[racket] Again on bindings visibility in eval

From: Markku Rontu (markku.rontu at iki.fi)
Date: Thu Jul 14 15:16:35 EDT 2011

On Thu, Jul 14, 2011 at 9:58 PM, Eli Barzilay <eli at barzilay.org> wrote:

> 25 minutes ago, Markku Rontu wrote:
> > > >
> http://hipster.home.xs4all.nl/lib/scheme/gauche/define-syntax-primer.txt
> > [...]
> >
> > Still, I count it as good eye opening material :)
> Well, it has a heavy focus on "computations via rewriting", which can
> of course be an eye opener, but off-topic when talking about practical
> macro writing.  (It still is practical, if you're confined to R5RS for
> portability, but we're now in a racket context.)
> Ok maybe that style as you say "computations via rewriting" is what brings
the bad C++ memories. But it opened my eyes as to what is possible with
syntax-rules. And sometimes I use it with syntax-case too.

> > Granted your cited blog post is good too but it's not a complete
> > tutorial.
> It's inteneded as a quick thing for people who are familiar with
> symbolic `defmacro' things.
> > > If you're working in Racket, it makes much more sense to really use
> > > its syntax system rather than stick to those tricks.
> > >
> > However, I find syntax-case to be very similar if you proceed to it
> > as next step from syntax-rules. I mean that if you take it as an
> > improved syntax-rules, you will likely end up using it just like
> > syntax-rules except for the part about breaking hygiene. This is
> > just my experience.
> Not at all.  The main addition is exactly what you complained about:
> having the full language.  Here's an implementation of a function
> application with reversed arguments, with `syntax-rules':
>  (define-syntax rapp
>    (syntax-rules ()
>      [(rapp f x ...) (rapp* f (x ...))]))
>  (define-syntax rapp*
>    (syntax-rules ()
>      [(rapp* f (x0 x ...) r ...) (rapp* f (x ...) x0 r ...)]
>      [(rapp* f () r ...) (f r ...)]))
> Can you see the reversal?  It's of course possible -- and even an
> instance of such eye openning, but I really wouldn't call that
> obvious.  (And un-obvious is bad -- it leads to surprising bugs.)
> Yeah, it's obvious after the tutorial I linked ;-)

> Here is the same using `syntax-case':
>  (define-syntax (rapp stx)
>    (syntax-case stx ()
>      [(rapp f x ...) #`(f #,@(reverse (syntax->list #'(x ...))))]))
> and now the reversal is explicit -- it says "reverse"...
> There's of course the naive translation of `syntax-rules' to
> `syntax-case', leading to the same bad code as the first example.
> That's why the first thing that should be made obvious is that there
> is no magic here -- these are just plain values.  Once *that* is
> clear, it should also be clear that such a translation is a bad idea.
I would not call this piece of code much more obvious, it has too much line
noise for my comfort :-)

> As an antidote to such illusions of black magic, here's another
> version that doesn't even use `syntax-case':
>  (define-syntax (rapp stx)
>    (let ([l (cdr (syntax->list stx))])
>      (datum->syntax stx (cons (car l) (reverse (cdr l))))))
Thanks anyway for these, they are more of what I was talking about. I do use
"the full power of Racket" when writing my macros. Like introducing new
names through string manipulation and syntax->datum and datum->syntax. Still
that part usually looks ugly and un-Rackety to me. Have to ponder more why
it feels so.

> > > Read that blog post: it shows you how you can use plain racket
> > > code to write macros -- and it introduces some of the tools that
> > > make it easy to deal with syntax values.
> >
> > I think what is mainly missing is a bit more positive publicity in
> > this area. A great collection of tools for writing macros exists,
> > and obviously great things are being written using them (TR
> > etc.). Just need to produce enough material so that the search
> > engines start to find the good stuff.
> That is a social problem:
> - There are many people who still think that macros are a bad idea in
>  general, and advocate that idea.
> - From the peole who manage to get passed that propaganda line, there
>  are people who think that there's nothing wrong with plain CPP-style
>  textual macros, and advocate that idea.
> - From the peole who manage to get passed that propaganda line, there
>  are people who think that symbolic macros are superior, and advocate
>  that idea.
> - From the peole who manage to get passed that propaganda line, there
>  are people who think that `syntax-rules' are better since they don't
>  get phases, and advocate that idea.
Who else to praise the awesomeness of Racket macros and syntax-case and
phase levels if not the Racket people? If you get them young then they might
not need to pass all those filters :-)

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