On 1/23/20 3:59 PM, Sean Kemplay wrote:
Hello,
I am exploring macros and am trying to define a variable at the top
level (with the goal in mind to dynamically define a group of functions
from a macro).
with-syntax works fine however I was just wondering if it is possible to
directly inject an identifier as syntax within syntax - something like
the following which does not work!
(define-syntax x
(lambda (x)
#`(define ,#'y "y val")))
(x)
y => y: undefined;
cannot reference an identifier before its definition
First, to escape a quasisyntax (#`) template you need to use unsyntax
(#,), not unquote (,).
Second, due to hygiene the y from the macro has an extra scope, so you
can't refer to it by typing y at the top level. But you can do this, for
example:
(define-syntax (x2 stx)
#`(begin (define #,#'y "y val") y))
Or you can write a macro that defines a y with the lexical context of
the macro use:
(define-syntax (x3 stx)
#`(define #,(datum->syntax stx 'y) "y val"))
You could also write this macro with with-syntax instead. The way that
you insert an identifier into a syntax template (quasisyntax/unsyntax vs
with-syntax) is independent of the way you create the identifier.
(Note: using the lexical context of the macro use works here, but it's
not always the right answer. Unhygienic macros are complicated.)
Ryan
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