: Learning GNU Emacs, 3rd Edition Using Lisp interaction mode Using Lisp interaction mode

An even more powerful feature is Lisp interaction mode. This is the mode the default buffer *scratch* is in. Filenames with no suffixes normally cause Emacs to go into Lisp interaction mode, though you can change this using the variable auto-mode-alist, described earlier in this chapter and in more detail in Chapter 10. You can also put any buffer in Lisp interaction mode by typing M-x lisp-interaction-mode Enter; to create an extra Lisp interaction buffer, just type C-x b (for switch-to-buffer), supply a buffer name, and put it in Lisp interaction mode.

Lisp interaction mode is identical to Emacs Lisp mode except for one important feature: C-j is bound to the command eval-print-last-sexp. This command takes the S-expression just before point, evaluates it, and prints the result in the buffer. To get the usual newline-and-indent functionality attached to C-j in other modes, you must press Enter, followed by Tab.

Remember that an S-expression is any syntactically valid expression in Lisp. Therefore, you can use C-j in Lisp interaction mode to check the values of variables, enter function definitions, run functions, and so on. For example, if you type auto-save-interval and press C-j, the value of that variable (300 by default) appears. If you type a defun and press C-j after the last right parenthesis, Emacs stores the function defined (for future invocation) and prints its name; in this case, C-j is similar to C-M-x (for eval-defun) except that the cursor must be after (as opposed to before or in the middle of) the function being defined. If you invoke a function, Emacs evaluates (runs) the expression and responds with whatever value the function returns.

C-j in Lisp interaction mode gives you an excellent way to play with, incrementally develop, and debug Emacs Lisp code, and since Emacs Lisp is "true" Lisp, it is even useful for developing some bits of code for other Lisp systems.

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