Racket macro for wrapping getter function around identifier - macros

Question

I would like to wrap a getter and setter function around an identifier, so that
(define x 1)
(set! x v) ; should expand to (custom-set! x v)
x ; should expand to (custom-get x)
where custom-put! and custom-get are defined somewhere else and add additional behavior such as logging.
In the racket guide (Section 16.1.6), there is an example for a Set!-Transformer which works for a setter and a getter with zero arguments. However, I need a macro which expands an identifier occurence to a getter function with the matched identifier as its argument.
I tried to adopt the macro in the linked section as follows:
(define-syntax-rule (generate-accessors id get put!)
(define-syntax id
(make-set!-transformer
(lambda (stx)
(syntax-case stx (set! get)
[id (identifier? (syntax id)) (syntax (get id))]
[(set! id e) (syntax (put! id e))])))))
The problem is that (syntax (get id)) leads to infinite expansion.
Is there a straightforward way to cut off recursion for a template expression? I also thought about generating a fresh identifier bound to id and including it in the literal list, but I do not know how to accomplish this.

Answer 1

The example you link to works with the code from the previous section
that defines get-val and put-val!:
(define-values (get-val put-val!)
(let ([private-val 0])
(values (lambda () private-val)
(lambda (v) (set! private-val v)))))
(define-syntax val
(make-set!-transformer
(lambda (stx)
(syntax-case stx (set!)
[val (identifier? (syntax val)) (syntax (get-val))]
[(set! val e) (syntax (put-val! e))]))))
val ;; 0
(set! val 42)
val ;; 42
Let's write the macro-writing macro, generate-accessors, that
parameterizes the identifier and accessors, and see that it works:
(define-syntax (generate-accessors stx)
(syntax-case stx ()
[(_ val get-val put-val!)
(syntax
(define-syntax val
(make-set!-transformer
(lambda (stx)
(syntax-case stx (set!)
[val (identifier? (syntax val)) (syntax (get-val))]
[(set! val e) (syntax (put-val! e))])))))]))
(generate-accessors foo get-val put-val!)
foo ;; 0
(set! foo 42)
foo ;; 42
Also, let's exercise it with another pair of accessors, that shows a
side-effect like "logging":
(define-values (custom-get custom-set!)
(let ([private-val 0])
(values (lambda () (println "get") private-val)
(lambda (v) (println "set") (set! private-val v)))))
(generate-accessors bar custom-get custom-set!)
bar ;; 0
(set! bar 42)
bar ;; 42
EDIT: In response to your comment, here's a variation where it generates a fresh pair of accessors (and value) for each invocation. It lets you supply a pair of runtime functions to be called, to do extra work (here just println). (You could simplify this to hardcode the println or log-debug or whatever, if you find it's always the same thing.)
(require (for-syntax racket/base
racket/syntax
syntax/parse))
(define-syntax (define/logged stx)
(syntax-parse stx
[(_ id:id init:expr on-get:expr on-set:expr)
#:with get (format-id stx "get-~a" #'id)
#:with set (format-id stx "set!-~a" #'id)
#'(begin
(define-values (get set)
(let ([v init])
(values (λ () (on-get 'id v) v)
(λ (e) (on-set 'id e) (set! v e)))))
(define-syntax id
(make-set!-transformer
(λ (stx)
(syntax-parse stx
[id:id #'(get)]
[(set! id e) #'(set e)])))))]))
(define (on-get id v)
(println `(get ,id ,v)))
(define (on-set! id v)
(println `(set! ,id ,v)))
(define/logged foo 0 on-get on-set!)
foo
(set! foo 42)
foo
;; prints:
;; '(get foo 0)
;; 0
;; '(set! foo 42)
;; '(get foo 42)
;; 42
I'm not 100% clear on the context of what you're trying to do. Maybe this is closer, or at least gives you some ideas.
Note here I'm switching to syntax-parse instead of trying to follow the original example from the Guide. Just because.