Ok, I oversimplified. The reader for `#hash` doesn't actually look for
`(`; it looks for any character that is mapped to the default meaning
of `(`.
For example, we could map `!` to the meaning of `(`, and the `#hash`
part of the reader would go along with that mapping:
(parameterize ([current-readtable
(make-readtable #f #\! #\( #f)])
(read (open-input-string "#hash!!x . 1))")))
So, the issue in your example is that `(` is mapped to something other
than the default meaning of `(`.
At Mon, 13 Apr 2015 16:20:04 -0400, "Alexander D. Knauth" wrote:
> But if it does that, then why should it care whether the readtable entry for
> `(` is exactly the same as the default?
>
>
> On Apr 13, 2015, at 1:00 PM, Matthew Flatt <mfl...@cs.utah.edu> wrote:
>
> > The reader for `#hash` parses parentheses and dots itself, instead of
> > recurring and checking the result, so that it can provide better error
> > reporting.
> >
> > For example, if I write
> >
> > #hash((x . 1) y (z . 2))
> >
> > then the reader can highlight y and complain about that part
> > specifically. If the `#hash` part of the reader instead inspected the
> > result of a recursive `read`, then it wouldn't have source location for
> > "y", or it would have to recur in a way that preserved locations even
> > though the locations would be later discarded.
> >
> > Also, checking after recurring would make it more difficult to report
> > errors left-to-right, such as getting an error about "]" instead of "y"
> > in
> >
> > #hash((x . 1) y ]
> >
> > It's difficult to say just how much those things matter, and there are
> > trade-offs in flexibility and error reporting, but that's why the
> > reader works the way it does currently.
> >
> > At Sun, 12 Apr 2015 08:54:16 -0400, "Alexander D. Knauth" wrote:
> >> For something like this:
> >> #lang racket
> >> (define orig-readtable (current-readtable))
> >> (parameterize ([current-readtable
> >> (make-readtable orig-readtable
> >> #\( 'terminating-macro
> >> (λ (c in src ln col pos)
> >> (read-syntax/recursive src in c
> >> orig-readtable)))])
> >> (println (read (open-input-string "()"))) ; ’()
> >> (println (read (open-input-string "#hash()")))) ; . read: bad syntax
> `#hash('
> >>
> >> I can understand why this could be a problem, but why doesn't it work so
> that
> >> if reading the () produces the correct association list, it still works?
> >>
> >> Otherwise is there any way around it without just reimplementing #hash(),
> >> #hasheq(), #hasheqv(), #(), etc. ?
> >>
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