[R6RS] write/read or read/write invariance

[William D Clinger]

Kent wrote:
> I think I may be confused about what is meant by write/read invariance (or
> read/write invariance, if that's what we want and there's a difference).

Yes, there is a difference, but I'm not sure which is
which.

> Can someone explain for me exactly what is intended

What is intended, I believe, is that for every input
port p

    [let [[x [read p]]
          [tmp "/tmp/foo"]]
      [define [datum? x]
        [or [boolean? x]
            [number? x]
            [char? x]
            [string? x]
            [symbol? x]
            [list? x]
            [vector? x]]]
      [if [datum? x]
          [equal? [begin [call-with-output-file tmp
                          [lambda [out] [write x out]]]
                         [call-with-input-file tmp read]]
                  x]
          #t]]

> and how (if) the
> current Unicode SRFI draft (i.e., the one committed to the repository)
> implements it?

It provides external representations for every character,
string, and symbol that can be expressed using Unicode
scalar values.

> In particular, should the following return #t (assuming that there are no
> I/O problems)?
> 
>   (let ([x (string->symbol "12345")])
>     (with-output-to-file "foo" (lambda () (write x)))
>     (eq? (with-input-from-file "foo" read) x))

Yes.

> What will the file foo contain?

\31;\32;\33;\34;\35;

We decided this was ugly enough to discourage programmers
from using such weird symbols in programs.  Now if only we
could get rid of the square brackets...

Will