On 07/28/2010 02:39 PM, Eric Tanter wrote:
Hi,
I have a problem with nested templates in macros.
Consider:
(define-syntax FOO
(lambda (stx)
(syntax-case stx ()
((FOO x)
#'(let-syntax
((test (λ (stx)
(syntax-case stx ()
((test a)
#'(list a))))))
x)))))
(FOO (test 2))
(2)
Now, I want the `test' macro to match on a variable number of arguments so as
to be able to write, eg: (FOO (test 1 2 3 4))
(define-syntax FOO
(lambda (stx)
(syntax-case stx ()
((FOO x)
#'(let-syntax
((test (λ (stx)
(syntax-case stx ()
((test a ...)
#'(list a ...))))))
x)))))
This fails with:
syntax: no pattern variables before ellipses in template in: ...
I understand that the error comes from the fact that the '...' are treated as
part of the first macro and therefore do not correspond to any pattern
variable, but, they do correspond to a pattern variable in the generated macro
(test).
Is there a clean solution to this issue?
You have to escape ellipses like so (... ...). You're pattern would be
(syntax-case stx ()
[(test a (... ...)) #'(list a (... ...))])
Ellipses mean something in a template, and you're pattern is the entire
`let-syntax' expression. When a new `syntax-case' expression occurs, its
not that the code inside it is hidden from the outer template. The code
inside the `syntax-case' expression is treated just the same as any
other code. It might be helpful to remember that #' is just a shorthand
for (syntax ...) so that
#'(list a ...) is just (syntax list a ...) and therefore `a' needs to be
a pattern variable in the template where the ellipses is bound. Using
just one ellipses means the variable has to be bound in the outer-most
template.
Anyway, I don't think what you are doing would work anyway becuase you
are trying to unhygienically introduce the `test' macro to expressions
inside the body of `FOO'. Your choices are:
a) use `syntax-local-introduce' to unhygienically add `test'
b) make the user of `FOO' pass in the `test' name to the macro: (FOO
test (test 1 2 3)) and then the macro can bind test in such a way that
the body can use it.
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