prop:rename-transformer caching results in macro expander

[Leif Andersen]

Hello,

I am trying to use a struct that is both mutable, and has
prop:rename-transformer, to create a mutable rename transformer. I am
able to update the value in the struct, and that update gets reflected
properly when I do `syntax-local-value/immediate`, but
`syntax-local-value` seems to keep around the previous definition.

Is prop:rename-transformer or syntax-local-value supposed to cache its
result, or would this be a bug?

My code so far is here: http://pasterack.org/pastes/41342

```
#lang racket

(define-syntax string1 "hello")
(define-syntax string2 "world")

(begin-for-syntax
(struct string-box (str)
#:mutable
#:property prop:rename-transformer
(lambda (inst)
(if (string-box-str inst)
#'string1
#'string2))))
(define-syntax (string-lookup stx)
(syntax-case stx ()
[(_ id)
#`'#,(syntax-local-value #'id)]))
(define-syntax (string-set! stx)
(syntax-case stx ()
[(_ box value)
(begin
(define-values (x y) (syntax-local-value/immediate #'box))
(set-string-box-str! x #'value)
#'(void))]))

(define-syntax current-str (string-box #t))
(string-lookup current-str)
(string-set! current-str #f)
(string-lookup current-str)
```

Thank you for your help.

~Leif Andersen

[Matthew Flatt]

The main problem is that you need to attach the 'not-free-identifier=?
property to the syntax object produced by your rename transformer:

(begin-for-syntax
(struct string-box (str)
#:mutable
#:property prop:rename-transformer
(lambda (inst)

(syntax-property

(if (string-box-str inst)
#'string1
#'string2)

'not-free-identifier=?
#t))))

Otherwise, `current-str` is created as an alias to the binding of the
initial result from `(string-box #t)`. That binding alias effectively
overrides rename transformer.


A second problem is that you want `(syntax-e #'value)` instead of just
`#'value`.

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[Leif Andersen]

That worked. Thank you Matthew.

~Leif Andersen

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