At Tue, 14 Apr 2015 21:04:38 -0400, "Alexander D. Knauth" wrote:
>
> On Apr 14, 2015, at 6:57 PM, Matthew Flatt <mfl...@cs.utah.edu> wrote:
>
> > This is the kind of problem that the readtable argument to
> > `read/recursive` was meant to solve, but I see that it doesn't work in
> > this case.
> >
> > I have in mind that you'd map `#` to use `read/recursive` with a
> > readtable that restores the default `(`. Mapping `#` would cover hash
> > tables, vectors, and prefab structures.
> >
> > As `read/recursive` is currently implemented, however, a given
> > readtable is would be used only for the initial `#`, so it doesn't
> > affect the `(`s that delimit hash-table entries.
>
> Do you mean 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))
> #\# 'non-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()")))) ; '#hash()
Yes.
> Because this worked! I’m surprised though; how is it communicating
> the orig-readtable to the #hash reader macro so that it can use that
> for parsing the ()?
Hm, right. The current reader does use the given readtable all the way
through the opening '(', but not after that --- which doesn't seem like
a good choice overall.
> This doesn’t work, though:
> (parameterize ([current-readtable ….])
> (println (read (open-input-string "#hash((a . b))")))) ; . read: expected
> or
> `[' or `{' to start a hash pair
> If I replace the inner set of parens with brackets, it works.
Yep.
> > It would make sense
> > --- and I think it would solve your problem --- if the given readtable
> > instead applied to all of the delimiters of the non-nested parts of the
> > first datum in the input.
>
> Would this require giving an extra argument to reader macro
> procedures, or am I misunderstanding something?
I had in mind an extra argument to `read/recursive` to enable the new
mode, with no change to the arguments to reader macros.
But, now that you say so, I see a potential need to send a readtable to
reader macros, too. That would be needed if you wanted to implement a
`#hash` macro from scratch that behaves the the built-in one.
The interface for reader macros is already complicated, but I think it
would work to add one extra optional-to-accept argument that is the
readtable to use for immediate delimiters.
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