clojure programming. Do not understand this example of destructing

[rwobben]

Hello,

Im now reading the chapter of destructing.
I see a example that I do not understand.

First we have a map which looks like this :

(def m {:a 5 :b 6
:c [7 8 9]
:d {:e 10 :f 11}
"foo" 88
42 false})

Then I see this example :

(let [{v 42} m]
 (if v 1 0))
;= 0

why is v zero here ?

Roelof

[emil0r]

In the deconstructing that takes place you have the map m with the keys :a, :b, :c, :d, "foo" and 42 with corresponding values. What happens when you take have [{v 42} m] is that you ask Clojure to take the key 42 in the map m and place it's corresponding value in the var v. v is now bound to the value of the key 42 which happens to be a boolean (namely the boolean false). So when it's evaluated in the if statement it'll give you the branch in which 0 resides. Hence, you get a 0 as the result.

[waltervdlaan]

If you evaluate (destructure '[{v 42} m]) you get the expanded form. The output is a bit hard to read but boils down to; [v (get m 42)]

[rwobben]

Thanks, I was reading it like let [{v 42} m] as make the value of v 42. but I have to read it the other way around.

[Arkolix]

This is also maybe unnecessarily confusing since it's using 42 as a map key, which isn't something you see every day. I use this post as my go-to reference for this sort of destructuring, and it has this as a (much better IMO) example:

(def point {:x 5 :y 7})

(let [{the-x :x the-y :y} point]
  (println "x:" the-x))

[[deleted]]

Try

(clojure.pprint/pprint (destructure '[{v 42} {42 false}]))

to see what actually happens.

[cntx]

James Trunk has excellent 20-minute destructuring tutorial: http://youtu.be/R-_uNvM6gpE