[r6rs-discuss] [Formal] I/O: Bytevector output ports and `set-port-position!'

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Submitter:      Ludovic Court?s
Email:          ludovic.courtes at laas.fr
Type of Issue:  Defect
Priority:       Major
Component:      I/O
Report:         Revised^5.92 Report on the Algorithmic Language Scheme,
                Standard Libraries
Summary:        The return value of a bytevector output port procedure
                is under-specified when `set-port-position!' is used.
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.
Thanks,
Ludovic.