I'm surprised by the behavior of using a pattern variable under one set of
ellipses in the pattern, and under two sets of ellipses in the template:
(require (for-syntax syntax/parse))
(define-syntax (test stx)
[(_ (x y ...) ...) #''(top (list (x y) ...) ...)]))
(test (a 1 2 3)
(b 4 5 6)
(c 7 8 9))
I would expect this to produce:
'(top (list (a 1) (a 2) (a 3))
(list (b 4) (b 5) (b 6))
(list (c 7) (c 8) (c 9)))
But instead, it produces:
'(top (list (a 1) (b 2) (c 3))
(list (a 4) (b 5) (c 6))
(list (a 7) (b 8) (c 9)))
I'm surprised by this for two reasons:
- What I thought was the obvious implementation for matching/substituting
with ellipses produces the top answer.
- It breaks some properties that I would expect ellipses to have, such as:
(define-syntax-rule (test (P ...)) (list Q ...))
(test (T1 T2))
(define-syntax-rule (test P P*) (list Q Q*))
(test T1 T2)
; for all patterns P and templates Q and terms T1 and T2, where P* and Q*
are a renamed version of P and Q.
Is this the expected behavior? And is there a good reason for it?
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