Racket's syntax-expander deliberately goes out of its way to break exactly
this example. ("hygiene")
The issue/bug/ambiguity it is protecting you from is:
(let ((send (lambda args (length args)) ))
(def (foo3 x y)
(send x + y)
))
(foo3 2 3) ;; In lisps with poor grooming, 5. In racket, 3. 3 is not
what you wanted this time, but it is the better default behavior overall.
There are many ways to get exactly the desired syntactic behavior; syntax
parameters work, as does datum->syntax.
If you're willing to bend a little, a more racket-y solution, it seems to
me, is to force the user of the syntax to give you a name, like:
(define-syntax-rule (with-SVO-as name body ...) (let-syntax ((name
(syntax-rules [(_ s v o ...) (v s o ...)] ))) body ...)
Which can be used like:
(define (foo4 x y) (with-SVO-as send (send x + y)))
Or like:
(define-syntax send (with-SVO-as n n))
(define (foo5 x y) (send x + y))
Or like:
(define (foo6 x y) (define-syntax send (with-SVO-as n n)) (send x + y))
That approach doesn't work so well for things like:
(with-slots (x y z) (person-x person-y person-z) the-person
(sqrt (* x x) (* y y) (* z z)))
That is, when there are lots of names to be bound, and they're pretty much
always the same, then the repetition/renaming at each use-site gets quite
tiresome; that is the problem that modules solve.
-Will
On Thu, Sep 3, 2015 at 9:59 AM, Brian Mastenbrook <br...@mastenbrook.net>
wrote:
> On Sep 3, 2015, at 11:44, Konrad Hinsen <konrad.hin...@fastmail.net>
> wrote:
>
> > Hi everyone,
> >
> > Here's another plea for help from the macro experts. I am trying to
> > write a macro whose expansion contains another macro, more precisely a
> > let-syntax form. My full example is attached below. In the first part,
> > I use a let-syntax directly. In the second part, I use a macro that
> > generates exactly the form that I wrote by hand in the first part -
> > but then the let-syntax seems to be ignored.
> >
> > The macro stepper isn't of much help here. It shows the expansion
> > of (defn (foo2 ...)) into essentially what I wrote by hand for foo1,
> > and then declares the expansion finished.
> >
> > Can anyone tell me (1) why this doesn't work and (2) how to fix it?
>
> It's a capture problem. In the first case, you're just binding the name
> "send" locally and all is well. In the second case, you're trying to
> introduce a binding for "send" that you didn't get from the input form.
> You're also getting a confusing error because "send" is already bound; try
> using a name that's not already defined and you should get an unbound
> identifier.
>
> You can fix this in one of two ways. Your `def' macro could take the name
> of the `send' macro it binds as a parameter, which gets a little ugly.
> Alternatively, you can use a syntax parameter, which is probably the ideal
> solution here.
>
> (require (for-syntax syntax/parse))
> (require racket/stxparam)
>
> (define-syntax-parameter send (lambda (stx) (raise-syntax-error 'send
> "send used out of context")))
>
> (define-syntax (def stx)
> (syntax-parse stx
> [(_ (fn-name:id arg:id ...) body ... )
> #'(define (fn-name arg ...)
> (syntax-parameterize ([send (λ (stx)
> (syntax-parse stx
> [(send obj:expr method:id x:expr
> (... ...))
> #'(method obj x (... ...))]))])
> body ...))]))
>
> (def (foo2 x y)
> (send x + y))
>
> (foo2 2 3)
>
>
> --
> Brian Mastenbrook
> br...@mastenbrook.net
> http://brian.mastenbrook.net/
>
> --
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