## 7 GNU Emacs Interface

There is an interface library, called `xscheme`, distributed with MIT/GNU Scheme and GNU Emacs, which facilitates running Scheme as a subprocess of Emacs. If you are using a recent version of GNU Emacs, say version 22 or later, use the `xscheme` file that comes with it. Otherwise use the one that is in the Scheme distribution.

This interface works under unix only, because it requires unix signals for its operation. Porting it to Windows would require reimplementing the interface to eliminate the use of signals. We have no plans to do this.

To invoke Scheme from Emacs, load the `xscheme` library, then use `M-x run-scheme`. You may give `run-scheme` a prefix argument, in which case it will allow you to edit the command line that is used to invoke Scheme. Do not remove the `--emacs` option!

Note carefully: In Emacs 19 and later, the `run-scheme` command exists, but is different from the one described here! In order to get this interface, you must load the `xscheme` library before executing `run-scheme`.

Scheme will be started up as a subprocess in a buffer called ‘`*scheme*`’. This buffer will be in `scheme-interaction-mode` and all output from the Scheme process will go there. The mode line for the ‘`*scheme*`’ buffer will have this form:

    --**-*scheme*: 1 [Evaluator]           (Scheme Interaction: input)------

The first field, showing ‘`1`’ in this example, is the level number.

The second field, showing ‘`[Evaluator]`’ in this example, describes the type of REPL that is running. Other values include:

    [Debugger]
    [Where]

The mode after ‘`Scheme Interaction`’ is one of:

‘`input`’

  Scheme is waiting for input.

‘`run`’

  Scheme is running an evaluation.

‘`gc`’

  Scheme is garbage collecting.

When `xscheme` is loaded, `scheme-mode` is extended to include commands for evaluating expressions (do `C-h m` in any `scheme-mode` buffer for the most up-to-date information):

`M-o`

  Evaluates the current buffer (`xscheme-send-buffer`).

`M-z`

  Evaluates the current definition (`xscheme-send-definition`). This is also bound to `C-M-x`.

`C-M-z`

  Evaluates the current region (`xscheme-send-region`).

`C-x C-e`

  Evaluates the expression to the left of point (`xscheme-send-previous-expression`). This is also bound to `M-RET`.

`C-c C-s`

  Selects the ‘`*scheme*`’ buffer and places you at its end (`xscheme-select-process-buffer`).

`C-c C-y`

  Yanks the most recently evaluated expression, placing it at point (`xscheme-yank-previous-send`). This works only in the ‘`*scheme*`’ buffer.

The following commands provide interrupt capability:

`C-c C-c`

  Like typing `C-g` when running Scheme without Emacs. (`xscheme-send-control-g-interrupt`)

`C-c C-x`

  Like typing `C-c C-x` when running Scheme without Emacs. (`xscheme-send-control-x-interrupt`)

`C-c C-u`

  Like typing `C-c C-u` when running Scheme without Emacs. (`xscheme-send-control-u-interrupt`)

`C-c C-b`

  Like typing `C-c C-b` when running Scheme without Emacs. (`xscheme-send-breakpoint-interrupt`)

`C-c C-p`

  Like evaluating ‘`(continue)`’. (`xscheme-send-proceed`)