Re: [Scheme-reports] What happened to (UNQUOTE <expression> ...)

[Andre van Tonder <andre@x>]

On Tue, 3 May 2011, Peter Bex wrote:

> On Tue, May 03, 2011 at 02:56:31PM +0900, Andrzej wrote:
>> The question is whether we want to do it at all. Note that e.g.
>> ``,,@(list) and ``,@,@(list) both expand into forms that are illegal
>> when expanded again. Perhaps that's not a big issue but is not a
>> particularly elegant design.
>
> Agreed.  This also bothered me about the design.

Note that Al* Petrofsky proposed a different solution that does not 
involve unquote lists, and that some may find more elegant.

>From his post here: 
http://www.rhinocerus.net/forum/lang-scheme/98742-quasiquote-syntax-rules-macro.html

> One way to
> make scheme compatible with common lisp would be to add the following
> to r5rs:

> When a comma at-sign and the expression that follows it are being
> replaced by the elements of the list that resulted from the
> expression's evaluation, any sequence of commas and comma at-signs
> that immediately preceeded the comma at-sign is also removed and is
> added to the front of each of the replacements.

> (let ((x '(a b c))) ``(,,x ,@,x ,,@x ,@,@x))
>   => `(,(a b c) ,@(a b c) ,a ,b ,c ,@a ,@b ,@c)

> ``(,,@'() ,@,@(list))
>  => `()

> `````(a ,(b c ,@,,@,@(list a b c)))
>  => ````(a ,(b c ,@,,@a ,@,,@b ,@,,@c))

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