package org.checkerframework.dataflow.qual;

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

/**
 * A method is called <em>side-effect-free</em> if it has no visible side-effects, such as setting a
 * field of an object that existed before the method was called.
 *
 * <p>Only the visible side effects are important. The method is allowed to cache the answer to a
 * computationally expensive query, for instance. It is also allowed to modify newly-created
 * objects, and a constructor is side-effect-free if it does not modify any objects that existed
 * before it was called.
 *
 * <p>This annotation is important to pluggable type-checking because if some fact about an object
 * is known before a call to such a method, then the fact is still known afterwards, even if the
 * fact is about some non-final field. When any non-{@code @SideEffectFree} method is called, then a
 * pluggable type-checker must assume that any field of any accessible object might have been
 * modified, which annuls the effect of flow-sensitive type refinement and prevents the pluggable
 * type-checker from making conclusions that are obvious to a programmer.
 *
 * <p>Also see {@link Pure}, which means both side-effect-free and {@link Deterministic}.
 *
 * <p><b>Analysis:</b> The Checker Framework performs a conservative analysis to verify a
 * {@code @SideEffectFree} annotation. The Checker Framework issues a warning if the method uses any
 * of the following Java constructs:
 *
 * <ol>
 *   <li>Assignment to any expression, except for local variables and method parameters.<br>
 *       (Note that storing into an array element, such a {@code a[i] = x}, is not an assignment to
 *       a variable and is therefore forbidden.)
 *   <li>A method invocation of a method that is not {@code @SideEffectFree}.
 *   <li>Construction of a new object where the constructor is not {@code @SideEffectFree}.
 * </ol>
 *
 * These rules are conservative: any code that passes the checks is side-effect-free, but the
 * Checker Framework may issue false positive warnings, for code that uses one of the forbidden
 * constructs but is side-effect-free nonetheless. In particular, a method that caches its result
 * will be rejected.
 *
 * <p>In fact, the rules are so conservative that checking is currently disabled by default, but can
 * be enabled via the {@code -AcheckPurityAnnotations} command-line option.
 *
 * <p>This annotation is inherited by subtypes, just as if it were meta-annotated with
 * {@code @InheritedAnnotation}.
 *
 * @checker_framework.manual #type-refinement-purity Side effects, determinism, purity, and
 *     flow-sensitive analysis
 */
// @InheritedAnnotation cannot be written here, because "dataflow" project cannot depend on
// "framework" project.
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD, ElementType.CONSTRUCTOR})
public @interface SideEffectFree {}