On this page:

16.3 Wills and Executors

A will executor manages a collection of values and associated will procedures (a.k.a. finalizers). The will procedure for each value is ready to be executed when the value has been proven (by the garbage collector) to be unreachable, except through weak references (see Weak Boxes) or as the registrant for other will executors. A will is useful for triggering clean-up actions on data associated with an unreachable value, such as closing a port embedded in an object when the object is no longer used.

Calling the will-execute or will-try-execute procedure executes a will that is ready in the specified will executor. A will executor is also a synchronizable event, so sync or sync/timeout can be used to detect when a will executor has ready wills. Wills are not executed automatically, because certain programs need control to avoid race conditions. However, a program can create a thread whose sole job is to execute wills for a particular executor.

If a value is registered with multiple wills (in one or multiple executors), the wills are readied in the reverse order of registration. Since readying a will procedure makes the value reachable again, the will must be executed and the value must be proven again unreachable through only weak references before another of the wills is readied or executed. However, wills for distinct unreachable values are readied at the same time, regardless of whether the values are reachable from each other.

A will executor’s registrant is held non-weakly until after the corresponding will procedure is executed. Thus, if the content value of a weak box (see Weak Boxes) is registered with a will executor, the weak box’s content is not changed to #f until all wills have been executed for the value and the value has been proven again reachable through only weak references.

A will executor can be used as a synchronizable event (see Events). A will executor is ready for synchronization when will-execute would not block; the synchronization result of a will executor is the will executor itself.

These examples show how to run cleanup actions when no synchronization is necessary. It simply runs the registered executors as they become ready in another thread.

> (define an-executor (make-will-executor))
> (void
    (λ ()
      (let loop ()
        (will-execute an-executor)
> (define (executor-proc v) (printf "a-box is now garbage\n"))
> (define a-box-to-track (box #f))
> (will-register an-executor a-box-to-track executor-proc)
> (collect-garbage)
> (set! a-box-to-track #f)
> (collect-garbage)

a-box is now garbage

Returns a new will executor with no managed values.


(will-executor? v)  boolean?

  v : any/c
Returns #t if v is a will executor, #f otherwise.


(will-register executor v proc)  void?

  executor : will-executor?
  v : any/c
  proc : (any/c . -> . any)
Registers the value v with the will procedure proc in the will executor executor. When v is proven unreachable, then the procedure proc is ready to be called with v as its argument via will-execute or will-try-execute. The proc argument is strongly referenced until the will procedure is executed.


(will-execute executor)  any

  executor : will-executor?
Invokes the will procedure for a single “unreachable” value registered with the executor executor. The values returned by the will procedure are the result of the will-execute call. If no will is ready for immediate execution, will-execute blocks until one is ready.


(will-try-execute executor)  any

  executor : any/c
Like will-execute if a will is ready for immediate execution. Otherwise, #f is returned.