_ unused

When unused values require a name, it is common to use _.

• (fn [_ _ a])) - ignore first two function arguments
• (let [[_ a _ b] ...]) - only bind names to 2nd and 4th values of a sequence

The compiler does not specially treat _. It is a valid symbol, thus it can serve as a name for a value. It is just convention to reserve this name for values that are not to be referenced.

Multiple _'s can be used since duplicate names shadow those that come before. Thus, _ actually holds the value of its last binding.

Create a function whose first two arguments are ignored:

(fn [_ _ a]
  (println a))

Ignore the first and third value of a destructured sequence:

(let [ [_ a b _ c]
       [1 2 3 4 5] ]
  (println a b c))
;; 2 3 5

Ignore return values of debug statements in a let block:

(let [a 1
      _ (println a)
      b (+ a 2)
      _ (println b)
      c (+ b 3)]
  ...)

It is common to use _ to ignore all but the latest value of a changing atom inside an add-watch callback:

(def a (atom 1))

(add-watch a :foo
  (fn [_ _ _ s]
    (println s)))

(reset! a 2)
;; 2