On Wed, 21 Feb 2007, Huazhong Wang wrote:
Hi, i want to display a message on my screen during run. my code is below:
...
(run-until 500 (at-every 100 (display "fine!")))
...
but program stops at point of time 100. the errors are:
ERROR: In procedure procedure-property:
ERROR: Wrong type argument in position 1: #<unspecified> who could help me
correct this code? thanks,
This is a perennial source of confusion: the things that you pass to (run
...) must be *functions*. What you are doing above is calling (display
"fine") and passing the *result* of this function (which is
#<unspecified>) to run-until. This is not what you want.
You want something like:
(run-until 500 (at-every 100 (lambda () (print "fine!\n"))))
where (lambda () ...) is a Scheme construct to define a function inline.
Alternatively, you could define the function on a separate line, e.g.
(define (print-my-message) (print "fine!\n"))
(run-until 500 (at-every 100 print-my-message))
Notice that we pass the *name* of the function, print-my-message, rather
than passing the *result* (print-my-message) of evaluating
print-my-message.
Note also that I recommend using libctl's "print" function instead of
"display". "print" allows you to print multiple arguments, and also does
the right thing on a multi-processor machine. Note also that you probably
want to print a newline "\n" at the end of your message.
Steven
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