eintr: Review
3.11 Review
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In the last few chapters we have introduced a macro and a fair number of
functions and special forms. Here they are described in brief, along
with a few similar functions that have not been mentioned yet.
‘eval-last-sexp’
Evaluate the last symbolic expression before the current location
of point. The value is printed in the echo area unless the
function is invoked with an argument; in that case, the output is
printed in the current buffer. This command is normally bound to
‘C-x C-e’.
‘defun’
Define function. This macro has up to five parts: the name, a
template for the arguments that will be passed to the function,
documentation, an optional interactive declaration, and the body of
the definition.
For example, in Emacs the function definition of
‘dired-unmark-all-marks’ is as follows.
(defun dired-unmark-all-marks ()
"Remove all marks from all files in the Dired buffer."
(interactive)
(dired-unmark-all-files ?\r))
‘interactive’
Declare to the interpreter that the function can be used
interactively. This special form may be followed by a string with
one or more parts that pass the information to the arguments of the
function, in sequence. These parts may also tell the interpreter
to prompt for information. Parts of the string are separated by
newlines, ‘\n’.
Common code characters are:
‘b’
The name of an existing buffer.
‘f’
The name of an existing file.
‘p’
The numeric prefix argument. (Note that this ‘p’ is lower
case.)
‘r’
Point and the mark, as two numeric arguments, smallest first.
This is the only code letter that specifies two successive
arguments rather than one.
Code Characters for ‘interactive’ (elisp)Interactive Codes,
for a complete list of code characters.
‘let’
Declare that a list of variables is for use within the body of the
‘let’ and give them an initial value, either ‘nil’ or a specified
value; then evaluate the rest of the expressions in the body of the
‘let’ and return the value of the last one. Inside the body of the
‘let’, the Lisp interpreter does not see the values of the
variables of the same names that are bound outside of the ‘let’.
For example,
(let ((foo (buffer-name))
(bar (buffer-size)))
(message
"This buffer is %s and has %d characters."
foo bar))
‘save-excursion’
Record the values of point and the current buffer before evaluating
the body of this special form. Restore the value of point and
buffer afterward.
For example,
(message "We are %d characters into this buffer."
(- (point)
(save-excursion
(goto-char (point-min)) (point))))
‘if’
Evaluate the first argument to the function; if it is true,
evaluate the second argument; else evaluate the third argument, if
there is one.
The ‘if’ special form is called a “conditional”. There are other
conditionals in Emacs Lisp, but ‘if’ is perhaps the most commonly
used.
For example,
(if (= 22 emacs-major-version)
(message "This is version 22 Emacs")
(message "This is not version 22 Emacs"))
‘<’
‘>’
‘<=’
‘>=’
The ‘<’ function tests whether its first argument is smaller than
its second argument. A corresponding function, ‘>’, tests whether
the first argument is greater than the second. Likewise, ‘<=’
tests whether the first argument is less than or equal to the
second and ‘>=’ tests whether the first argument is greater than or
equal to the second. In all cases, both arguments must be numbers
or markers (markers indicate positions in buffers).
‘=’
The ‘=’ function tests whether two arguments, both numbers or
markers, are equal.
‘equal’
‘eq’
Test whether two objects are the same. ‘equal’ uses one meaning of
the word “same” and ‘eq’ uses another: ‘equal’ returns true if the
two objects have a similar structure and contents, such as two
copies of the same book. On the other hand, ‘eq’, returns true if
both arguments are actually the same object.
‘string<’
‘string-lessp’
‘string=’
‘string-equal’
The ‘string-lessp’ function tests whether its first argument is
smaller than the second argument. A shorter, alternative name for
the same function (a ‘defalias’) is ‘string<’.
The arguments to ‘string-lessp’ must be strings or symbols; the
ordering is lexicographic, so case is significant. The print names
of symbols are used instead of the symbols themselves.
An empty string, ‘""’, a string with no characters in it, is
smaller than any string of characters.
‘string-equal’ provides the corresponding test for equality. Its
shorter, alternative name is ‘string=’. There are no string test
functions that correspond to >, ‘>=’, or ‘<=’.
‘message’
Print a message in the echo area. The first argument is a string
that can contain ‘%s’, ‘%d’, or ‘%c’ to print the value of
arguments that follow the string. The argument used by ‘%s’ must
be a string or a symbol; the argument used by ‘%d’ must be a
number. The argument used by ‘%c’ must be an ASCII code number; it
will be printed as the character with that ASCII code. (Various
other %-sequences have not been mentioned.)
‘setq’
‘set’
The ‘setq’ function sets the value of its first argument to the
value of the second argument. The first argument is automatically
quoted by ‘setq’. It does the same for succeeding pairs of
arguments. Another function, ‘set’, takes only two arguments and
evaluates both of them before setting the value returned by its
first argument to the value returned by its second argument.
‘buffer-name’
Without an argument, return the name of the buffer, as a string.
‘buffer-file-name’
Without an argument, return the name of the file the buffer is
visiting.
‘current-buffer’
Return the buffer in which Emacs is active; it may not be the
buffer that is visible on the screen.
‘other-buffer’
Return the most recently selected buffer (other than the buffer
passed to ‘other-buffer’ as an argument and other than the current
buffer).
‘switch-to-buffer’
Select a buffer for Emacs to be active in and display it in the
current window so users can look at it. Usually bound to ‘C-x b’.
‘set-buffer’
Switch Emacs’s attention to a buffer on which programs will run.
Don’t alter what the window is showing.
‘buffer-size’
Return the number of characters in the current buffer.
‘point’
Return the value of the current position of the cursor, as an
integer counting the number of characters from the beginning of the
buffer.
‘point-min’
Return the minimum permissible value of point in the current
buffer. This is 1, unless narrowing is in effect.
‘point-max’
Return the value of the maximum permissible value of point in the
current buffer. This is the end of the buffer, unless narrowing is
in effect.
Info Catalog
eintr: save-excursion
eintr: Writing Defuns
eintr: defun Exercises