On Jan 12, 6:49 pm, Rock <rocco.ro...@gmail.com> wrote:
> After a fair amount of macro writing in CL, I think I got the
> backquote expansion mechanism pretty much nailed down (after all, it
> is quite clear in the HyperSpec). Now, I thought I understood the
> Clojure algorithm just as well. It looked very similar (concat instead
> of append and so on). Apart from the namespace resolution business,
> and a few other irrelevant details, the essence looked the same.
>
> But I did the following just for fun, and I got a surprising result:
>
> > (def x `a)
> > (def a 5)
> > `(list `(list ,,x))
>
> What I theoretically was expecting was the outcome of this:
>
> (concat (list `list) (list (concat (list `concat) (list (concat (list
> `list) (list ``list))) (list (concat (list `list) (list x))))))
>
> In other words, behind the curtains, the latter was supposed to be
> evaluated. Now, as Guy Steele points out, with a k-level nested
> backquote, you are guaranteed to get the same result only after k
> evaluations. So evaluating twice the result is:
>
> ((clojure.core/list 5))
>
> I checked this in Common Lisp and the result was sure enough exactly
> the equivalent, as I expected.
>
> Yet, in clojure this is what happens (of course you see the first
> evaluation directly in this case):
>
> > `(list `(list ,,x))
>
> (clojure.core/list (clojure.core/concat (clojure.core/list (quote
> clojure.core/list)) (clojure.core/list (quote user/x))))
>
> Another evaluation gives:
> ((clojure.core/list user/x))
>
> That's ALMOST the same but not quite. See the "user/x"? It came from a
> very strange (quote user/x) above which wasn't supposed to be there. I
> mean user/x should not have been quoted.
>
> Is this a bug?
The unquote character in Clojure is ~ not comma.
user=> (eval `(list `(list ~~x)))
((clojure.core/list 5))
Rich
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