> 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
    [(_ (~or (~seq #:return val)
             (~and (~seq) (~parse val #'42))))
     #:declare val (expr/c #'(or/c list? #f))

This raises the contract violation you expected.

Alex Knauth

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