groovy.transform.stc.ClosureParams Maven / Gradle / Ivy

/*
 * Copyright 2003-2013 the original author or authors.
 *
 * 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.
 */
package groovy.transform.stc;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * Parameter annotation aimed at helping the IDEs or the static type checker to infer the
 * parameter types of a closure. Without this annotation, a method signature may look like
 * this:
 * public <T,R> List<R> doSomething(List<T> source, Closure<R> consumer)
 * 

 * 
The problem this annotation tries to solve is to define the expected parameter types of the
 * consumer closure. The generics type defined in Closure<R> correspond to the
 * result type of the closure, but tell nothing about what the closure must accept as arguments.
 * 
 * There's no way in Java or Groovy to express the type signature of the expected closure call method from
 * outside the closure itself, so we rely on an annotation here. Unfortunately, annotations also have limitations
 * (like not being able to use generics placeholder as annotation values) that prevent us from expressing the
 * type directly.
 * Additionally, closures are polymorphic. This means that a single closure can be used with different, valid,
 * parameter signatures. A typical use case can be found when a closure accepts either a {@link java.util.Map.Entry}
 * or a (key,value) pair, like the {@link org.codehaus.groovy.runtime.DefaultGroovyMethods#each(java.util.Map, groovy.lang.Closure)}
 * method.
 * For those reasons, the {@link ClosureParams} annotation only takes two arguments:
 * 

 *     {@link ClosureParams#value()} defines a {@link groovy.transform.stc.ClosureSignatureHint} hint class
 *     that the compiler will use to infer the parameter types
 *     {@link ClosureParams#options()}, a set of options that are passed to the hint when the type is inferred
 * 
 * 
 * As a result, the previous signature can be written like this:
 * public <T,R> List<R> doSomething(List<T> source, @ClosureParams(FirstParam.FirstGenericType.class) Closure<R> consumer)
 * Which uses the {@link FirstParam.FirstGenericType} first generic type of the first argument hint to tell that the only expected
 * argument type corresponds to the type of the first generic argument type of the first method parameter.
 *
 * @author Cédric Champeau
 */
@Target(ElementType.PARAMETER)
@Retention(RetentionPolicy.RUNTIME)
public @interface ClosureParams {
    Class value();
    String[] options() default {};
}