Re: parallel with no exprs

[Mikael Djurfeldt]

Kevin Ryde <address@hidden> writes:

> I tried using parallel with no expressions,
>
>       (use-modules (ice-9 threads))
>       (call-with-values
>           (lambda ()
>             (parallel))
>         (lambda ()
>           (display "hi\n")))
>
> but got
>
>       ERROR: In procedure lambda:
>       ERROR: bad body
>
> Is parallel allowed to be called with no expressions?  It'd be pretty
> silly to write that deliberately, but perhaps it could arise from a
> simple-minded macro expansion or something.
>
>         * threads.scm (parallel): For no forms, use `(values)' not `(begin)'.
>
>         * tests/threads.test: New file, exercising "parallel".
>
>
> --- threads.scm.~1.21.~       2003-04-28 07:51:19.000000000 +1000
> +++ threads.scm       2003-06-08 14:31:57.000000000 +1000
> @@ -182,7 +182,7 @@
>       %thread-handler)))
>  
>  (define-macro (parallel . forms)
> -  (cond ((null? forms) '(begin))
> +  (cond ((null? forms) '(values))
>       ((null? (cdr forms)) (car forms))
>       (else
>        (let ((vars (map (lambda (f)

Please don't apply this patch.

The original code is correct.  The error instead lies in the use of
parallel.  You'll find that for example

        (use-modules (ice-9 threads))
        (call-with-values
            (lambda ()
              (display "hello\n")
              (parallel))
          (lambda ()
            (display "hi\n")))

will be accepted.  (lambda () (parallel)) correctly yields an error
message since that is a lambda with no expressions in the body.

Best regards,
Mikael