The macro expander merges syntax properties between the input of a macro and
the output of a macro using `cons`. That means that if there is an
identity-macro call like this:
(identity-macro x)
where both `x` and `(identity-macro x)` have the property `'prop = 5`, the
final result has the property `'prop = (cons 5 5)`.
To avoid this merging, I would like to "delete" the `'prop` property on the
`x`. According to the docs for syntax property, it returns #false if no value
is associated with the key. So it seems like the way to "delete" it is to set
it to #false.
However, this method of "deleting" doesn't get rid of the merging. Instead I
get `'prop = (cons #false 5)`. Is there a better way of deleting properties?
How do I get rid of this merging? Here's my attempt:
#lang racket
(require syntax/parse/define)
(define-simple-macro (id-macro x) x)
(define-syntax-parser get-prop
[(_ x)
#`'#,(syntax-property (local-expand #'x 'expression '()) 'prop)])
(define-syntax set-up
(λ (stx)
(syntax-property
#`(id-macro #,(syntax-property #'(void) 'prop #f))
'prop
5)))
(get-prop (set-up))
; output: (cons #f 5)
Alex Knauth
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