On May 1, 2009, at 12:30 PM, Ramana Kumar wrote:
If I understand correctly, this is the 90% solution
You can start with:
(define (proc x stx)
(if (pair? x) (car x) (error 'car "not a pair" p stx)))
(define-syntax car+
(lambda (x)
(with-syntax ([stx x])
(syntax-case x ()
[(_ e)
#'(proc e #'stx)]
[id (identifier? #'id)
#'(lambda (x) (proc x #'stx))]))))
which avoids excessive code growth at each use of car+.
Of course you probably want to do cdr too, and you don't want
proc to be visible, so, you do a little abstraction there.
(define-syntax define-pair-checked ;;; can't think of good names
(syntax-rules ()
[(_ car+ car)
(begin
(define (proc x stx)
(if (pair? x) (car x) (error 'car "not a pair" p stx)))
(define-syntax car+
(lambda (x)
(with-syntax ([stx x])
(syntax-case x ()
[(_ e)
#'(proc e #'stx)]
[id (identifier? #'id)
#'(lambda (x) (proc x #'stx))])))))]))
(define-pair-checked car+ car)
(define-pair-checked cdr+ cdr)
You can take this as far as you please.
Aziz,,,
