Using expr/c to attach a contract to a macro sub-pattern doesn't seem to
work with ~optional, even when the attribute is bound with #:defaults.
For example, this program:
> #lang racket
(require (for-syntax syntax/parse))
> (define-syntax (example stx)
> (syntax-parse stx
> [(_ (~optional (~seq #:return val)
> #:defaults ([val #'42])))
> #:declare val (expr/c #'(or/c list? #f))
reports the following error:
> val.c: bad attribute value for syntax template
> attribute value: #f
> expected for attribute: syntax
> sub-value: #f
> expected for sub-value: syntax in: val.c
Is there a better way to do this?
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