/*
* Copyright (c) 2014, the Dart project authors.
*
* Licensed under the Eclipse Public License v1.0 (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.eclipse.org/legal/epl-v10.html
*
* 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.
*/
package com.google.dart.engine.type;
import java.util.Set;
/**
* A flat immutable union of {@code Type}s. Here "flat" means a union type never contains another
* union type.
*/
// Other operations we may want:
//
// - lub: fold lub over the elements of the union. This will be less useful with
// the spec-defined lub, but more useful with a custom lub that e.g. defines
// [lub(List<int>, List<double>) = List<dynamic>].
// Discussion
//
// The union type is not really like other types in the implementation:
//
// - union types are never associated with [Element]s.
// - no user program ever declares one.
// - no value ever has union type.
// - they are never components of other types (e.g., never have [G<X|Y>]).
//
// With this in mind, I was initially inclined to not make [UnionType] a subtype of [Type].
// However, that causes problems if we want to make the propagated type a union type, since
// much existing code depends on the propagated type being a [Type].
// Moreover, by making [UnionType] a [Type], we can more easily do
// other type-related things with it in the future. For example, if we were doing type
// inference for functions, then we might infer that a function returned a union type. So,
// [UnionType extends Type].
public interface UnionType extends Type {
/**
* @return an immutable view of the types in this union type.
*/
public Set<Type> getElements();
}