Hello,
I have now pushed an implementation of syntax-local-binding to
stable-2.0, with the following documentation. In the spirit of Eli's
note on Racket's syntax-local-value, it also works with identifiers that
are bound at the module level or the top level. Comments and patches
welcome.
Cheers,
Andy
-- Scheme Procedure: syntax-local-binding id
Resolve the identifer ID, a syntax object, within the current
lexical environment, and return two values, the binding type and a
binding value. The binding type is a symbol, which may be one of
the following:
`lexical'
A lexically-bound variable. The value is a unique token (in
the sense of `eq?') identifying this binding.
`macro'
A syntax transformer, either local or global. The value is
the transformer procedure.
`pattern-variable'
A pattern variable, bound via syntax-case. The value is an
opaque object, internal to the expander.
`displaced-lexical'
A lexical variable that has gone out of scope. This can
happen if a badly-written procedural macro saves a syntax
object, then attempts to introduce it in a context in which
it is unbound. The value is `#f'.
`global'
A global binding. The value is a pair, whose head is the
symbol, and whose tail is the name of the module in which to
resolve the symbol.
`other'
Some other binding, like `lambda' or other core bindings. The
value is `#f'.
This is a very low-level procedure, with limited uses. One case in
which it is useful is to build abstractions that associate
auxiliary information with macros:
(define aux-property (make-object-property))
(define-syntax-rule (with-aux aux value)
(let ((trans value))
(set! (aux-property trans) aux)
trans)))
(define-syntax retrieve-aux
(lambda (x)
(syntax-case x ()
((x id)
(call-with-values (lambda () (syntax-local-binding #'id))
(lambda (type val)
(with-syntax ((aux (datum->syntax #'here
(and (eq? type 'macro)
(aux-property
val)))))
#''aux)))))))
(define-syntax foo
(with-aux 'bar
(syntax-rules () ((_) 'foo))))
(foo)
=> foo
(retrieve-aux foo)
=> bar
`syntax-local-binding' must be called within the dynamic extent of
a syntax transformer; to call it otherwise will signal an error.
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