What may confuse is that map destructuring swaps the positions of keys
and values as compared to map literals:
user> (let [{x :body} {:body 42}]
x)
42
It does conform to the pattern that the bound variable precedes the
value to bind in forms like let. A benefit of this ordering is that
destructuring patterns like {:keys [a b c]} are unambiguous.
Your posted example performs two levels of destructuring. It first
extracts the value associated with the key :body in the map passed to
the function as the first argument. It then extracts the head and the
tail of this value (which is therefore assumed to be a sequence) and
binds them to the local variables 'head' and 'body'.
-Per
On Fri, Apr 16, 2010 at 3:59 PM, Bytesource <stefan.rohlf...@gmail.com> wrote:
> Hi,
>
> I am currently reading "Programming Clojure" but got stuck at the
> destructuring done in the "head-overlaps-body?" function call that is
> part of the "snake" game:
>
> (defn head-overlaps-body? [{[head & body] :body}]
> (includes? body head))
>
> ;; page 200 of the pdf version
>
> I can not figure out what {[head & body] :body} actually means here.
>
> "head-overlaps-body?" is called inside "lose?" which is called with a
> "snake" object:
>
> (def lose? head-overlaps-body?)
>
> (defn create-snake []
> {:body (list [1 1])
> :dir [1 0]
> :type :snake
> :color (Color. 15 160 70)})
>
> I read the the information about the "let" binding form on clojure.org/
> special_forms but still have no clue how the above destructuring is
> done.
>
> Hope someone can give me a hint.
>
> Stefan
>
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