emacs: Lisp Eval
27.9 Evaluating Emacs Lisp Expressions
======================================
Emacs Lisp mode is the major mode for editing Emacs Lisp. Its mode
command is ‘M-x emacs-lisp-mode’.
Emacs provides several commands for evaluating Emacs Lisp
expressions. You can use these commands in Emacs Lisp mode, to test
your Emacs Lisp code as it is being written. For example, after
re-writing a function, you can evaluate the function definition to make
it take effect for subsequent function calls. These commands are also
available globally, and can be used outside Emacs Lisp mode.
‘M-:’
Read a single Emacs Lisp expression in the minibuffer, evaluate it,
and print the value in the echo area (‘eval-expression’).
‘C-x C-e’
Evaluate the Emacs Lisp expression before point, and print the
value in the echo area (‘eval-last-sexp’).
‘C-M-x’ (in Emacs Lisp mode)
‘M-x eval-defun’
Evaluate the defun containing or after point, and print the value
in the echo area (‘eval-defun’).
‘M-x eval-region’
Evaluate all the Emacs Lisp expressions in the region.
‘M-x eval-buffer’
Evaluate all the Emacs Lisp expressions in the buffer.
‘M-:’ (‘eval-expression’) reads an expression using the minibuffer,
and evaluates it. (Before evaluating the expression, the current buffer
switches back to the buffer that was current when you typed ‘M-:’, not
the minibuffer into which you typed the expression.)
The command ‘C-x C-e’ (‘eval-last-sexp’) evaluates the Emacs Lisp
expression preceding point in the buffer, and displays the value in the
echo area. When the result of an evaluation is an integer, it is
displayed together with the value in other formats (octal, hexadecimal,
and character).
If ‘M-:’ or ‘C-x C-e’ is given a prefix argument, it inserts the
value into the current buffer at point, rather than displaying it in the
echo area. If the prefix argument is zero, any integer output is
inserted together with its value in other formats (octal, hexadecimal,
and character). Such a prefix argument also prevents abbreviation of
the output according to the variables ‘eval-expression-print-level’ and
‘eval-expression-print-length’ (see below).
The ‘eval-defun’ command is bound to ‘C-M-x’ in Emacs Lisp mode. It
evaluates the top-level Lisp expression containing or following point,
and prints the value in the echo area. In this context, a top-level
expression is referred to as a “defun”, but it need not be an actual
‘defun’ (function definition). In particular, this command treats
‘defvar’ expressions specially. Normally, evaluating a ‘defvar’
expression does nothing if the variable it defines already has a value.
But this command unconditionally resets the variable to the initial
value specified by the ‘defvar’; this is convenient for debugging Emacs
Lisp programs. ‘defcustom’ and ‘defface’ expressions are treated
similarly. Note that the other commands documented in this section do
not have this special feature.
With a prefix argument, ‘C-M-x’ instruments the function definition
for Edebug, the Emacs Lisp Debugger. 
Instrumenting for Edebug
(elisp)Instrumenting.
The command ‘M-x eval-region’ parses the text of the region as one or
more Lisp expressions, evaluating them one by one. ‘M-x eval-buffer’ is
similar but evaluates the entire buffer.
The options ‘eval-expression-print-level’ and
‘eval-expression-print-length’ control the maximum depth and length of
lists to print in the result of the evaluation commands before
abbreviating them. Supplying a zero prefix argument to
‘eval-expression’ or ‘eval-last-sexp’ causes lists to be printed in
full. ‘eval-expression-debug-on-error’ controls whether evaluation
errors invoke the debugger when these commands are used; its default is
‘t’.
Info Catalog
emacs: Lisp Libraries
emacs: Building
emacs: Lisp Interaction