Formal comment #89 (simplification)

Leave readers and writers out of the report
Reported by:	William D Clinger

Component:	i/o
Version:	5.91

Section 15.2 of the draft R6RS specifies reader and writer objects,
which represent arbitrary sources and sinks for binary data. These
objects are not used anywhere else in the report, do not appear to
have been used in the field, and well-known principles of software
reuse would argue for making them ports instead of a separate data
type.

So long as they are a separate data type, however, they could be
specified by a SRFI and implemented by a portable reference
implementation. It appears that no significant efficiency would be
lost by providing them via a portable reference implementation, since
the operations on file readers and writers could just delegate to an
encapsulated binary port.

If they were ports, they might be useful. Including them in the R6RS
as a separate data type might interfere with making them ports in the
R7RS.

RESPONSE:

The comment's statement that the ports "are not used anywhere else in
the report" is false: They appear in Section 15.3.8, "Ports from
Readers and Writers."  Readers and writers are the only means
specified in the 5.91 draft for obtaining ports from custom input
sources and output sinks.

The next draft of the report will eliminate readers and writers, but
provide means for obtaining ports from custom input sources and output
sinks via the procedures outlined below, obtained by composing the
`open-...-port' procecures from Section 15.3.8 with the
`make-simple-reader' and `make-simple-writer' from the Primive I/O
library, and replacing the procedures of Section 15.3.8 with them.
The exact specification of these procedures may change slightly
because of other changes planned for the I/O libraries.

(open-custom-input-port id descriptor
                        chunk-size read! available
                        get-position set-position! end-position
                        close)

(open-custom-output-port id descriptor
                         chunk-size write!
                         get-position set-position! end-position
                         close)

(open-custom-input/output-port id descriptor
                               chunk-size read! write!
                               get-position set-position! end-position
                               close)

The various arguments to these procedures are as specified for
`make-simple-reader' and `make-simple-writer' in Section 15.2.

In addition to these changes, file options and the I/O condition types
described in Section 15.1 will be moved to the (r6rs i/o ports)
library.