unwind-protect
type: special form (fsubr) location: built-in source file: xlcont.c Common LISP compatible: yes supported on: all machines
SYNTAX
(unwind-protect <protect-form> <clean-up-form> ... )
<protect-form> - a form that is to be protected
<clean-up-form> - a sequence forms to execute after <protect-form>
DESCRIPTION
The UNWIND-PROTECT special form allows the protecting (trapping) of all forms of exit from the <protect-form>. The exits that are trapped include errors, THROW, RETURN and GO. The <clean-up-form> will be executed in all cases - when there is an exit from <protect-form> and when the form does not have exit. UNWIND-PROTECT will return the result from the <protect-form>, not from the <clean-up-form>s. Errors or exits that occur in the <clean-up-form> are not protected. It is possible to trap these with another UNWIND-PROTECT.
EXAMPLES
(unwind-protect ;
(+ 2 2) ; protected form
(print "an exit")) ; clean up form
; prints "an exit"
; returns 4
;
(nodebug) ; to turn off break loop traps
(unwind-protect ;
(+ 1 "2") ; protected form
(print "something happened")) ; clean up form
; error: bad argument type - "2"
; prints "something happened"
;
(catch 'mytag ;
(unwind-protect ;
(throw 'mytag) ; protected form
(print "an exit") ) ) ; clean up form
; prints "an exit"
;
(nodebug) ; to turn off break loop traps
(unwind-protect ;
(throw 'notag) ; protected form
(print "an exit")) ; clean up form
; error: no target for THROW
; prints "an exit"
;
(prog () ;
(print "start") ;
(unwind-protect ;
(go end) ; protected form
(print "an exit")) ; clean-up form
end (print "end") ) ; prints "start"
; prints "an exit"
; prints "end"
;
(prog () ;
(print "start") ;
(unwind-protect ;
(return "I'm done") ; protected form
(print "but first")) ; clean-up form
(print "won't get here") ) ; prints "start"
; prints "but first"
; returns "I'm done"