Is it possible to break out of a dolist loop? if not, are there alternatives to dolist that don't involve a while loop and indexing a list?
2 Answers
One option is to use catch / throw:
https://www.gnu.org/software/emacs/manual/html_node/elisp/Catch-and-Throw.html
(catch 'foo
(dolist (x '("a" "b" "c" "d" "e" "f" "g"))
(when (equal x "d")
(throw 'foo x))))
The catch statement evaluates to the value thrown (x in this case), or the value of the last body form if the throw function never executes. In the case at hand the dolist form is the last entry in the body. The optional RESULT argument of (dolist (VAR LIST [RESULT]) BODY...) is not specified. Therefore, it evaluates to nil.
As pointed out by @Drew and @Tobias in comments below, the dolist function that is available without loading the cl library is defined in subr.el. Once the cl library is loaded, however, describing the function with C-h f reflects that dolist has been modified as follows: :around advice: ‘cl--wrap-in-nil-block’. The function return (which is an alias for cl-return that is defined in the cl library) can be used to break the dolist loop. To load the cl library, begin with (require 'cl):
(dolist (x '("a" "b" "c" "d" "e" "f" "g"))
(when (equal x "d")
(return x)))
-
2+1. Maybe make clearer that without loading library
clthe version ofdolistis different (not an alias forcl-dolist). You can usereturn(cl-return) only with the version ofdolistprovided by librarycl(it is an alias forcl-dolist). You more or less say that, but it could be clearer - there are two differentdolists, and only one lets you usereturn.– DrewNov 13, 2019 at 5:05 -
1@Drew
dolistis never an alias forcl-dolist.cl.elrather wrapsdolistinto a nil-block by an around advice and(cl-return val)is essentially(throw nil val). So there is not much difference between the two solutions of lawlist. The second solution is just more elegant if cl is available.– TobiasNov 13, 2019 at 5:55 -
1Do not require
cl. That is depreciated. Better requirecl-macsorcl-liband usecl-dolistandcl-returninstead ofdolistandreturn. Maybe, explicitly state that and give both alternative solutions.– TobiasNov 13, 2019 at 6:03 -
1Citation from the cl info file: "Since the old
cl.eldoes not use a clean namespace, Emacs has a policy that packages distributed with Emacs must not loadclat run time. (It is ok for them to loadclat compile time, witheval-when-compile, and use the macros it provides.) There is no such restriction on the use ofcl-lib. New code should usecl-librather thancl."– TobiasNov 13, 2019 at 6:33 -
1@lawlist here is the thread about cl.el = declaring obsolete: emacs.1067599.n8.nabble.com/…– HubisanNov 13, 2019 at 9:23
The Common Lisp Cookbook has an extensive section on loops. Among other things, as a substitute for dolist you could try a "for" loop and use "when" to exit. For example:
(require 'cl-lib)
(cl-loop for i in '(123 3 22 1 309)
when (< i 2)
return -500
do (print i))
... output ...
123
3
22
-500
whileloop is excluded. IMHO it is the only pre-defined loop avoiding non-local exit when breaking out of a list iteration. It can be head or tail controlled or anything in between.The catch statement evaluates to the value thrown (x in this case), or nil if no value is thrown.to lawlist's answer. How do you throw without passing a value? TheVALUEarg ofthrowis obligatory. Maybe, you meancl-returnwhere it is optional.