(emacs.info) Lisp Eval

 
 Evaluating Emacs-Lisp Expressions
 =================================
 
    Lisp programs intended to be run in Emacs should be edited in
 Emacs-Lisp mode; this happens automatically for file names ending in
 `.el'.  By contrast, Lisp mode itself is used for editing Lisp programs
 intended for other Lisp systems.  To switch to Emacs-Lisp mode
 explicitly, use the command `M-x emacs-lisp-mode'.
 
    For testing of Lisp programs to run in Emacs, it is often useful to
 evaluate part of the program as it is found in the Emacs buffer.  For
 example, after changing the text of a Lisp function definition,
 evaluating the definition installs the change for future calls to the
 function.  Evaluation of Lisp expressions is also useful in any kind of
 editing, for invoking noninteractive functions (functions that are not
 commands).
 
 `M-:'
      Read a single Lisp expression in the minibuffer, evaluate it, and
      print the value in the echo area (`eval-expression').
 
 `C-x C-e'
      Evaluate the Lisp expression before point, and print the value in
      the echo area (`eval-last-sexp').
 
 `C-M-x'
      Evaluate the defun containing or after point, and print the value
      in the echo area (`eval-defun').
 
 `M-x eval-region'
      Evaluate all the Lisp expressions in the region.
 
 `M-x eval-current-buffer'
      Evaluate all the Lisp expressions in the buffer.
 
    `M-:' (`eval-expression') is the most basic command for evaluating a
 Lisp expression interactively.  It reads the expression using the
 minibuffer, so you can execute any expression on a buffer regardless of
 what the buffer contains.  When the expression is evaluated, the current
 buffer is once again the buffer that was current when `M-:' was typed.
 
    In Emacs-Lisp mode, the key `C-M-x' is bound to the command
 `eval-defun', which parses the defun containing or following point as a
 Lisp expression and evaluates it.  The value is printed in the echo
 area.  This command is convenient for installing in the Lisp environment
 changes that you have just made in the text of a function definition.
 
    `C-M-x' treats `defvar' expressions specially.  Normally, evaluating
 a `defvar' expression does nothing if the variable it defines already
 has a value.  But `C-M-x' unconditionally resets the variable to the
 initial value specified in the `defvar' expression.  This special
 feature is convenient for debugging Lisp programs.
 
    The command `C-x C-e' (`eval-last-sexp') evaluates the Lisp
 expression preceding point in the buffer, and displays the value in the
 echo area.  It is available in all major modes, not just Emacs-Lisp
 mode.  It does not treat `defvar' specially.
 
    If `C-M-x', `C-x C-e', or `M-:' is given a numeric argument, it
 inserts the value into the current buffer at point, rather than
 displaying it in the echo area.  The argument's value does not matter.
 
    The most general command for evaluating Lisp expressions from a
 buffer is `eval-region'.  `M-x eval-region' parses the text of the
 region as one or more Lisp expressions, evaluating them one by one.
 `M-x eval-current-buffer' is similar but evaluates the entire buffer.
 This is a reasonable way to install the contents of a file of Lisp code
 that you are just ready to test.  Later, as you find bugs and change
 individual functions, use `C-M-x' on each function that you change.
 This keeps the Lisp world in step with the source file.
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