-This is ../info/xemacs-faq.info, produced by makeinfo version 4.0 from
+This is ../info/xemacs-faq.info, produced by makeinfo version 4.0b from
xemacs-faq.texi.
INFO-DIR-SECTION XEmacs Editor
END-INFO-DIR-ENTRY
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-File: xemacs-faq.info, Node: Q5.1.3, Next: Q5.1.4, Prev: Q5.1.2, Up: Miscellaneous
-
-Q5.1.3: Could you explain `read-kbd-macro' in more detail?
-----------------------------------------------------------
-
- The `read-kbd-macro' function returns the internal Emacs
-representation of a human-readable string (which is its argument).
-Thus:
-
- (read-kbd-macro "C-c C-a")
- => [(control ?c) (control ?a)]
-
- (read-kbd-macro "C-c C-. <up>")
- => [(control ?c) (control ?.) up]
-
- In GNU Emacs the same forms will be evaluated to what GNU Emacs
-understands internally--the sequences `"\C-x\C-c"' and `[3 67108910
-up]', respectively.
-
- The exact "human-readable" syntax is defined in the docstring of
-`edmacro-mode'. I'll repeat it here, for completeness.
-
- Format of keyboard macros during editing:
-
- Text is divided into "words" separated by whitespace. Except for
- the words described below, the characters of each word go directly
- as characters of the macro. The whitespace that separates words is
- ignored. Whitespace in the macro must be written explicitly, as in
- `foo <SPC> bar <RET>'.
-
- * The special words `RET', `SPC', `TAB', `DEL', `LFD', `ESC',
- and `NUL' represent special control characters. The words
- must be written in uppercase.
-
- * A word in angle brackets, e.g., `<return>', `<down>', or
- `<f1>', represents a function key. (Note that in the standard
- configuration, the function key `<return>' and the control key
- <RET> are synonymous.) You can use angle brackets on the
- words <RET>, <SPC>, etc., but they are not required there.
-
- * Keys can be written by their ASCII code, using a backslash
- followed by up to six octal digits. This is the only way to
- represent keys with codes above \377.
-
- * One or more prefixes `M-' (meta), `C-' (control), `S-'
- (shift), `A-' (alt), `H-' (hyper), and `s-' (super) may
- precede a character or key notation. For function keys, the
- prefixes may go inside or outside of the brackets: `C-<down>'
- == `<C-down>'. The prefixes may be written in any order:
- `M-C-x' == `C-M-x'.
-
- Prefixes are not allowed on multi-key words, e.g., `C-abc',
- except that the Meta prefix is allowed on a sequence of
- digits and optional minus sign: `M--123' == `M-- M-1 M-2 M-3'.
-
- * The `^' notation for control characters also works: `^M' ==
- `C-m'.
-
- * Double angle brackets enclose command names: `<<next-line>>'
- is shorthand for `M-x next-line <RET>'.
-
- * Finally, `REM' or `;;' causes the rest of the line to be
- ignored as a comment.
-
- Any word may be prefixed by a multiplier in the form of a decimal
- number and `*': `3*<right>' == `<right> <right> <right>', and
- `10*foo' == `foofoofoofoofoofoofoofoofoofoo'.
-
- Multiple text keys can normally be strung together to form a word,
- but you may need to add whitespace if the word would look like one
- of the above notations: `; ; ;' is a keyboard macro with three
- semicolons, but `;;;' is a comment. Likewise, `\ 1 2 3' is four
- keys but `\123' is a single key written in octal, and `< right >'
- is seven keys but `<right>' is a single function key. When in
- doubt, use whitespace.
-
-\1f
-File: xemacs-faq.info, Node: Q5.1.4, Next: Q5.1.5, Prev: Q5.1.3, Up: Miscellaneous
-
-Q5.1.4: What is the performance hit of `let'?
----------------------------------------------
-
- In most cases, not noticeable. Besides, there's no avoiding
-`let'--you have to bind your local variables, after all. Some pose a
-question whether to nest `let's, or use one `let' per function. I
-think because of clarity and maintenance (and possible future
-implementation), `let'-s should be used (nested) in a way to provide
-the clearest code.
-
-\1f
-File: xemacs-faq.info, Node: Q5.1.5, Next: Q5.1.6, Prev: Q5.1.4, Up: Miscellaneous
-
-Q5.1.5: What is the recommended use of `setq'?
-----------------------------------------------
-
- * Global variables
-
- You will typically `defvar' your global variable to a default
- value, and use `setq' to set it later.
-
- It is never a good practice to `setq' user variables (like
- `case-fold-search', etc.), as it ignores the user's choice
- unconditionally. Note that `defvar' doesn't change the value of a
- variable if it was bound previously. If you wish to change a
- user-variable temporarily, use `let':
-
- (let ((case-fold-search nil))
- ... ; code with searches that must be case-sensitive
- ...)
