At Thu, 1 Oct 2009 15:01:56 -0500, Casey Klein wrote:
> Can someone explain why this macro expands without error in a module
> but not in the REPL?
>
> (define-syntax (m stx)
> (syntax-case stx ()
> [(_ x)
> (with-syntax ([(y) (generate-temporaries (syntax (x)))])
> (syntax (define (y) y)))]))
>
> > (m q)
> compile: unbound identifier (and no #%top syntax transformer is bound) in: q1
Yes, it's a hopeless-top-level sort of problem. At the point where the
expander is trying to figure out where `y' comes from, there is no
suitable `y', yet, and there won't be until the `define' is actually
evaluated.
There's a special hack to solve this problem: when a top-level
`define-syntaxes' gets zero results from the RHS expression, it just
makes the identifiers exist (in a suitable way) without defining
anything. So, this works:
(define-syntax (m stx)
(syntax-case stx ()
[(_ x)
(with-syntax ([(y) (generate-temporaries (syntax (x)))])
(syntax
(begin
(define-syntaxes (y) (values))
(define (y) y))))]))
I see that the problem and workaround are not documented (at least not
where I can find it), so I'll work on the docs.
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