Re: Backquote simplification

[Hans Aberg]

On 11 Dec 2010, at 00:12, Neil Jerram wrote:

> > [Your reply does not seem to be on the list, so I cc it.]
> >
> > Thanks. I might try an iterated cons, that is a function f such such
> > that
> >  (f x1 ... xk y) --> (cons x1 ... (cons xk y) ...))
>
> Isn't that just `list'?
>
> More generally: I've been reading your emails, but I'm afraid I have  
> no
> idea what you are trying to do.  Perhaps you could step back and  
> explain
> that overall, before continuing with details.

The reply I got was helpful, but I decided to settle for a macro  
implementation:

(use-syntax (ice-9 syncase))

(define-syntax tuple
  (syntax-rules ()
    ((tuple xs ...)
     `(tuple ,xs ...))
    ((tuple x1 x2 . y)
     (append `(tuple ,x1 ,x2) y))
    ((tuple x1 . y)
     (append `(tuple ,x1) y))
))

It behaves as I want in my context, a functional language on top of  
Guile. I decided to implement the construct (lambda (x_1 ... x_k . y)  
f) using an improper list (x_1 ... x_k . y); when it is a proper list,  
one just gets the fixed number of arguments construct.

Then the object (x_1 ... x_k . y) also become available, so it is  
possible to form
  (define f (lambda (x_1 ... x_k . y) (x_1 ... x_k . y)))
Then one would expect f(a_1, ..., a_n), for n >= k, to be (a_1, ...,  
a_n) - this a form of the identity.

So to get this effect, I need a function g that can call improper  
lists (g x_1, ..., x_k . y), where y is a list. For some reason, the  
substitution of the list y to get a list for the function g does not  
work in this situation, exception for the macro definition above, g =  
tuple.

Then, there is another problem with this macro: if having the ".", one  
cannot have "...", like would be needed say when implementing (define  
(f x_1 ... x_k . y) ...) expanding to (define f (lambda (x_1 ... x_k .  
y) ...) for any k. That is, the following isn't legal:

(define-syntax tuple
  (syntax-rules ()
    ((tuple xs ...)
     `(tuple ,xs ...))
    ((tuple xs ... . y)
     `(tuple ,xs ... . ,y))
))

But if it would have been, it would produce what I want.