2.1.1 The Prompt and Level Number

The REPL prompt normally has the form

1 ]=>

The ‘1’ in the prompt is a level number, which is always a positive integer. This number is incremented under certain circumstances, the most common being an error. For example, here is what you will see if you type f o o RET after starting Scheme:

;Unbound variable: foo
;To continue, call RESTART with an option number:
; (RESTART 3) => Specify a value to use instead of foo.
; (RESTART 2) => Define foo to a given value.
; (RESTART 1) => Return to read-eval-print level 1.

2 error>

In this case, the level number has been incremented to ‘2’, which indicates that a new REPL has been started (also the prompt string has been changed to remind you that the REPL was started because of an error). The ‘2’ means that this new REPL is “over” the old one. The original REPL still exists, and is waiting for you to return to it, for example, by entering ‘(restart 1)’. Furthermore, if an error occurs while you are in this REPL, yet another REPL will be started, and the level number will be increased to ‘3’. This can continue ad infinitum, but normally it is rare to use more than a few levels.

The normal way to get out of an error REPL and back to the top level REPL is to use the C-g interrupt. This is a single-keystroke command executed by holding down the CTRL key and pressing the G key. C-g always terminates whatever is running and returns you to the top level REPL immediately.

Note: The appearance of the ‘error>’ prompt does not mean that Scheme is in some weird inconsistent state that you should avoid. It is merely a reminder that your program was in error: an illegal operation was attempted, but it was detected and avoided. Often the best way to find out what is in error is to do some poking around in the error REPL. If you abort out of it, the context of the error will be destroyed, and you may not be able to find out what happened.