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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 side-effect-free if it has no visible side-effects, such as setting a
* field of an object that existed before the method was called.
*
* 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.
*
*
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.
*
*
Also see {@link Pure}, which means both side-effect-free and {@link Deterministic}.
*
*
Analysis: 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:
*
*
* - Assignment to any expression, except for local variables and method parameters.
* (Note that storing into an array element, such a {@code a[i] = x}, is not an assignment to
* a variable and is therefore forbidden.)
* - A method invocation of a method that is not {@code @SideEffectFree}.
*
- Construction of a new object where the constructor is not {@code @SideEffectFree}.
*
*
* 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.
*
* 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.
*
*
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.CLASS)
@Target({ElementType.METHOD, ElementType.CONSTRUCTOR})
public @interface SideEffectFree {}