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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.
eval-expression).
eval-last-sexp).
eval-defun).
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. See Instrumenting for Edebug.
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.