# Make-variable-like-transformer **URL:** \ **Category:** Questions & Answers\ **Tags:** question\ **Created:** [July 20, 2026, 11:04am UTC](https://racket.discourse.group/t/make-variable-like-transformer/4323 "2026-07-20T11:04:54Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![joskoot](https://yyz2.discourse-cdn.com/free1/user_avatar/racket.discourse.group/joskoot/32/1964_2.png) [@joskoot](https://racket.discourse.group/u/joskoot)\ **Post date:** [July 20, 2026, 11:04am UTC](https://racket.discourse.group/t/make-variable-like-transformer/4323/1 "2026-07-20T11:04:54Z")
Why does the following give an error? state-a: undefined; cannot reference an identifier before its definition in module: 'anonymous-module Wrong use of datum-\>syntax? How to fix this? ```scheme #lang racket (require (for-syntax (only-in syntax/transformer make-variable-like-transformer))) (struct state (a) #:mutable #:constructor-name make-state #:omit-define-syntaxes) (define state (make-state 1)) (define-syntax (state-component stx) (syntax-case stx () ((_ id) (with-syntax ((state-id (datum->syntax stx (string->symbol (format "state-~s" (syntax-e #'id))))) (set-state-id! (datum->syntax stx (string->symbol (format "set-state-~s!" (syntax-e #'id)))))) #'(define-syntax id (make-variable-like-transformer (state-id state) (λ (v) (set-state-id! state v)))))))) (state-component a) ``` ---
**Author:** ![LiberalArtist](https://yyz2.discourse-cdn.com/free1/user_avatar/racket.discourse.group/liberalartist/32/151_2.png) [@LiberalArtist](https://racket.discourse.group/u/LiberalArtist)\ **Post date:** [July 20, 2026, 2:01pm UTC](https://racket.discourse.group/t/make-variable-like-transformer/4323/2 "2026-07-20T14:01:22Z")
I think the problem is easier to see if we replace `(state-component a)` with its expansion, as reported by the macro stepper: ```scheme #lang racket (require (for-syntax (only-in syntax/transformer make-variable-like-transformer))) (struct state (a) #:mutable #:constructor-name make-state #:omit-define-syntaxes) (define state (make-state 1)) (define-syntax a (make-variable-like-transformer (state-a state) (λ (v) (set-state-a! state v)))) ``` This is trying to use `state`, `state-a`, and `set-state-a!` at compile time, but those values don’t exist until runtime. You need to pass `make-variable-like-transformer` a syntax-quoted `#'(state-a state)`\[1\] that it should expand to, not the value of the expression `(state-a state)`. Here is a working version, with only two other changes I couldn’t resist: generating names is much better with `format-id`, and using the scope from `#'id` rather than `stx` is more robust if some other macro might expand to `state-component`: ```scheme #lang racket (require (for-syntax (only-in racket/syntax format-id) (only-in syntax/transformer make-variable-like-transformer))) (module+ test (require rackunit) (check-eqv? a 1) (set! a 5) (check-eqv? a 5)) (struct state (a) #:mutable #:constructor-name make-state #:omit-define-syntaxes) (define state (make-state 1)) (define-syntax (state-component stx) (syntax-case stx () [(_ id) (with-syntax ([state-id (format-id #'id "state-~a" #'id)] [set-state-id! (format-id #'id "set-state-~a!" #'id)]) #`(define-syntax id (make-variable-like-transformer #'(state-id state) #'(λ (v) (set-state-id! state v)))))])) (state-component a) ``` * * * Looking beyond the immediate problem, I’d guess that you have written `define-state-component` because, in reality, you have several components. I am allergic to writing out lists of fields multiple times—in the `struct` declaration, in the arguments to `make-state`, and in the uses of `define-state-component`—and I’d be sad that DrRacket’s scope-aware renaming couldn’t handle `a`. I would be inclined to do something like this: ```scheme #lang racket (require (for-syntax racket/syntax syntax/transformer) syntax/parse/define) (module+ test (require rackunit) (check-eqv? a 1) (set! a 5) (check-eqv? a 5) (check-equal? b '(a b c)) (set! b 'done) (check-eq? b 'done)) (define-syntax-parse-rule (define-state name:id (~seq field:id init-val:expr) ...) #:with make-name (format-id #'name "make-~a" #'name #:subs? #t) #:with ((~seq field-ref set-field!) ...) (for*/list ([fld (in-list (attribute field))] [fmt `("~a-~a" "set-~a-~a!")]) (format-id fld fmt #'name fld)) (begin (struct name (field ...) #:mutable #:constructor-name make-name #:omit-define-syntaxes) (define name (make-name init-val ...)) (define-syntax field (make-variable-like-transformer (quote-syntax (field-ref name)) (quote-syntax (λ (v) (set-field! name v))))) ...)) (define-state state a 1 b '(a b c)) ``` In real life, I [did something like this](https://github.com/racket/web-server/blob/080d2ea7dfc8b8e9a147f2d3155970098eac69b0/web-server-lib/web-server/safety-limits.rkt#L29) to define the web server’s [“safety limits”](https://docs.racket-lang.org/web-server-internal/dispatch-server-unit.html#%28part._safety-limits%29) construct with `make-safety-limits` and `make-unlimited-safety-limits`. Often, when we talk about why macros are so useful, we give examples of using macros to implement new linguistic constructs. But I think the macro that is used exactly once is an underappreciated genre! * * * 1. More correctly, `(quote-syntax (state-a state))`, but we can ignore that in this case. ---
**Author:** ![joskoot](https://yyz2.discourse-cdn.com/free1/user_avatar/racket.discourse.group/joskoot/32/1964_2.png) [@joskoot](https://racket.discourse.group/u/joskoot)\ **Post date:** [July 20, 2026, 3:09pm UTC](https://racket.discourse.group/t/make-variable-like-transformer/4323/3 "2026-07-20T15:09:59Z")
Thanks for your rapìd reply. It works (of course) Including the definition of the struct in the macro such as to avoid repeated lists of field-names, is a good idea, Makes adding or removing fields in new versions easier and less work. Jos Koot