Книга: Learning GNU Emacs, 3rd Edition

9.8.5.1 Commands for evaluating a line of Lisp

9.8.5.1 Commands for evaluating a line of Lisp

The first of these is the command M-: (for eval-expression). This command enables you to type a one-line Lisp expression of any kind in the minibuffer; the expression is evaluated, and the result is printed in the minibuffer. This is an excellent, quick way to check the values of Emacs variables and to experiment with "internal" Emacs functions that aren't bound to keys or that require arguments. You can use the symbol completion command M-Tab while you are using eval-expression.

Unfortunately (or fortunately, depending on your point of view), Emacs doesn't normally let you use eval-expression. If you try pressing M-:, you will see the message loading novice ... in the minibuffer. Then a window pops up with a message on the order of, "You didn't really mean to type that, did you?" You get three options: press Space to try the command only once, y to try it and enable it for future use with no questions asked, or n to do nothing.

If you want to use eval-expression, type y. This command actually results in the following line being put in your .emacs file:

(put 'eval-expression 'disabled nil)

If you are a knowledgeable Lisp programmer, you will understand that this addition sets the property disabled of the symbol eval-expression to nil. In other words, Emacs considers certain commands to be verboten to novice users and thus allows commands to be disabled. If you want to skip this entire procedure and just use eval-expression, simply put the above line in your .emacs file yourself (make sure you include the single quotes).

Another feature that helps you exercise Emacs Lisp code is C-x C-e (for eval-last-sexp). This command runs the line of Lisp that your cursor is on and prints its value in the minibuffer. C-x C-e is handy for testing single lines of code in an Emacs Lisp file.

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