[plt-scheme] macros: incompatible ellipsis match counts

--- Jon Rafkind <workmin at ccs.neu.edu> wrote:

> I was playing around with macros and am having trouble
> understanding 
> whats going wrong here. This macro works
> 
> (define-syntax foo2
>   (syntax-rules ()
>     ((_ (z ...) f ...)
>      (begin
>        (begin z f ...) ...))))
> 
> (foo2 (1 2 3) 4 5 6)
> 
> The macro stepper says it evaluates to
> (begin (begin 1 4 5 6) (begin 2 4 5 6) (begin 3 4 5 6))
> 
> But if I put another ... after (z ...) f ... then things break.
> 
> (define-syntax foo3
>   (syntax-rules ()
>     ((_ ((z ...) f ...) ...)
>      (begin
>        (begin
>          (begin z f ...) ...)
>        ...)
>        )))

This 'syntax-rules' expression is wrong. The 'z' pattern variable has
an ellipsis-depth of 2, and the 'f' pattern variable also has an
ellipsis-depth of 2. In the template, though, you're using 'f' at a
depth of 3. Usually that kind of error is caught at compile time. You
seem to have found a bug in mzscheme's syntax-rules/syntax-case.

> 
> (foo3 ((1 2 3) 4 5 6))
> 
> syntax: incompatible ellipsis match counts for template in: ...
> 
> Somehow this works if I don't have the ... after f, only using the
> ... 
> after the inner-most begin to match all the parts.
> 
> (define-syntax foo3
>   (syntax-rules ()
>     ((_ ((z ...) f ...) ...)
>      (begin
>        (begin
>          (begin z f) ...)
>        ...))))

Here both 'z' and 'f' have ellipsis-depth 2 in both the pattern and
the template. So this is okay. This macro basically zips the z-list
and the f-list together...

> 
> Which the macro stepper says is
> (begin (begin (begin 1 4) (begin 2 5) (begin 3 6)))
> 
> Which makes sense but that only works if the z's and f's have the
> same arity. If I use
> (foo3 ((1 2 3) 4 5))
> 
> then it breaks:
> syntax: incompatible ellipsis match counts for template in: ...

... which is why it breaks if you have different numbers. The macro
is just pairing the 'f's and the 'z's together, and it complains if
they don't match up.

Ryan


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