I think I found a clearer way of talking about this. This rule:
(define-syntax-rule
(test (x y ...) ...)
'(((x y) ...) ...))

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Has more than one possible meaning. But it can be clarified using explicit
subscripts. It could mean either this (which is what I expected):
(define-syntax-rule
(test (x_j y_j_i ...i) ...j)
'(((x_j y_j_i) ...i) ...j))
Or this (which is what Racket does):
(define-syntax-rule
(test (x_j y_j_i ...i) ...j)
'(((x_i y_j_i) ...i) ...j))
I'm surprised that x's subscript changed between the pattern and the
template.
Ryan, thanks for the interesting example. Made explicit, it becomes:
((x_j x_i ...i) ...j)
although figuring out how to label that was tricky and I don't know how to
do it in general, or even if it can be done for all valid syntax rules.
Matthias: the algorithm I was thinking of is the Macro-by-Example algorithm
that Ryan pointed out, which is neither of yours. Decompose the environment
at the outer ellipsis. I take back my claim that it's the most obvious
algorithm, though.
On Tuesday, March 27, 2018 at 6:05:12 PM UTC-4, Ryan Culpepper wrote:
>
> On 03/27/2018 11:46 PM, Ryan Culpepper wrote:
> > On 03/27/2018 10:01 PM, Justin Pombrio wrote:
> >> 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:
> >> [...]
>
> BTW, it looks like Macro-By-Example[1] describes the behavior that you
> expected. See the definitions in the second column of page 4.
>
> Ryan
>
> [1] "Macro-By-Example: Deriving Syntactic Transformations from their
> Specifications" by Eugene Kohlbecker and Mitchell Wand.
> ftp://www.cs.indiana.edu/pub/techreports/TR206.pdf
>
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