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tag—a go tag; not evaluated.
statement—a compound form; evaluated as described below.
Executes zero or more statements in a lexical environment that provides for control transfers to labels indicated by the tags.
The statements in a
tagbody are evaluated in order
from left to right, and their values are discarded. If at any time
there are no remaining statements,
) is evaluated, control jumps to the
part of the body labeled with the tag. (Tags are compared with
A tag established by
tagbody has lexical scope
and has dynamic extent. Once
tagbody has been exited,
it is no longer valid to
go to a tag in its body.
It is permissible for
go to jump to a
tagbody that is
not the innermost
tagbody containing that
the tags established by a
tagbody only shadow
other tags of like name.
The determination of which elements of the body are tags and which are statements is made prior to any macro expansion of that element. If a statement is a macro form and its macro expansion is an atom, that atom is treated as a statement, not a tag.
(let (val) (tagbody (setq val 1) (go point-a) (incf val 16) point-c (incf val 04) (go point-b) (incf val 32) point-a (incf val 02) (go point-c) (incf val 64) point-b (incf val 08)) val) → 15 (defun f1 (flag) (let ((n 1)) (tagbody (setq n (f2 flag #'(lambda () (go out)))) out (prin1 n)))) → F1 (defun f2 (flag escape) (if flag (funcall escape) 2)) → F2 (f1 nil) ▷ 2 → NIL (f1 t) ▷ 1 → NIL
The macros in the next figure have implicit tagbodies.