On 09/23/2016 02:43 PM, 'William J. Bowman' via Racket Users wrote:
Under certain conditions, the value of a syntax property is duplicated.
I can't figure out why, or if this is a bug, and any advice would be
appreciated.
I've attached the smallest program that generates this behavior that I've
managed to find.
The conditions seem to be:
1. Write a syntax object that
- represents an application of constructor for a struct to some value.
- has an attached syntax property; suppose this property's key is 'x and
value is '1.
- does not manually use `#%app` to apply the constructor.
2. Use `local-expand` on a use of this macro.
Before using `local-expand`, the syntax object has a property 'x with value '1.
The result of `local-expand` will have a property 'x with a value of '(1 . 1).
If you manually use `#%app` to apply the constructor, or apply a procedure
instead of a constructor,
then the result of `local-expand` will have a property 'x and a value of '1.
It appears that the constructor macro (implemented by
self-ctor-transformer in racket/private/define-struct.rkt) transfers the
syntax properties from the macro use to its expansion (see the
datum->syntax in the first syntax-case clause). Then the properties get
duplicated because of the merging step described here:
http://docs.racket-lang.org/reference/stxprops.html.
Perhaps that datum->syntax shouldn't copy the properties over?
Ryan
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