Kazimir Majorinc wrote at 05/14/2011 10:55 PM:
How would you do that "idiomatically"?
One way is to use syntax extension. Below, I have simplified your
syntax for "counted" a little, although I could have also done your
exact syntax by adding it to the "(_ VAR EXPR)" form.
;;;; BEGIN
#lang racket/base
(define-syntax counted
(syntax-rules ()
((_ VAR EXPR)
(let ((i 0))
(lambda (VAR)
(set! i (+ 1 i))
(display i)
(newline)
EXPR)))))
(define f (counted x (+ x x)))
(f 3)
(f 9)
(f 27)
;;;; END
An arguably more idiomatic way to do this particular thing is to not use
syntax extension, but first-class closures:
;;;; BEGIN
#lang racket/base
(define (make-counted proc)
(let ((i 0))
(lambda (val)
(set! i (+ 1 i))
(display i)
(newline)
(proc val))))
(define f (make-counted (lambda (x) (+ x x))))
(f 3)
(f 9)
(f 27)
;;;; END
The name "make-counted" is convention, but you could also name it "counted".
--
http://www.neilvandyke.org/
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