clojure - How does this destructuring let form work? -

consider form:

(def v [42 "foo" 99.2 [5 12]]) 

i've read if have declare unused variables in let form should denote them _ in destructuring form:

(let [[x _ _ [y z]] v]   (+ x y z)) 

my question how assignment _ happens? since not throw exception assume 2nd _ overrides first 1 i'm not sure. how work?

this use of _ purely conventional: clojure's point of view, it's regular symbol can used name local. so, can check value of _ directly confirm understanding:

(let [[x _ _ [y z]] v]   _) ;= 99.2 

as goes on under hood, simplest way check macroexpand let form:

(macroexpand-1 '(let [[x _ _ [y z]] v] _)) 

the result of above, reformatted clarity, looks this:

(let* [vec__7 v        x (clojure.core/nth vec__7 0 nil)        _ (clojure.core/nth vec__7 1 nil)        _ (clojure.core/nth vec__7 2 nil)        vec__8 (clojure.core/nth vec__7 3 nil)        y (clojure.core/nth vec__8 0 nil)        z (clojure.core/nth vec__8 1 nil)]   _) 

so second _ shadows first one.

let* implementation detail behind let; special form directly understood compiler, let macro adds destructuring support.