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Re: About `catch' and `throw'
From: |
Pascal J. Bourguignon |
Subject: |
Re: About `catch' and `throw' |
Date: |
Fri, 21 Dec 2012 20:08:33 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.2 (gnu/linux) |
Xue Fuqiao <xfq.free@gmail.com> writes:
> I'm reading the Emacs Lisp Reference Manual, and I met a problem. In the
> node
> 10.5.1(http://www.gnu.org/software/emacs/manual/html_node/elisp/Catch-and-Throw.html#Catch-and-Throw),
> it says:
>
> 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))
> ...)
>
> but the `throw' is used outside the `catch', I'm confused. Can anybody help?
The point is that catch/throw is a dynamic mechanism, not a lexical one.
So it is bad to say "outside", since this has a spacial conotation.
throw can be used WHEN catch is executing, or WHEN it is not.
If you call (throw 'something result) WHEN there's a (catch 'something …)
executing, then the control will pass from the throw expression to the
catch expression, and the catch expression will return the result
passed to throw.
It's only a question of time, of WHEN catch is being evaluated and WHEN
throw is evaluated.
I could talk you about block and return-from which are a lexical
mechanism, with which what matters is WHERE the return-from is relative
to the lexical scope defined by block, but the current implementation
(even in emacs-24) is full of bugs. If you want, ask again on
news:comp.lang.lisp and we'll tell you about catch/throw and
block/return-from in Common Lisp.
--
__Pascal Bourguignon__ http://www.informatimago.com/
A bad day in () is better than a good day in {}.