On Mon, 1 Nov 2010 23:41:47 -0700 (PDT)
Meikel Brandmeyer <m...@kotka.de> wrote:
> Hi,
>
> On 2 Nov., 03:25, Mike K <mbk.li...@gmail.com> wrote:
>
> > (print-value-a [:b 7 :a 3])
> > ; actually prints nil
>
> You have to use apply. (apply print-value-a [:b 7 :a 3]).
>
> Furthermore: how could (let [{a :a} [:b 7 :a 3]] [a]) possibly work?
The same way this one works:
> (defn foo [& {:keys [a b]}] ...)
>
> is equivalent to
>
> (defn foo [& options#] (let [{:keys [a b]} (apply hash-map
> options#)] ...))
This only happens if the rest argument destructuring is a hash map -
if I use a vector or a symbol there, then the values don't get turned
into a map. Can't that same mechanism be used in the case where some
non-rest argument is a hash-map trying to destructure a sequence?
So that:
(let [{a :a} [:b 7 :a 3]] ...)
would be equivalent to
(let [x# [:b 7 :a 3]] (let [{a :a} (apply hash-map x#)] ...)
> user=> (let [{a :a} [:b 7 :a 3]] [a])
> [nil]
> user=> (let [{a 2} [:b 7 :a 3]] [a])
> [:a]
>
> The map destructuring in the defn is a special case of defn, not
> destructuring itself.
Interesting. This means you can't use your second example to
destructure a rest argument:
user=> ((fn [& {a 2}] [a]) :b 7 :a 3)
[nil]
But that's the same behavior as you got with 1.1. But this case:
user=> ((fn [& {a 2}] [a]) 1 7 2 3)
[3] ;; 1.2 behavior; 1.1 returns [nil]
Changed between 1.1 and 1.2.
As much as I hate special cases when they aren't needed, I'm not
arguing that this should change. Handling rest arguments that way is
very useful. It's not at all clear there's a use for doing this
anywhere case, other than to scratch that consistency itch.
<mike
--
Mike Meyer <m...@mired.org> http://www.mired.org/consulting.html
Independent Network/Unix/Perforce consultant, email for more information.
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