Amalie,
Thank you, this is impressive and helpful. I did not know about
syntax-local-value (I also am not certain that I know it now good enough).
However, I did not mean to use the (my-let)'s "definition" of x to pass
information to (access). (Also, can (access y) fail in a customizable
way?) I was wondering if there is a way to just pass any information
(list of variables, which can be empty, or whatever else) from (my-let)
to (access).
Best regards,
Dmitry
On 17.04.2018 1:18, Dionna Amalie Glaze wrote:
syntax-local-value is likely what you'll want to use here. You can
bind your x to a compile-time value that you can access with
syntax-local-value. You can even make that value a struct with
#:procedure prop:procedure and have its procedure be a syntax
transformer (so it can expand into something meaningful outside its
use by access).
#lang racket
(require (for-syntax syntax/parse racket/list syntax/transformer))
(begin-for-syntax
(struct communicate (syntax value) #:prefab))
(define-syntax (access stx)
(syntax-parse stx
[(_ x:id)
#:fail-unless (communicate? (syntax-local-value #'x))
(format "Expected ~a to be bound by my-let" (syntax-e #'x))
(define slv (syntax-local-value #'x))
#`(list #,(communicate-syntax slv) 'has-tree-nodes
'#,(communicate-value slv))]))
(define-syntax (my-let stx)
(syntax-parse stx
[(_ ([x:id e:expr] ...) body:expr)
#'(let-syntax ([x (communicate #'e
(length (flatten (syntax->datum #'e))))] ...)
body)]))
(my-let ([x '(a b ((c d) 4) (5 9))])
(access x))
;=>
'((a b ((c d) 4) (5 9)) has-tree-nodes 8)
(the answer is 8 and not 7 because it's counting the 'quote in the syntax)
On Mon, Apr 16, 2018 at 2:16 PM Dmitry Pavlov <dpav...@iaaras.ru
<mailto:dpav...@iaaras.ru>> wrote:
Hello,
I would like to write two seemingly simple macros and I found
no way to do it.
(my-let ((x 2))
(begin
(begin
(begin
(access x)
(access y)))))
I would like the (access) macro to know at compile (expansion)
time that x is up there in (my-let) macro and y is not.
I tried the following:
- save information about 'x during (my-let) expansion
in a parameter (present in expander but not in
expanded code) and check that parameter in (access).
This does not seem to work, probably because of
the way expansion works.
- attach information as a property to syntax object
generated by (my-let), then bring it all the way down
(that requires redefining begin) and get it back
from syntax object in (access). That works, but is
a pain.
- explicitly call (expand-syntax) for (my-let)'s body
in (my-let)'s expansion. Does not work because (access)
is not yet available at that point.
- find and read "Macros that work together" my Flatt et al.
Well, I did try to find what I need, until the code
has switched from Racket to some other fancy language
and then I am stuck.
Any advice please?
Best regards,
Dmitry
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