syntax-parse ellipsis question

[Jonathan Simpson]

Given this macro that I'm experimenting with:

(syntax-parse #'(1 2 2 a 2 2 b 2 c) [(1 (~seq n:nat ...+ x) ...) #'((n ... x) ...)])

How would I change it so that it returns #'(2 2 a 2 2 b 2 c) instead of #'((2 2 a) (2 2 b) (2 c)) ?

I don't want the parens around the individual sequences that I'm matching, but I haven't figured out a way to do this without using a pattern that forces me to add them in the body of the syntax-parse clause. How can I group the n and x pattern variables in the body without adding parens around them?

If could get a syntax class that replaces the (~seq n:nat ...+ x) that might work but I haven't been able to get a syntax class that will do that either.

Thanks,

Jonathan

[Jens Axel Søgaard]

Two options:

(syntax-parse #'(1 2 2 a 2 2 b 2 c)

  [(1 (~seq n:nat ...+) x ...) #'(n ... x ...)])

(syntax-parse #'(1 2 2 a 2 2 b 2 c)

  [(1 n:nat ...+ x ...) #'(n ... x ...)])

/Jens Axel

[Philip McGrath]

Use `~@` (sine Racket 7.0):

(syntax-parse #'(1 2 2 a 2 2 b 2 c)
  [(1 (~seq n:nat ...+ x) ...)

   #'((~@ n ... x) ...)])

-Philip

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[Ryan Culpepper]

On 9/21/19 10:15 PM, Jonathan Simpson wrote:
> Given this macro that I'm experimenting with:
>
> (syntax-parse #'(1 2 2 a 2 2 b 2 c) [(1 (~seq n:nat ...+ x) ...) #'((n
> ... x) ...)])
>
> How would I change it so that it returns #'(2 2 a 2 2 b 2 c) instead of
> #'((2 2 a) (2 2 b) (2 c)) ?
>
> I don't want the parens around the individual sequences that I'm
> matching, but I haven't figured out a way to do this without using a
> pattern that forces me to add them in the body of the syntax-parse
> clause. How can I group the n and x pattern variables in the body
> without adding parens around them?

Use `~@` in the template:

#'((~@ n ... x) ...)

Ryan

[Jonathan Simpson]

Thanks. I'm currently running 6.12, so this may be what convinces me to go ahead and upgrade.

-- Jonathan