From 754bbf7a25a8dda49b5d08ef0d0443bbf5af0e36 Mon Sep 17 00:00:00 2001 From: Craig Jennings Date: Sun, 7 Apr 2024 13:41:34 -0500 Subject: new repository --- devdocs/elisp/examples-of-catch.html | 34 ++++++++++++++++++++++++++++++++++ 1 file changed, 34 insertions(+) create mode 100644 devdocs/elisp/examples-of-catch.html (limited to 'devdocs/elisp/examples-of-catch.html') diff --git a/devdocs/elisp/examples-of-catch.html b/devdocs/elisp/examples-of-catch.html new file mode 100644 index 00000000..d10b0589 --- /dev/null +++ b/devdocs/elisp/examples-of-catch.html @@ -0,0 +1,34 @@ +

Examples of catch and throw

One way to use catch and throw is to exit from a doubly nested loop. (In most languages, this would be done with a goto.) Here we compute (foo i j) for i and j varying from 0 to 9: 

(defun search-foo ()
+  (catch 'loop
+    (let ((i 0))
+      (while (< i 10)
+        (let ((j 0))
+          (while (< j 10)
+            (if (foo i j)
+                (throw 'loop (list i j)))
+            (setq j (1+ j))))
+        (setq i (1+ i))))))
+
+

If foo ever returns non-nil, we stop immediately and return a list of i and j. If foo always returns nil, the catch returns normally, and the value is nil, since that is the result of the while.

Here are two tricky examples, slightly different, showing two return points at once. First, two return points with the same tag, hack: 

(defun catch2 (tag)
+  (catch tag
+    (throw 'hack 'yes)))
+⇒ catch2
+
+ +
(catch 'hack
+  (print (catch2 'hack))
+  'no)
+-| yes
+⇒ no
+
+

Since both return points have tags that match the throw, it goes to the inner one, the one established in catch2. Therefore, catch2 returns normally with value yes, and this value is printed. Finally the second body form in the outer catch, which is 'no, is evaluated and returned from the outer catch.

Now let’s change the argument given to catch2: 

(catch 'hack
+  (print (catch2 'quux))
+  'no)
+⇒ yes
+
+

We still have two return points, but this time only the outer one has the tag hack; the inner one has the tag quux instead. Therefore, throw makes the outer catch return the value yes. The function print is never called, and the body-form 'no is never evaluated.

+

+ Copyright © 1990-1996, 1998-2022 Free Software Foundation, Inc.
Licensed under the GNU GPL license.
+ https://www.gnu.org/software/emacs/manual/html_node/elisp/Examples-of-Catch.html +

+
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