I think the following program illustrates the idea, though it doesn't
really work:
#lang racket
(begin-for-syntax
(define (syntax-pair->cons stx)
(define datum (syntax-e stx))
(cond
[(list? datum)
(map syntax-pair->cons datum)]
[(pair? datum)
(cons (syntax-pair->cons (car datum))
(syntax-pair->cons (cdr datum)))]
[else stx]))
(define (ir-macro-transformer proc)
(lambda (stx)
(define transformed-s-expr
(proc (syntax-pair->cons stx)
(λ (id) (datum->syntax stx id))))
(datum->syntax #'here transformed-s-expr)))) ;; hack
(define-syntax loop
(ir-macro-transformer
(lambda (expr inject)
(let ((body (cdr expr)))
`(call-with-current-continuation
(lambda (,(inject 'exit))
(let f () ,@body (f))))))))
(let ([i 0])
(loop
(printf "i = ~a\n" i)
(set! i (+ i 1))
(when (>= i 5)
(exit 'ok))))
Syntax objects are the representation of syntaxes used in expansion
and compilation. Racket doesn't just use quoted expressions since the
expander and the compiler need to keep track of lexical information,
source location and other properties. The form #' and #` are
constructors of syntax objects that take templates and produce the
syntax objects with the desired shape, just like ' and ` are
constructors of quoted s-expressions.
If you want to provide a function that manipulates lists and symbols
as a transformer, ir-macro-transformer would at least need to be a
function that maps between syntax objects and quoted expressions. Such
maps are imperfect though.
One problem I found with the above program is that `inject' doesn't
have the desired lexical context, so the while example does not work.
phase 1 phase 0
+------------ syntax object (instead of quoted s-expression)
|
transformer
(lambda functions bound
using define-syntax)
|
+-----------> syntax object
On Wed, Aug 29, 2018 at 7:25 AM, Philip McGrath
<phi...@philipmcgrath.com> wrote:
> For future reference, you should try the wonderful macro stepper in
> DrRacket, which shows you exactly how expansion happens. It can even handle
> buggy, non-terminating examples like the version of `test` you wrote.
> -Philip
>
>
> On Wed, Aug 29, 2018 at 7:20 AM Philip McGrath <phi...@philipmcgrath.com>
> wrote:
>>
>> I'm not familiar with how `ir-macro-transformer` is supposed to work, but
>> your macro is currently fails for essentially the same reason as:
>> (define-syntax (diverge stx)
>> stx
>>
>> The `expr` given to `test` is a syntactic list beginning with the
>> identifier `test`, so including it in the output triggers another expansion
>> of `test`, infinitely.
>>
>> -Philip
>>
>>
>> On Wed, Aug 29, 2018 at 7:06 AM t791bc via Racket Users
>> <racket-users@googlegroups.com> wrote:
>>>
>>> Hi,
>>>
>>> while syntax-case has some advantages, I was trying to implement a
>>> Chicken style ir-macro-transformer in Racket so that I can write macros that
>>> will run both on Racket and systems like Chicken/Chibi Scheme.
>>>
>>> My first attempt was as follows:
>>>
>>> (begin-for-syntax
>>> (require (for-syntax racket/base))
>>> (define-syntax (ir-macro-transformer stx)
>>> (syntax-case stx ()
>>> [(_ ir-trans)
>>> #'(lambda (x)
>>> #`(ir-trans #,(syntax->datum x) 23))])))
>>>
>>> (define-syntax test
>>> (ir-macro-transformer
>>> (lambda (expr inject)
>>> expr)))
>>>
>>> The result was "Background expansion terminated abnormally (out of
>>> memory)". If the #, is taken out I get an error that x is unbound. As should
>>> be obvious from the above, I find it hard to reason about all those syntax
>>> effects - which only makes me want the ir-macro-transformer more despite all
>>> its shortcomings.
>>>
>>> Any help would be highly appreciated.
>>>
>>>
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