Re: [Chicken-users] Expanding ellipsis on lists of different length

[Tobia Conforto]

Elf wrote:
> Tobia Conforto wrote:
> > (define-syntax test
> >   (syntax-rules ()
> >     ((test (a ...) (b ...))
> >      (quote ((a b) ...)))))
>
> i believe that this is an error in chicken.  there are two ellipses  
> in the pattern and only one in the output spec, so in both cases it  
> should error with an invalid transformer spec notice.

I beg to differ.  This is a perfectly valid syntax-rule.  The number  
of ellipses in the pattern doesn't need to match those in the  
template, not in this way.  For example this is valid too--it does the  
opposite of test:

(define-syntax untest
  (syntax-rules ()
    ((untest (a b) ...)
     (quote ((a ...) (b ...))))))

(untest (1 10) (2 20) (3 30))  =>  ((1 2 3) (10 20 30))


> additionally, the 'a' isnt followed by an ellipsis, nor is the 'b'  
> followed by an ellipsis, which is specifically the error referred to  
> here: 'pattern variables that occur in subpatterns followed by one  
> or more instances of the identifier ... are allowed only in  
> subtemplates that are followed by as many instances of ... . they  
> are replaced in the output by all of the subforms they match in the  
> input, distributed as indicated.

I think you are reading it wrong.  Let's apply it to "test":

(define-syntax test
  (syntax-rules ()
    ((test (a ...) (b ...))
     (quote ((a b) ...)))))

Pattern variables [for example, the 'b inside (b ...)] that occur in  
subpatterns [the same 'b is also the subpattern in this case] followed  
by one or more instances of the identifier "..." [one instance] are  
allowed *only* in subtemplates [(a b) is the subtemplate] that are  
followed by as many instances of "..." [again one instance.]

Applied to "untest":

(define-syntax untest
  (syntax-rules ()
    ((untest (a b) ...)
     (quote ((a ...) (b ...))))))

Pattern variables [for example, the 'a in (a b)] that occur in  
subpatterns [(a b) is the subpattern] followed by one or more  
instances of the identifier "..." [one instance] are allowed *only* in  
subtemplates [the 'a inside (a ...) is the subtemplate] that are  
followed by as many instances of "..." [again one instance.]


AFAICS, the only possible error that can happen at macro expansion  
time, related to the ellipsis, is to take a macro combining several  
pattern-ellipses into one template-ellipsis (such as "test" above) and  
apply it to lists of different length.  What I'm proposing is to  
always signal it as an error, not just half of the times.


Tobia