syntax closures

[Stefan Israelsson Tampe <stefan.itampe@gmail.com>]

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____ FUN with syntax case ____

Hi all, I wanted to resurrect the idea of a syntactic closure. I did some
thinking and
decided on the following system:

The low level routine is a special macro 'syntax-closure' put in (ice-9
syntax-closure)
The macro has the following ideom

  (syntax-closure closure-object template1 ...)

e.g.

  #`(syntax-closure #,(lambda (x y) (f x 1 y)) #'(+ x z) #'w)

1. Note that we use #, to insert a function object into the syntax
expression
2. The idea is to capture the syntactic environment in template1 ... and
 call the
    closure with the captured syntactic elements and insert the resulting
syntax into
.

The code for syntax-closure is simple:

(define-syntax-rule (syntax-closure f x ...)
  (let-syntax ((capture (lambda (stx)
                          (syntax-case stx ()
                            ((_ y (... ...))
                             (apply f #'(y (... ...))))))))
    (capture x ...)))

And with this tool we can device a very useful reader extension #_ e.g.

(define (syntax-closure-reader chr port)
  (define (g-loop x)
    (let loop ((x x) (a '()) (g '()) (t '()))
      (match x
        (((and stx ('syntax x)) . l)
         (let ((gen (gensym "template")))
           (loop l (cons gen a) (cons gen g) (cons x t))))
        ((x . l)
         (loop l (cons x a) g t))
        (()
         (values (reverse a) (reverse g) (reverse t))))))

  (let ((code (read port)))
    (match code
      ((F X ...)
       (call-with-values (lambda () (g-loop X))
         (lambda (args gensyms template)
           `(syntax-closure
             (unsyntax (lambda ,gensyms
                         (,F ,@args)))
             ,@template))))
      (_ (error "wrong format in syntax-closure-reader")))))

(read-hash-extend #\_ syntax-closure-reader)

And with this we can now do

  (define (g x) x)
  (define-syntax f (lambda (x) #`(let ((x 1)) #_(g #'x))))

leading to

scheme@(guile-user)> f
$2 = 1

The reader extension will be of the form
#_(F a ...)

And all (syntax x) arguments in a ... will be automatically captured from
the environment
e.g. #_ is as #, but with the syntax arguments correctly captured from the
surrounding
environment.

With this tool we will get close enough to srfi-72 and that code will not
be needed.

I'm I out of the blue here?

The question for me is how to treat this code, as a library of it's own
supporting e.g.
racket and guile and other schemes with the syntax case system, or should
we try to make this reader extension as nice as possible and incorporate
the code in guile. Is #_ ok? do you have any other suggestion?

Regards
/Stefan

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