On 10/16/2012 03:54 PM, J. Ian Johnson wrote:
I'm doing a bit of macrobatics where I'm creating a series of definitions,
during which I want to also do define-for-syntax for some syntax transformers
that are used in a produced syntax-parameterize form.
In a top level call to this macro, things would be fine. However, I set up some
syntax parameters first with splicing-syntax-parameterize.
Now I get an error of the form
begin-for-syntax: not in a definition context
in: (begin-for-syntax (define-values (foo) foo-def))
context...:
backtrace
From a quick glance at the implementation, this looks like a bug in
'splicing-syntax-parameterize'. I'll fix it.
This boils down to the fact that
(let () (define-for-syntax (foo x) x) 0)
is not legal, because the internal definition context set up is only for phase
0.
How can I get around this? Is this an intended limitation?
Yes. (Ish. For now, anyway.)
There is more odd behavior to this. Consider the following interaction:
Welcome to Racket v5.3.0.24.
(let () (define-syntax foo (λ (stx) (syntax-case stx () [(_ e) #'e]))) (foo 0))
0
(let ()
(define-for-syntax foo-def (λ (stx) (syntax-case stx () [(_ e) #'e])))
(define-syntax foo foo-def)
(foo 0))
foo-def: undefined;
cannot reference undefined identifier
context...:
/home/ianj/plt/pltgit/collects/racket/private/misc.rkt:87:7
(let () (define-for-syntax foo-def (λ (stx) (syntax-case stx () [(_ e) #'e])))
0)
begin-for-syntax: not in a definition context
in: (begin-for-syntax (define-values (foo-def) (λ (stx) (syntax-case stx ()
((_ e) (syntax e))))))
context...:
/home/ianj/plt/pltgit/collects/racket/private/misc.rkt:87:7
If the define-for-syntax is just not allowed, then shouldn't I get an error the
form of the latter instead of the former?
Yes, although I'm not sure it's possible to reliably detect that
situation. (I believe that checking syntax-local-context might cause a
macro to raise an error if it's used within a splicing form, even if the
splicing form is itself at module level. But I haven't tried it.)
Ryan
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