Clojure: What does [_] do in a Functions Argument

问题:

I am working through the joy of clojure and am wondering what the _ syntax does in a functions argument vector.

Example:

(def available-processors
    (.availableProcessors (Runtime/getRuntime)))

(prn "available processors: " available-processors)

(def pool
    (Executors/newFixedThreadPool (+ 2 available-processors)))

(defn dothreads!
    [func & {thread-count :threads exec-count :times :or {thread-count 1 exec-count 1}}]
    (dotimes [t thread-count]
        (.submit pool #(dotimes [_ exec-count] (func)))))

What is the underscore doing in the form:

#(dotimes [_ exec-count] (func))

回答1:

I believe that underscore is used in Clojure, by convention, as a placeholder for a required but unused argument. As Keith Bennet puts it:

In Clojure, the underscore is used idiomatically to indicate that the argument it identifies is not subsequently used.

Your example is consistent with this "usage," since the first argument to dotimes, which is an indexer, is not needed, but the binding is required by the form.

回答2:

Nothing special about it, it is just a convention for naming something that you don't care about, but still it is a name and can be used like a normal name.

(defn hello [_] (+ 1 _))
(hello 10)

UPDATE

Doing this:

(defn hello [a a]  (+ a a))

doesn't produce an error and hence you can use as many _ as you want ;).

NOTE: The above isn't the case with Scheme or CL... Hmm what was the rational behind it in clojure then???

回答3:

The previous answers are good, but since I needed some extra clarification, here's my answer.

(defn blah[_] (str "the value you sent is " _)

is identical to

(defn blah[my-arg] (str "the value you sent is " my-arg)

There is no difference. _ is just a way to let the person looking at the code to know that the parameter is not intended to be used.

So for example, this is fine programatically:

(dotimes [_ 5] (println (str "I'm going to print this 5 times, and this is index # " _)))

But someone looking at the code would think you aren't planning on using the _. So it would be better to use 'n' or 'ind' or whatever, instead of _, just to be clear. If you aren't using that value, like below...

(dotimes [_ 5] (println "I'm going to print this 5 times"))

Then it makes since to bind your parameter to _, as you are indicating that you aren't using it.

And one last thing, if bindings have the same name, the last one wins. So the following will print "4last4last4last4last".

(defn will-print [_ a _ a _ a _ a] (println (str _ a _ a _ a _ a)))
(will-print 1 "a" 2 "b" 3 "c" 4 "last")

So in the println block '_' is bound to 4, and 'a' is bound to 'last'. All of the previous values sent are ignored/overwritten.