> On Mar 20, 2017, at 11:04 AM, Philip McGrath <phi...@philipmcgrath.com> wrote:
>
> 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))
> #'val.c]))
> (example)
> 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?
This is because the `val` in the #:defaults is treated as just an attribute
name, not a variable pattern, and syntax-classes like `expr/c` can only apply
to variable patterns. One way to work around this is to use ~or and ~parse to
put it in a pattern position:
#lang racket
(require (for-syntax syntax/parse))
(define-syntax example
(syntax-parser
[(_ (~or (~seq #:return val)
(~and (~seq) (~parse val #'42))))
#:declare val (expr/c #'(or/c list? #f))
#'val.c]))
(example)
This raises the contract violation you expected.
Alex Knauth
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