-
- You will notice the user-variables by their docstrings beginning
- with an asterisk (a convention).
-
- * Local variables
-
- Bind them with `let', which will unbind them (or restore their
- previous value, if they were bound) after exiting from the `let'
- form. Change the value of local variables with `setq' or whatever
- you like (e.g. `incf', `setf' and such). The `let' form can even
- return one of its local variables.
-
- Typical usage:
-
- ;; iterate through the elements of the list returned by
- ;; `hairy-function-that-returns-list'
- (let ((l (hairy-function-that-returns-list)))
- (while l
- ... do something with (car l) ...
- (setq l (cdr l))))
-
- Another typical usage includes building a value simply to work
- with it.
-
- ;; Build the mode keymap out of the key-translation-alist
- (let ((inbox (file-truename (expand-file-name box)))
- (i 0))
- ... code dealing with inbox ...
- inbox)
-
- This piece of code uses the local variable `inbox', which becomes
- unbound (or regains old value) after exiting the form. The form
- also returns the value of `inbox', which can be reused, for
- instance:
-
- (setq foo-processed-inbox
- (let .....))
-
-\1f
-File: xemacs-faq.info, Node: Q5.1.6, Next: Q5.1.7, Prev: Q5.1.5, Up: Miscellaneous
-
-Q5.1.6: What is the typical misuse of `setq' ?
-----------------------------------------------
-
- A typical misuse is probably `setq'ing a variable that was meant to
-be local. Such a variable will remain bound forever, never to be
-garbage-collected. For example, the code doing:
-
- (defun my-function (whatever)
- (setq a nil)
- ... build a large list ...
- ... and exit ...)
-
- does a bad thing, as `a' will keep consuming memory, never to be
-unbound. The correct thing is to do it like this:
-
- (defun my-function (whatever)
- (let (a) ; default initialization is to nil
- ... build a large list ...
- ... and exit, unbinding `a' in the process ...)
-
- Not only is this prettier syntactically, but it makes it possible for
-Emacs to garbage-collect the objects which `a' used to reference.
-
- Note that even global variables should not be `setq'ed without
-`defvar'ing them first, because the byte-compiler issues warnings. The
-reason for the warning is the following:
-
- (defun flurgoze nil) ; ok, global internal variable
- ...
-
- (setq flurghoze t) ; ops! a typo, but semantically correct.
- ; however, the byte-compiler warns.
-
- While compiling toplevel forms:
- ** assignment to free variable flurghoze
-
-\1f
-File: xemacs-faq.info, Node: Q5.1.7, Next: Q5.1.8, Prev: Q5.1.6, Up: Miscellaneous
-
-Q5.1.7: I like the the `do' form of cl, does it slow things down?
------------------------------------------------------------------
-
- It shouldn't. Here is what Dave Gillespie has to say about cl.el
-performance:
-
- Many of the advanced features of this package, such as `defun*',
- `loop', and `setf', are implemented as Lisp macros. In
- byte-compiled code, these complex notations will be expanded into
- equivalent Lisp code which is simple and efficient. For example,
- the forms
-
- (incf i n)
- (push x (car p))
-
- are expanded at compile-time to the Lisp forms
-
- (setq i (+ i n))
- (setcar p (cons x (car p)))
-
- which are the most efficient ways of doing these respective
- operations in Lisp. Thus, there is no performance penalty for
- using the more readable `incf' and `push' forms in your compiled
- code.
-
- _Interpreted_ code, on the other hand, must expand these macros
- every time they are executed. For this reason it is strongly
- recommended that code making heavy use of macros be compiled. (The
- features labelled "Special Form" instead of "Function" in this
- manual are macros.) A loop using `incf' a hundred times will
- execute considerably faster if compiled, and will also
- garbage-collect less because the macro expansion will not have to
- be generated, used, and thrown away a hundred times.
-
- You can find out how a macro expands by using the `cl-prettyexpand'
- function.
-
-\1f
-File: xemacs-faq.info, Node: Q5.1.8, Next: Q5.1.9, Prev: Q5.1.7, Up: Miscellaneous
-
-Q5.1.8: I like recursion, does it slow things down?
----------------------------------------------------
-
- Yes. Emacs byte-compiler cannot do much to optimize recursion. But
-think well whether this is a real concern in Emacs. Much of the Emacs
-slowness comes from internal mechanisms such as redisplay, or from the
-fact that it is an interpreter.
-
- Please try not to make your code much uglier to gain a very small
-speed gain. It's not usually worth it.
-
-\1f
File: xemacs-faq.info, Node: Q5.1.9, Next: Q5.1.10, Prev: Q5.1.8, Up: Miscellaneous
Q5.1.9: How do I put a glyph as annotation in a buffer?
projects / chise / xemacs-chise.git- / blobdiff