Re: Redefine read-eval-print loop?

From: Erick Gallesio <>
Date: Mon, 24 Mar 1997 10:51:10 +0100

> David Fox writes
> ] I have a need to alter the STk read-eval-print loop in a way that
> ] prevents it from printing after evaluation.
> You could write your own:
> (let loop ()
> (display "STk> ")
> (flush (current-output-port))
> (eval (read))
> (loop))

Of course, you can do it in Scheme ;-) I should have answered this first.
However, I still think that the current implementaion was not coherent when
the interpreter is not used in interactive mode (i.e the prompt is not
printed and the result is).

Hereafter is a better version of the scheme solution, imho; the problem with
David's version is that it breaks the loop when an error occurs.
If you don't want this behaviour, you can use the following version:

    (define (my-toplevel)
      (let ((toplevel (lambda ()
                        (let loop ()
                          (display "> ")
                          (flush (current-output-port))
                          (let ((expr (read)))
                            (unless (eof-object? expr)
                              (eval expr)
        (dynamic-wind (lambda () #f)

You need to enter an eof to exit this loop and the dynamic-wind avoids to
break it prematurely whence an error occurs.

                -- Erick
Received on Mon Mar 24 1997 - 10:51:43 CET

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