30 minutes ago, Markku Rontu wrote:
> Well one problem is that all these syntax objects have introduced a
> new language, with accessors and functions that are completely new
> that must be learnt. And programs using these new functions look
> quite different to standard data structure (i.e., cons cell)
> manipulation. [...]
Ah, but it's not a new language -- just a new type. It's a good hook
to explain it in a different way -- it's as if you had this:
(provide syntax? syntax-e syntax->datum)
(struct syntax (e))
;; a wrapper for syntax values, holding a single value which is a
;; plain sexpr -- don't give out the `syntax' constructor.
(define (syntax->datum stx)
(unless (syntax? stx)
(raise-type-error 'syntax->datum "syntax" stx))
(let loop ([d stx])
(cond [(syntax? d) (loop (syntax-e d))]
[(pair? d) (cons (loop (car d) (cdr d)))]
[(vector? d) ...]
...
[else d])))
and then extended it with some opaque lexical context field:
(provide syntax? syntax-e syntax->datum datum->syntax)
(struct syntax (e ctx))
;; but don't provide `syntax-ctx', instead have a function that
;; creates a syntax using the context of another:
(define (datum->syntax c dtm)
(syntax (syntax-ctx c) dtm))
and this goes on.
(This is a nice way to explain it, since many explanations take
`syntax-case' as the primitive and the rest is derived from it.)
--
((lambda (x) (x x)) (lambda (x) (x x))) Eli Barzilay:
http://barzilay.org/ Maze is Life!
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