ANSI Common Lisp   5 Data and Control Flow

5.2 Transfer of Control to an Exit Point

When a transfer of control is initiated by go, return-from, or throw the following events occur in order to accomplish the transfer of control. Note that for go, the exit point is the form within the tagbody that is being executed at the time the go is performed; for return-from, the exit point is the corresponding block form; and for throw, the exit point is the corresponding catch form.

• 1. Intervening exit points are "abandoned" (i.e., their extent ends and it is no longer valid to attempt to transfer control through them).

• 2. The cleanup clauses of any intervening unwind-protect clauses are evaluated.

• 3. Intervening dynamic bindings of special variables, catch tags, condition handlers, and restarts are undone.

• 4. The extent of the exit point being invoked ends, and control is passed to the target.

The extent of an exit being "abandoned" because it is being passed over ends as soon as the transfer of control is initiated. That is, event 1 occurs at the beginning of the initiation of the transfer of control. The consequences are undefined if an attempt is made to transfer control to an exit point whose dynamic extent has ended.

Events 2 and 3 are actually performed interleaved, in the order corresponding to the reverse order in which they were established. The effect of this is that the cleanup clauses of an unwind-protect see the same dynamic bindings of variables and catch tags as were visible when the unwind-protect was entered.

Event 4 occurs at the end of the transfer of control.