return-from (original) (raw)

ANSI Common Lisp 5 Data and Control Flow 5.3 Dictionary of Data and Control Flow

Syntax:

return-from _name [result] |

Arguments and Values:

name - a block tag; not evaluated.

result - a form; evaluated. The default is nil.

Description:

Returns control and _multiple values_2 from a lexically enclosing block.

A block form named name must lexically enclose the occurrence of return-from; any values _yielded_by the evaluation of result are immediately returned from the innermost such lexically enclosing block.

The transfer of control initiated by return-from is performed as described in Section 5.2 Transfer of Control to an Exit Point.

Examples:

(block alpha (return-from alpha) 1) NIL (block alpha (return-from alpha 1) 2) 1 (block alpha (return-from alpha (values 1 2)) 3) 1, 2 (let ((a 0)) (dotimes (i 10) (incf a) (when (oddp i) (return))) a) 2 (defun temp (x) (if x (return-from temp ')) 44) TEMP (temp nil) 44 (temp t) DUMMY (block out (flet ((exit (n) (return-from out n))) (block out (exit 1))) 2) 1 (block nil
(unwind-protect (return-from nil 1) (return-from nil 2))) 2 (dolist (flag '(nil t)) (block nil (let ((x 5)) (declare (special x)) (unwind-protect (return-from nil) (print x)))) (print 'here)) 5 HERE 5 HERE NIL (dolist (flag '(nil t)) (block nil (let ((x 5)) (declare (special x)) (unwind-protect (if flag (return-from nil)) (print x)))) (print 'here)) 5 HERE 5 HERE NIL

The following has undefined consequences because the block _form_exits normally before the return-from form is attempted.

(funcall (block nil #'(lambda () (return-from nil)))) is an error.

See Also:

block,return, Section 3.1 Evaluation

Allegro CL Implementation Details:

None.