This works for me:
#lang racket
(define (hex:char x)
(if (number? x)
x
(string->number (symbol->string x) 16)))
(define-syntax-rule (hex num ...) (bytes (hex:char (quote num)) ...))
(hex a b c 1 2 3) ; #"\n\v\f\1\2\3"
It’s almost always a mistake to use a function that manipulate syntax
object (syntax-e, etc.) inside syntax-rules, because syntax-rules don’t
give you an access to the syntax object. If you do want to manipulate
syntax object, use syntax-cases instead. In your case, however, the problem
is easy enough that you don’t need to directly manipulate syntax objects.
On Mon, Sep 9, 2019 at 12:12 PM Simon Haines <
simon.hai...@con-amalgamate.net> wrote:
> I'm trying to write a macro that will turn a list of hex literals into a
> byte string.
>
> (hex a b c 1 2 3) ; #"\n\v\f\1\2\3"
>
> After many hours I have finally come up with this:
>
> #lang racket
> (define-syntax hex
> (syntax-rules ()
> [(_ num ...)
> (bytes
> (let ([e (syntax-e #'num)])
> (if (number? e) num
> (string->number (symbol->string e) 16))) ...)]))
>
> (hex a b c 1 2 3)
>
> Of course there are many issues with checking the parameters etc. My
> problem is this generates "a: unbound identifier in: a" because the
> arguments are evaluated? If I remove the last line it works in the REPL OK.
>
> I suspect this is a small matter of my phases being mixed up, or a
> misunderstanding of when macros can be defined and used, or just outright
> ignorance on my part. I couldn't find any clues in the many, many
> references and tutorials I have read. I want to master them but I loathe
> creating macros, they always make me feel like an idiot, and I hope Racket2
> simplifies them somehow.
>
> Thanks for any help sorting this one out.
>
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