1. Overall, I think the treatment of eof objects is fine, but there is one
deficiency in the current draft. Generally, Scheme tries to provide type
discriminators, constructors, accessors, and mutators for its types; however,
there is no constructor for the eof object. Yet in fact an eof object can be
constructed in standard portable R7RS Scheme, using the idiom which I think
many Schemers are familiar with:
(let* ((p (open-input-string ""))
(x (read p)))
(close-port p)
x)
I'd therefore suggest adding a procedure named make-eof-object or some such,
and defined to return one of the set of eof objects for the implementation.
This isn't just an attempt to create a vain orthogonality; there are good
reasons why arbitrary code might wish to return an eof object. For example, a
DBMS interface might have a routine that returns one row, as a list or a
vector, at a time; after the last, it is perfectly reasonable to return an eof
object.
An argument against providing this is that the constructor may be trivially
written, as shown above. A similar argument could be applied to zero?, newline,
quotient, remainder, and modulo, among others. R7RS is not afraid to provide
easy-to-implement procedures in the name of simplicity, orthogonality, or
historical compatibility. I think the lack of an eof constructor is worth
remedying.
2. I'm not sure I understand the difference between `input port' and `port
capable of doing input operations'. The draft offers input-port? and
output-port? that are true if the argument is an input (or output port). Yet
close-input-port and close-output-port are clearly defined to shut down the
input or output side of a port, leaving the other side still functioning, and
explicitly work on things that are not input (or output) ports. This is all
good language design, and I'm fine with the functionality described here.
However, if an implementation offers some kind of input/output port, I'd ask
`is this an input port?' and `is this an output port?' I think the draft would
say No (which I would not agree with, but appears to be the intention in the
{input,output}-port? entry), but I'm not sure. Could I therefore ask that the
entry for {input,output}-port? include a sentence or two that is comparable to
that for close-{input,output}-port saying what happens in such cases.
> (define p (open-input/output-file "/dev/tty")) ; implementation-provided
port type.
> (input-port? p)
???
> (output-port? p)
???
> (input/output-port? p) ; implementation-provided
procedure.
#t
I'd like the ???s to be replaced with #t, but even if the result is required to
be #f at least people will know what these type discriminators do.
-- vincent
_______________________________________________
Scheme-reports mailing list
[email protected]
http://lists.scheme-reports.org/cgi-bin/mailman/listinfo/scheme-reports
