gets the Optional value.
-   *
-   * Throws [StateError] if [value] is null.
-   */
-  T get value {
-    if (this._value == null) {
-      throw new StateError('value called on absent Optional.');
-    }
-    return _value;
-  }
-
-  /**
-   * Executes a function if the Optional value is present.
-   */
-  void ifPresent(void ifPresent(T value)) {
-    if (isPresent) {
-      ifPresent(_value);
-    }
-  }
-
-  /**
-   * Execution a function if the Optional value is absent.
-   */
-  void ifAbsent(void ifAbsent()) {
-    if (!isPresent) {
-      ifAbsent();
-    }
-  }
-
-  /**
-   * Gets the Optional value with a default.
-   *
-   * The default is returned if the Optional is [absent()].
-   *
-   * Throws [ArgumentError] if [defaultValue] is null.
-   */
-  T or(T defaultValue) {
-    if (defaultValue == null) {
-      throw new ArgumentError('defaultValue must not be null.');
-    }
-    return _value == null ? defaultValue : _value;
-  }
-
-  /**
-   * Gets the Optional value, or [null] if there is none.
-   */
-  T get orNull => _value;
-
-  /**
-   * Transforms the Optional value.
-   *
-   * If the Optional is [absent()], returns [absent()] without applying the transformer.
-   *
-   * The transformer must not return [null]. If it does, an [ArgumentError] is thrown.
-   */
-  Optional transform(dynamic transformer(T value)) {
-    return _value == null
-        ? new Optional.absent()
-        : new Optional.of(transformer(_value));
-  }
-
-  /**
-   * Delegates to the underlying [value] hashCode.
-   */
-  int get hashCode => _value.hashCode;
-
-  /**
-   * Delegates to the underlying [value] operator==.
-   */
-  bool operator ==(o) => o is Optional && o._value == _value;
-
-  String toString() {
-    return _value == null
-        ? 'Optional { absent }'
-        : 'Optional { value: ${_value} }';
-  }
-}