在Clojure函数中使用过滤器

Question

Why

(filter even? (range 10))

gives (0 2 4 6 8)

but

((fn [x] filter even? x)
          (range 10))

gives

(0 1 2 3 4 5 6 7 8 9)

?

Answer 1

There is an issue in your anonymous function - let's write it in a more readable way:

((fn [x]
   (do filter
       even?
       x))
 (range 10))

Now I think it is obvious why it returns the whole range sequence: value of (range 10) is bound to argument x and is returned as the result from your function.

The correct version would be:

((fn [x] (filter even? x))
 (range 10))
;; => (0 2 4 6 8)