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(emacs-lisp-intro.info)simplified-beginning-of-buffer

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A Simplified `beginning-of-buffer' Definition
=============================================

   The `beginning-of-buffer' command is a good function to start with
since you are likely to be familiar with it and it is easy to
understand.  Used as an interactive command, `beginning-of-buffer'
moves the cursor to the beginning of the buffer, leaving the mark at the
previous position.  It is generally bound to `M-<'.

   In this section, we will discuss a shortened version of the function
that shows how it is most frequently used.  This shortened function
works as written, but it does not contain the code for a complex option.
In another section, we will describe the entire function.  (Note:
Complete Definition of `beginning-of-buffer'.)

   Before looking at the code, let's consider what the function
definition has to contain: it must include an expression that makes the
function interactive so it can be called by typing `M-x
beginning-of-buffer' or by typing a keychord such as `C-<'; it must
include code to leave a mark at the original position in the buffer;
and it must include code to move the cursor to the beginning of the
buffer.

   Here is the complete text of the shortened version of the function:

     (defun simplified-beginning-of-buffer ()
       "Move point to the beginning of the buffer;
     leave mark at previous position."
       (interactive)
       (push-mark)
       (goto-char (point-min)))

   Like all function definitions, this definition has five parts
following the special form `defun':

  1. The name: in this example, `simplified-beginning-of-buffer'.

  2. A list of the arguments: in this example, an empty list, `()',

  3. The documentation string.

  4. The interactive expression.

  5. The body.

In this function definition, the argument list is empty; this means that
this function does not require any arguments.  (When we look at the
definition for the complete function, we will see that it may be passed
an optional argument.)

   The interactive expression tells Emacs that the function is intended
to be used interactively.  In this example, `interactive' does not have
an argument because `simplified-beginning-of-buffer' does not require
one.

   The body of the function consists of the two lines:

     (push-mark)
     (goto-char (point-min))

   The first of these lines is the expression, `(push-mark)'.  When
this expression is evaluated by the Lisp interpreter, it sets a mark at
the current position of the cursor, wherever that may be.  The position
of this mark is saved in the mark ring.

   The next line is `(goto-char (point-min))'.  This expression jumps
the cursor to the minimum point in the buffer, that is, to the
beginning of the buffer (or to the beginning of the accessible portion
of the buffer if it is narrowed.  *Note Narrowing and Widening:
Narrowing & Widening.)

   The `push-mark' command sets a mark at the place where the cursor
was located before it was moved to the beginning of the buffer by the
`(goto-char (point-min))' expression.  Consequently, you can, if you
wish, go back to where you were originally by typing `C-x C-x'.

   That is all there is to the function definition!

   When you are reading code such as this and come upon an unfamiliar
function, such as `goto-char', you can find out what it does by using
the `describe-function' command.  To use this command, type `C-h f' and
then type in the name of the function and press <RET>.  The
`describe-function' command will print the function's documentation
string in a `*Help*' window.  For example, the documentation for
`goto-char' is:

     One arg, a number.  Set point to that number.
     Beginning of buffer is position (point-min),
     end is (point-max).

(The prompt for `describe-function' will offer you the symbol under or
preceding the cursor, so you can save typing by positioning the cursor
right over or after the function and then typing `C-h f <RET>'.)

   The `end-of-buffer' function definition is written in the same way as
the `beginning-of-buffer' definition except that the body of the
function contains the expression `(goto-char (point-max))' in place of
`(goto-char (point-min))'.