Very clear, much appreciate!
On Jun 4, 2:55 am, Chouser <chou...@gmail.com> wrote:
> On Thu, Jun 3, 2010 at 9:38 AM, YD <ydong.pub...@gmail.com> wrote:
> > Hi,
>
> > When 'destructure' is doing a map destructuring, 'pmap' is the
> > function to use. 'pmap' will do some kind of process to the given
> > bindings using these lines of code:
> > bes (reduce
> > (fn [bes entry]
> > (reduce #(assoc %1 %2 ((val
> > entry) %2))
> > (dissoc bes (key
> > entry))
> > ((key entry) bes)))
> > (dissoc b :as :or)
> > {:keys #(keyword (str %)), :strs
> > str, :syms #(list `quote %)})
>
> > I'm confused. Since every time ((key entry) bes) evaluates to nil, the
> > inner reduce will never really do some useful thing. What's the
> > purpose of this piece of code?
>
> Yes, if ((key entry) bes) is nil then the inner reduce simply
> returns the binding map as it stands. The purpose of this code
> is to support the :keys, :strs, and :syms keys in map
> destructuring:
>
> (let [{:keys [a b]} {:a 1 :b 2}] [a b])
> ;=> [1 2]
>
> In this example, (key entry) will be :keys for one of the
> iterations of the inner reduce. When (key entry) is :keys, ((key
> entry) bes) will be [a b], and the inner reduce will process each
> item of that vector, finally returning {a :a b :b} as the whole
> binding map.
>
> Which means the above example does exactly the same things as:
>
> (let [{a :a b :b} {:a 1 :b 2}] [a b])
> ;=> [1 2]
>
> ...which is what you'd do if there was no support for :keys
>
> --Chouserhttp://joyofclojure.com/
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