[plt-scheme] Re: How to use a custom procedure as read?

On Sep 30, 2:54 pm, Eli Barzilay <e... at barzilay.org> wrote:
> On Sep 30, Ali wrote:
>
>
>
>
>
> > On Sep 30, 1:12 pm, "Noel Welsh" <noelwe... at gmail.com> wrote:
> > > On Tue, Sep 30, 2008 at 3:45 AM, Ali <emailalicl... at gmail.com> wrote:
> > > >> > (swap-the-reader-in)
> > > >> > ... code using custom reader ...
> > > >> > (swap-the-reader-out)
> > > >> > ... normal reader again ...
> > > ...
> > > > Thanks. Do I have to write the code in a module? I'd much rather be
> > > > able to swap in and out of the custom reader mode as with above.
>
> > > You have to write your code in a separate module.  If you did not,
> > > Scheme would have to evaluate your code as the module was loaded to
> > > work out that you were swapping in a new reader.  This would break a
> > > lot of things, the module system being the prime one.
>
> > Okay, fair enough. So to take this example of a module named canvas,
> > how could I modify it to specify that to use my custom-reader?
>
> > (module canvas mzscheme
>
> >   ... code implementing canvas using custom-reader as the reader ...
>
> >   (provide new-canvas draw-dot! display-canvas))
>
> 1. You write a reader function, it will often use `read', so you'll
>    want to call it something else, say `read*'; you also need to do
>    the same for `read-syntax'.
>
> 2. You provide them as `read' and `read-syntax':
>      (provide (rename-out [read* read] [read-syntax* read-syntax]))
>
> 3. You can now use
>      #reader "path-to-your-module"
>    and the following expression will be read using your reader.  In
>    your case, the expression will be the whole module, for example:
>
>      #reader "my-reader.ss"
>      module foo scheme/base
>        define blah 123
>        ...
>
> There is also a way to change the current readtable, but that's a
> really bad idea, and is almost never what you want it to be.
>
> > > Note the documentation you are reading is for v4, while you're
> > > using v3.  I suggest you upgrade your installation.
>
> > Thanks. I upgraded Mzscheme. Tried a few things, but I've still got
> > no idea what code I have to run, do I need a file in a certain place
> > etc.  Can't really remember the last time I was this confused, so at
> > least its an experience for me!)
>
> [FWIW, my guess is that what I wrote above is not going to help much
> unless you know how to use the module system.]
>
> > > Finally, writing a custom reader is a fairly advanced task.  What is
> > > the problem you're trying to solve?  Perhaps list members can suggest
> > > a simpler way of solving it.
>
> > I'd like to try writing code using SRFI 49
> > (http://srfi.schemers.org/srfi-49/srfi-49.html). With a couple of
> > minor changes I can run the example implementation in guile, using
> > (setf! read sugar-read). Works fine. With a few more changes,
> > Mzscheme can load the reader. All I need now is the Mzscheme
> > equivalent for (setf! read sugar-read), which apparently doesn't do
> > anything in mzscheme.
>
> Yes, mzscheme tends to be more well behaved in this regard.
>
> > Sorry for not saying this explicitly before, but I was concerned the
> > discussion would be side-tracked to reasons why I shouldn't be using
> > SRFI 49 in the first place.
>
> [A valid concern, since it is a bad idea.  Even worse than you think.]
>
> > Anyway, thanks for telling me about the docs version, and making it
> > clear that I need to use a module. Really hope someone can give just
> > the smallest bit of example usage, just an equivalent for (set! read
> > sugar-read), where sugar-read is a user defined procedure. Or point
> > me to an existing example usage. Sorry if my irritation shows, I am
> > actually very grateful for your time, and I know there are more
> > enjoyable things one can do with it!
>
> You can look at "collects/scribble/reader.ss" in the distribution --
> that implements the scribble syntax, and can be used as:
>
>   #reader scribble/reader <expression-in-scribble-syntax>
>
> Note that getting a `read' function is usually just half of the work.
> MzScheme uses `read-syntax' to read code, which returns syntactic
> objects that contain source information.  Getting `read-syntax'
> working is usually more difficult.  You can try to make it the same as
> `read' just to experiment with things, but syntax errors are not going
> to have source information.
> --
>           ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
>                  http://www.barzilay.org/                Maze is Life!
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Excellent, thanks. It still doesn't work, but I'm hopeful that after
I've written read-syntax it might work, and this clears up the idea of
putting the code in a module definition.