Re: Identifier equality of identifiers stashed in preserved syntax properties

On 10/25/2016 08:04 PM, Alexis King wrote:
> That makes sense; thank you for your quick reply. It might be
> possible to do something like what you describe, but I do have a
> little more context that makes this sort of tricky. I’m trying to
> not just store identifiers but also store prefab structs containing
> identifiers. The issue I’m running into is that embedding prefab
> structs in syntax objects seems to “flatten” the contained syntax
> objects. For example, if I do something like this:
>
>    (begin-for-syntax
>      (struct prefab-box (val) #:prefab)
>      (syntax-parse
>          (local-expand
>           #`(let-syntax ([x (λ (stx)
>                               (syntax-property #'(void) 'prop
>                                                #,(prefab-box #'add1)))])
>               x)
>           'expression '())
>        #:literals [let-values]
>        [(let-values () (let-values () x))
>         (prefab-box-val (syntax-property #'x 'prop))]))
>
> The result of this is not the syntax #'add1, but the symbol 'add1.

Consider that (eval (list 'quote #'+)) also evaluates to a symbol, for 
about the same reason.

Try local-expanding either of these terms instead:

   #`(let-syntax ([x (λ (stx)
                       (syntax-property #'(void) 'prop
                                        (prefab-box #'add1)))])
       x)

or

   #`(let-syntax ([x (λ (stx)
                        #'#,(syntax-property #'(void) 'prop
                                             (prefab-box #'add1)))])
        x)

and the box will have an identifier in it.

Ryan

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