Re: Difference between values->list and list

[Sebastian Egner <sebastian.egner@xxxxxxxxxxx>]

Hello Christian,

No VALUES->LIST and LIST do different
things.

To understand the difference, let's
have a look at an implementation:

(define list
   (lambda xs xs))

(define-syntax values->list
  (syntax-rules ()
    ((values->list x)
     (call-with-values (lambda () x)
list))))

As you can see, LIST is a procedure
returning the list of its arguments.

VALUES-LIST on the other hand is a macro
evaluating its single
argument (called x above) by calling
the produce procedure (lambda () x).
This evaluation may result in several
values, on which call-with-values
calls the consumer procedure LIST.

The important thing is that "producing
several values" is not the
same as "returning a list with
several elements". You can imagine
the process transfering several values
from a produce to a consumer
as follows: (lambda () x) executes machine
instructions that leave
several results in a couple of CPU registers
used for results. Call-with-values
then moves all the data from the result
registers into the argument
registers for the next procedure and
calls the consumer procedure.
In practice, this may not be the way
it is happening because the
Scheme system can implement the transfer
of values in many
different ways, but I hope you get the
picture.

Sebastian.