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8.5 REPL Mode

Edwin provides a special mechanism for interacting with Scheme read-eval-print loops: REPL buffers. A REPL buffer is associated with a Scheme REPL running in a separate thread of execution; because of this, expressions may be evaluated in this buffer while you simultaneously do other things with the editor. A REPL buffer captures all printed output from an evaluated expression, as well as supporting interactive programs such as debug. For these and other reasons, REPL buffers are the preferred means for interacting with the Scheme interpreter.

When Edwin starts, it has one buffer: a REPL buffer called ‘*scheme*’. The command M-x repl selects this buffer, if it exists; otherwise it creates a new REPL buffer. If you want two REPL buffers, just rename the ‘*scheme*’ buffer to something else and run M-x repl again.

REPL buffers support all the same evaluation commands that Scheme mode does; in fact, REPL buffers use a special mode called REPL mode that inherits from Scheme mode. Thus, any key bindings defined in Scheme mode are also defined in REPL mode. One exception to this is the M-o command, which is deliberately undefined in REPL mode; otherwise it would be too easy to re-evaluate all the previously evaluated expressions in the REPL buffer.

In addition to evaluation commands, REPL mode provides a handful of special commands for controlling the REPL itself. The commands C-c C-x and C-c C-u abort the current evaluation, returning to the current or previous REPL levels, respectively. The command C-c C-b interrupts the current evaluation, entering a breakpoint.

Each REPL buffer maintains a history of the expressions that were typed into it. Several commands allow you to access the contents of this history. The command M-p moves backwards through the history, inserting previously evaluated expressions at point. Likewise, M-n moves forward through the history. The commands C-c C-r and C-c C-s search backward and forward through the history for a particular string. The command C-c C-o deletes any output from the previous evaluation; use this command with care since it cannot be undone. The command C-c C-l finds the most recent expression in the buffer and moves point there.

When an expression that you evaluate signals an error, the REPL buffer notices this and offers to run the debugger for you. Answer this question with a ‘y’ or ‘n’ response. You can start the debugger whenever the REPL buffer is listening by executing the C-c C-d command. In either case, this starts the Edwin debugger, which pops up a new window containing the debugger. Your REPL buffer remains in the error state, allowing you to examine it further if you wish.