37
votes
Optional parameter defaults
Unless you use Common Lisp extensions as suggested by @legoscia, you need to check if the optional argument was specified. Note that you don't really need to use let here. This seems more idiomatic to ...
- 20.2k
33
votes
Accepted
How do I use nadvice?
All information you need is included in C-h f add-function which
describes the underlying mechanism of advice-add.
The new advice system basically acts like replacing the current
definition of a ...
- 1,561
30
votes
Optional parameter defaults
You can use cl-defun, which lets you specify a default value for optional arguments:
(cl-defun command (a &optional (b default-b))
(command-body a b))
The default value, in this case default-b,...
- 6,022
26
votes
How to check in elisp if a string is a substring of another string?
cl-search can do that (and also returns the index of the substring, if found):
ELISP> (cl-search "f t" "df tj")
1 (#o1, #x1, ?\C-a)
ELISP> (cl-search "ft" "df tj")
nil
- 6,022
26
votes
Accepted
How to properly use defcustom?
The customization system is a built-in feature of Emacs designed to solve precisely the problem you describe—programming may not be the ideal way for the average user to configure their editor.
The ...
- 396
22
votes
Accepted
How can I find the path to an executable with Emacs Lisp?
@Sigma's answer is a good start, but it doesn't filter by executability, nor does it allow for extra suffixes. On windows, for example, running a can invoke a.exe, if that's what's in your path.
So ...
- 3,944
21
votes
Swap two variables in Elisp
This is the elegant idiom I use ;-).
(setq a (prog1 b (setq b a)))
- 75.8k
20
votes
How can I evaluate elisp in an orgmode file when it is opened?
This solution requires no change in init.el (with minor modifications). It involves file-local evaluations, though - but that's exactly what the OP asked for. Advantages of the solution are:
asks for ...
- 311
18
votes
Accepted
Move selected lines up and down
drag-stuff
Check out the drag-stuff package (also available on Melpa).
You can then select a region and use drag-stuff-up/drag-stuff-down to move that region up/down.
Alternative behavior when ...
- 25.2k
18
votes
Accepted
Swap two variables in Elisp
If memory serves me well and you're willing to use cl-lib then:
(cl-rotatef a b)
Note that this is Common Lisp way of solving the problem.
- 4,913
17
votes
Accepted
When should sharp quotes be used?
#' is just shorthand for function, just as ' is shorthand for quote.
You can use it anywhere where you want to indicate to the byte-compiler or the interpreter or a human reader that its argument is ...
- 75.8k
16
votes
Accepted
Emacs Lisp comment conventions
Actually, 3-and-more semi-colons stand for headings, where the more semi-colons you put the deeper the nesting of the heading. So it should look like
;;; Main heading
;;;; Sub heading
;;;;; Sub sub ...
- 26.2k
15
votes
Meaning of period in (. 123)
It looks like Emacs simply reads (. 123) as 123, what happened?
That's exactly what happened. To back it up with sources:
if (ch == '.')
{
if (!NILP (tail))
XSETCDR (tail, read0 (...
- 21.8k
14
votes
Accepted
Simple request.el usage
This is because '(("address" . address)) is quoted (i.e. prefixed by the ' operator), which means that what is inside does not get evaluated. The request function thus gets called with an argument ...
- 5,927
13
votes
When should sharp quotes be used?
#' (aka function) can be used in front of (lambda ...) but it's redundant there, so the only place where it's really meaningful is in front of a symbol, as in #'car. In ELisp, #'car and 'car are ...
- 26.2k
12
votes
How to replace an element of an alist?
The OP asks for a solution which handles alists that have string keys. To handle that, see this question. If by chance you only need to handle alists with symbol keys, then as of Emacs 25 you can use:
...
- 1,512
12
votes
Accepted
Run code right after compilation
Try using compilation-finish-functions:
(defun my-compilation-finish-function (buffer desc)
(message "Buffer %s: %s" buffer desc))
(add-hook 'compilation-finish-functions 'my-compilation-finish-...
- 21.8k
12
votes
How to edit elisp without getting lost in the parentheses
There are a number of add-on packages that might help, such as paredit, smartparens, and lispy. These packages make it easier to navigate and manipulate lisp code so that you can think in s-...
- 20.2k
12
votes
Accepted
How can I modify Elisp's reader?
Turns out that the manual implies that you can't actually do reader macros.
According to Appendix C Porting Common Lisp:
Reader macros. Common Lisp includes a second type of macro that works at ...
Dan♦
- 32.6k
12
votes
How can I modify Elisp's reader?
Apparently it can be done, but prepare for a lot of work. (or don't, because I'm gonna tackle it some weekend ;-).
(defvar *orig-read* (symbol-function 'read))
(defun read (&optional in)
(...
- 249
12
votes
Accepted
An efficient set data structure in elisp
The standard way to handle sets in lisps is to use hash tables. In fact, this is how sets are implemented in languages which do provide a dedicated interface (like Python). This offers O(1) access ...
- 5,948
11
votes
Accepted
Why doesn't "complement" work as a function?
Yes, this behavior is explained by differences in Variable Scoping between Emacs Lisp and Common Lisp.
In Common Lisp (a lexically scoped lisp) a lambda returned by complement you defined is turned ...
- 9,072
11
votes
An efficient set data structure in elisp
There are many different ways to implement sets, with very different efficiency profiles. In Elisp, the two simplest ways to implement sets are probably:
As a list: This tends to have poor ...
- 26.2k
10
votes
`cond` with less redundancy
Traditional Emacs Lisp "in my time we didn't have that new-fangled CL stuff" style:
(catch 'foo
(dolist (str (list str1 str2 str3 str4))
(if (firstFunction str)
(throw 'foo (...
- 5,680
10
votes
Accepted
Why does `eval-when-compile` run at file load and byte-compiled to .elc?
Did you ask Emacs what eval-when-compile is supposed to do?
C-h f eval-when-compile tells you that it evaluates the arg also at load time:
eval-when-compile is a Lisp macro in `byte-run.el'.
(eval-...
- 75.8k
10
votes
Accepted
Are local nested functions possible in elisp?
You can also do it without resorting to cl-lib as follows:
(defun my-fn (x y)
(let ((my-local-fn (lambda (a b) (message "%S" (- a b)))))
(funcall my-local-fn x y)
(funcall my-local-fn y x))...
- 26.2k
9
votes
Accepted
How to get exit code of command supplied to `compile' function?
There is no variable set to the last error code. Such a thing can be emulated though by setting compilation-exit-message-function to something custom:
(defvar compilation-exit-message-function nil "\...
- 21.8k
9
votes
Accepted
How to select text found by `re-search-forward`?
One way to do it:
(defun my-search-forward (str)
(interactive "smy-search-forward: ")
(when (re-search-forward str nil t)
(setf
(point) (match-beginning 0)
(mark) (match-end 0))))
- 10.3k
Only top scored, non community-wiki answers of a minimum length are eligible