Re: [Chicken-hackers] CR: (#439) quasiquote changes

[Peter Bex]

On Tue, Dec 07, 2010 at 12:46:29PM +0100, Peter Bex wrote:
> R5RS explicitly mentions that nested quasiquote, unquote or
> unquote-splicing calls in positions other than the head of a proper
> list which contains two elements is undefined and can lead to
> strange behaviour:
> http://www.schemers.org/Documents/Standards/R5RS/HTML/r5rs-Z-H-7.html#%_sec_4.2.6

I checked R6RS, which extends the unquote/unquote-splicing syntaxes
to allow for multiple arguments:
http://www.r6rs.org/final/html/r6rs/r6rs-Z-H-14.html#node_sec_11.17

However, this is rather non-untuitive IMO:

(let ((a 1) (b 2) (c 3))
  (quasiquote ((unquote a b c)))) => (1 2 3)

It's like unquote also sort-of splices its arguments into the containing
list.  In other words, it's identical to

(let ((a 1) (b 2) (c 3))
  (quasiquote ((unquote a) (unquote b) (unquote c)))) => (1 2 3)

I find this _very_ weird.  It feels as if they made (+ (values 1 2 3))
be identical to (+ 1 2 3) - sorry if that's a weird analogy, but it's
exactly the same to my brain.

Also, I'm unclear as to what happens when you leave off the outer
parentheses:

(let ((a 1) (b 2) (c 3))
  (quasiquote (unquote a b c))) => ; error?

If I interpret the spec correctly, it says the implementation should
throw an error in this case.  Testing with Racket confirms this.
However, (quasiquote (quasiquote (unquote a b c)))  is accepted by
Racket, but that doesn't seem correct if I understand the spec.
The quotation grammar in the spec doesn't make much sense to me though,
so maybe someone can help out in explaining it?

Quasiquote itself is not allowed to have multiple arguments,
which is also a bit inconsistant (but logical).  Racket does allow
multiple nested arguments to quasiquotes.  I haven't been able to test
with any other r6rs Scheme because Racket is the only one available in
pkgsrc and I'm not willing to mess around with building one from source.
Extra data points would be appreciated if someone else reading this
happens to have another r6 implementation at hand.

Anyway, I'd like to add:

Option 7:
Do it the R6RS way.  We'd have to flesh out what exactly happens when
you do (quasiquote (quasiquote a b c)) or similar, and what happens when
you pass multiple arguments to unquote in a non-list context.


BTW, I also checked R7 but couldn't find anything related to quotation
on their lists.

Cheers,
Peter
-- 
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