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### 27.11 Running an External Lisp

Lisp mode is the major mode for editing programs written in general-purpose Lisp dialects, such as Common Lisp. Its mode command is `M-x lisp-mode`. Emacs uses Lisp mode automatically for files whose names end in `.l`, `.lsp`, or `.lisp`.

You can run an external Lisp session as a subprocess or inferior process of Emacs, and pass expressions to it to be evaluated. To begin an external Lisp session, type `M-x run-lisp`. This runs the program named `lisp`, and sets it up so that both input and output go through an Emacs buffer named `*inferior-lisp*`. To change the name of the Lisp program run by `M-x run-lisp`, change the variable `inferior-lisp-program`.

The major mode for the `*lisp*` buffer is Inferior Lisp mode, which combines the characteristics of Lisp mode and Shell mode (see Shell Mode). To send input to the Lisp session, go to the end of the `*lisp*` buffer and type the input, followed by `RET`. Terminal output from the Lisp session is automatically inserted in the buffer.

When you edit a Lisp program in Lisp mode, you can type `C-M-x` (`lisp-eval-defun`) to send an expression from the Lisp mode buffer to a Lisp session that you had started with `M-x run-lisp`. The expression sent is the top-level Lisp expression at or following point. The resulting value goes as usual into the `*inferior-lisp*` buffer. Note that the effect of `C-M-x` in Lisp mode is thus very similar to its effect in Emacs Lisp mode (see Lisp Eval), except that the expression is sent to a different Lisp environment instead of being evaluated in Emacs.

The facilities for editing Scheme code, and for sending expressions to a Scheme subprocess, are very similar. Scheme source files are edited in Scheme mode, which can be explicitly enabled with `M-x scheme-mode`. You can initiate a Scheme session by typing `M-x run-scheme` (the buffer for interacting with Scheme is named `*scheme*`), and send expressions to it by typing `C-M-x`.

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