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(elisp)Catch and Throw

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Explicit Nonlocal Exits: `catch' and `throw'
--------------------------------------------

   Most control constructs affect only the flow of control within the
construct itself.  The function `throw' is the exception to this rule
of normal program execution: it performs a nonlocal exit on request.
(There are other exceptions, but they are for error handling only.)
`throw' is used inside a `catch', and jumps back to that `catch'.  For
example:

     (defun foo-outer ()
       (catch 'foo
         (foo-inner)))
     
     (defun foo-inner ()
       ...
       (if x
           (throw 'foo t))
       ...)

The `throw' form, if executed, transfers control straight back to the
corresponding `catch', which returns immediately.  The code following
the `throw' is not executed.  The second argument of `throw' is used as
the return value of the `catch'.

   The function `throw' finds the matching `catch' based on the first
argument: it searches for a `catch' whose first argument is `eq' to the
one specified in the `throw'.  If there is more than one applicable
`catch', the innermost one takes precedence.  Thus, in the above
example, the `throw' specifies `foo', and the `catch' in `foo-outer'
specifies the same symbol, so that `catch' is the applicable one
(assuming there is no other matching `catch' in between).

   Executing `throw' exits all Lisp constructs up to the matching
`catch', including function calls.  When binding constructs such as
`let' or function calls are exited in this way, the bindings are
unbound, just as they are when these constructs exit normally (Note:
Local Variables).  Likewise, `throw' restores the buffer and position
saved by `save-excursion' (Note: Excursions), and the narrowing
status saved by `save-restriction' and the window selection saved by
`save-window-excursion' (Note: Window Configurations).  It also runs
any cleanups established with the `unwind-protect' special form when it
exits that form (Note: Cleanups).

   The `throw' need not appear lexically within the `catch' that it
jumps to.  It can equally well be called from another function called
within the `catch'.  As long as the `throw' takes place chronologically
after entry to the `catch', and chronologically before exit from it, it
has access to that `catch'.  This is why `throw' can be used in
commands such as `exit-recursive-edit' that throw back to the editor
command loop (Note: Recursive Editing).

     Common Lisp note: Most other versions of Lisp, including Common
     Lisp, have several ways of transferring control nonsequentially:
     `return', `return-from', and `go', for example.  Emacs Lisp has
     only `throw'.

 - Special Form: catch tag body...
     `catch' establishes a return point for the `throw' function.  The
     return point is distinguished from other such return points by
     TAG, which may be any Lisp object except `nil'.  The argument TAG
     is evaluated normally before the return point is established.

     With the return point in effect, `catch' evaluates the forms of the
     BODY in textual order.  If the forms execute normally (without
     error or nonlocal exit) the value of the last body form is
     returned from the `catch'.

     If a `throw' is executed during the execution of BODY, specifying
     the same value TAG, the `catch' form exits immediately; the value
     it returns is whatever was specified as the second argument of
     `throw'.

 - Function: throw tag value
     The purpose of `throw' is to return from a return point previously
     established with `catch'.  The argument TAG is used to choose
     among the various existing return points; it must be `eq' to the
     value specified in the `catch'.  If multiple return points match
     TAG, the innermost one is used.

     The argument VALUE is used as the value to return from that
     `catch'.

     If no return point is in effect with tag TAG, then a `no-catch'
     error is signaled with data `(TAG VALUE)'.