Re: [racket-users] help with splicing syntax class error report

[Stephen Chang]

You get the dreaded "bad syntax" because syntax-parse doesnt know
which pattern failed. There a couple of things you can do:

1) Use a commit pattern, which tells syntax-parse not to backtrack
past the commit point. You have to change the order of patterns for
this to work.

  (define-splicing-syntax-class maybe
#:datum-literals (:thing)
(pattern (~seq) #:attr n #'0)
(pattern (~seq ~! :thing n:id)))

2) Use the ~post pattern, which essentially tells syntax-parse to give
priority to a particular pattern.

  (define-splicing-syntax-class maybe
#:datum-literals (:thing)
(pattern (~post (~seq :thing n:id)))
(pattern (~seq) #:attr n #'0))

On Tue, Mar 14, 2017 at 1:13 PM, Dan Liebgold
 wrote:
> Hi -
>
> I have some legacy code syntax I'm retrofitting with syntax-parse. Is there a 
> simple way to have this type of syntax error report a better error message:
>
> http://pasterack.org/pastes/59739
>
> I'd like it to point directly to ":thung" and say it expected ":thing" or 
> nothing there if possible.
>
> Thanks,
> Dan
>
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[racket-users] help with splicing syntax class error report

[Dan Liebgold]

Hi -

I have some legacy code syntax I'm retrofitting with syntax-parse. Is there a 
simple way to have this type of syntax error report a better error message:

http://pasterack.org/pastes/59739

I'd like it to point directly to ":thung" and say it expected ":thing" or 
nothing there if possible.

Thanks,
Dan

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