How to flatten nested lists with syntax templates?

Question

I'm working through the examples in Mythical Macros and am completely stumped on result6.

The goal is to replace the ???-pattern and ???-template in the following snippet in order to make it evaluate to true.

(define result6
  (with-syntax
    ([???-pattern (syntax ((a (1 "alpha") (2 "beta"))
                           (b (3 "gamma") (4 "delta"))))])
    (syntax ???-template)))
 
(equal? (syntax->datum result6)
        '((a b) (1 2 3 4) ("alpha" "beta" "gamma" "delta")))

I think I've got the pattern correct but am lost for how to do the template and probably splicing. My best iteration:

(define result6
  (with-syntax
    ([  ((l (n text) ...) ...)
      #'((a (1 "alpha") (2 "beta"))
         (b (3 "gamma") (4 "delta")))])
    #'( (l ...)  (~@ n ...) ... ) ))

This yields:

`((a b) 1 2 3 4)

I've not gotten the text template to work with anything I try. How do you solve this? What am I missing about this?

Answer 1

Got it!

I made the pattern to parse more explicit and then fiddled my way through the template.

(define syntax-example
  #'((a (1 "alpha") (2 "beta")) (b (3 "gamma") (4 "delta"))))

(define result6
  (with-syntax
    ([ ( (l (n1 t1) (n2 t2)) ...) syntax-example])
    #'( (l ...) ((~@ n1 n2) ...) ((~@ t1 t2) ... ))))