Formal comment #200 (defect)

I/O: Bytevector output ports and `set-port-position!'
Reported by: 	Ludovic Courtès
Version: 	5.92

Summary

The return value of a bytevector output port procedure is
under-specified when `set-port-position!' is used.

Description

Section 7.2.10 of the report (page 32) explains that binary bytevector
output ports "will support the port-position' and set-port-position!'
operations.". It then defines the associated procedure as follows:

    When the procedure is called without arguments, it returns a
    bytevector consisting of the port's accumulated data and removes
    the accumulated data from the port.

According to this definition, the procedure returns all the data
accumulated instead of just the data _up to_ the current port
position. I believe the latter would be a more appropriate definition
since otherwise the bytevector returned by the procedure differs from
the what was intended by the port user.

Example:

  (let-values (((port get-content)
                (open-bytevector-output-port)))
    (put-bytevector port (u8-list->bytevector '(1 2 3 4 5 6)))
    (set-port-position! port 0)
    (put-bytevector port (u8-list->bytevector '(0 0 0)))
    (let ((first (get-content)))
      (let ((second (get-content)))
        (set-port-position! port (+ (port-position port) 3))
        (list first second (get-content)))))

Using the current semantics, FIRST and SECOND would be equal to
`#vu8(0 0 0 4 5 6)' and `#vu8()', respectively, and the following
`set-port-position!' would raise an error.

Instead, I believe it should return `(#vu8(0 0 0) #vu8() #vu8(4 5
6))', which is closer to the user intention.

Thus, the aforementioned paragraph could be rewritten along the
following lines:

    When the procedure is called without arguments, it returns a
    bytevector consisting of the port's accumulated data up to its
    current position (when it supports port-position' and
    set-port-position!'). It removes all the accumulated data up to
    the port's current position from the port. Accumulated data that
    is beyond the port's current position should be kept by the
    port. Invoking the procedure also sets the port position to zero
    (if the port supports `port-position').

I'm not sure about this last sentence since it would make bytevector
output ports behave differently from "regular" output ports, which
might be an issue.

An alternative solution might be to disallow `set-port-position!'
altogether.


RESPONSE:

It is possible that a programmer would wish for a bytevector port's
procedure to return only the accumulated data up to the current position
(the proposed behavior).  It is also possible that a programmer would want
all of the accumulated data (current behavior).  The proposed behavior is
more complex to implement and potentially more costly.  Furthermore, in
cases where the proposed behavior is desired, it can be simulated via
bytevector-copy! and put-bytevector.  Therefore, the current behavior
will be retained in the next draft of R6RS.  The description will be
clarified to make the behavior clear.