readline eats previous text on line

[Jon Wilson <j85wilson@fastmail.fm>]

Hi,
I'm trying to read user input (interactive) using guile.  I'd like to use 
(ice-9 readline), so that the user can use backspace and history and such. 
  I'd also like to have a prompt at the beginning of the line.  These last 
two desires seem to be incompatible, at least mod my best efforts.

For instance, please consider the following:

(use-modules (ice-9 rdelim) (ice-9 format) (ice-9 readline))
(activate-readline)

(define (test)
   (define str #f)
   (display "prompt: ")
   (let loop ((n 1000000))
         (if (not (zero? n)) (loop (1- n))))
   (set! str (read-line))
   (newline)
   (display str))

(test)

This prints "prompt: " on a line, and then after a short time, it is 
erased.  The mess with loop in the middle is of course a crufty time delay, 
without which I can't see "prompt: " at all before it disappears.  After it 
has disappeared, I can type whatever I like, and then press enter. 
Whatever I have typed is then printed as expected, on a new line.

This behavior is unchanged (except for how much text is actually read) when 
I substitute read-char or read in for read-line.

This behavior is gone when I remove (activate-readline).

However, without readline, as I said above, the user cannot backspace, 
which is a great annoyance.

Obviously, however, when the normal guile repl uses readline, it has a 
prompt which is not eaten.  How is this accomplished?

Regards,
Jon


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