Re: [racket] Semaphore obscurities

[Ryan Culpepper]

On 09/26/2012 04:46 PM, Norman Gray wrote:

Greetings

The behaviour of call-with-semaphore doesn't appear to match the
documentation, or else I'm misunderstanding the documentation.

Consider:

(define (printit m)
   (printf "msg begin:~a~%" m))
(define try
   (let ((sema (make-semaphore 1)))
     (λ (msg)
       (call-with-semaphore sema
                            printit
                            ;#f
                            msg))))
(try "second")

With #f commented out, I get (with Racket 5.3):

call-with-semaphore: procedure arity does not match extra-argument count
   procedure: #<procedure:printit>
   extra-argument count: 0


With #f uncommented, it works as expected:

Welcome to DrRacket, version 5.3 [3m].
Language: racket; memory limit: 128 MB.
msg begin:second


The documentation says:

(call-with-semaphore            sema                            
                proc                            
         [      try-fail-thunk]                         
                arg ...)                →               any

However it appears that the try-fail-chunk argument is not in fact optional.

Here's what the documentation means: There must be at least two 
arguments. The first two arguments are 'sema' and 'proc'. If there are 
three or more arguments, the third argument is 'try-fail-thunk'. If 
there are four or more arguments, all arguments starting with the fourth 
argument are collected in a list for the 'args' rest argument.

----

Separately, it appears that CALL-WITH-SEMAPHORE returns the value
that (proc args…) returns.  However the documentation doesn't
actually say that.  Is that just an oversight, or is the return value
of CALL-WITH-SEMAPHORE undefined?

It's probably an oversight.

Ryan

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