[racket] Catching lifts and let

I think that `#%top` shouldn't be there, and I'll look into removing
it.

At Thu, 24 Jul 2014 16:32:58 -0400, Asumu Takikawa wrote:
> Hi all,
> 
> I have a question about syntax lifting and catching those lifts. Here's
> an example program that illustrates my concern:
> 
>   #lang racket
> 
>   ;; do the lifting
>   (define-syntax (m stx)
>     (syntax-local-lift-expression #'(+ 1 1)))
> 
>   ;; catch the lift, try to put definitions in a let
>   (define-syntax (catch stx)
>     (syntax-case stx ()
>       [(_ body ...)
>        (syntax-case (local-expand/capture-lifts #'(body ...) 
> (syntax-local-context) null)
>          (define-values begin)
>          [(begin (define-values (x ...) e ...) ... exp)
>           #'(let () (define-values (x ...) e ...) ... exp)])]))
> 
>   (catch (+ 1 (m)))
> 
> I'd like this program to work and produce `3` in the end, but it produces an
> unbound identifier error.
> 
> The trouble is that when a syntax lift is done, the identifier produced
> by the lift is a top-level identifier and gets wrapped with #%top. That
> means that putting the caught definitions in a `let` can't work, because
> those become local let-bindings.
> 
> In the example above, the relevant excerpt of the expansion is this:
> 
>   (letrec-values (((lifted.0) (#%app + '1 '1))) ; does not bind below
>     (#%app (#%app + '1 (#%top . lifted.0))))
> 
> Does the design of syntax lifting preclude this kind of macro
> interaction?
> 
> Cheers,
> Asumu
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