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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