如何可以通过reduce来定义map，filter和reverse等操作？

问题描述

在此博客条目中，JavaScript中的CSP和换能器 a>，作者指出：

My question is: How can operations like map, filter and reverse can be defined in terms of a reduce? Could you provide examples in Clojure?

解决方案

This is true, if we don't care about laziness. In Clojure, map and filter are lazy, but reduce is eager. Not only is reverse not lazy, but the standard definition uses reduce. Modulo the laziness, we can get equivalent results for the others:

user> (defn eager-map [f coll]
        (reduce (fn [acc v] (conj acc (f v)))
        []
        coll))
#'user/eager-map
user> (eager-map inc (range 10))
[1 2 3 4 5 6 7 8 9 10]

user> (defn eager-filter [f coll]
         (reduce (fn [acc v] (if (f v) (conj acc v) acc))
                 []
                 coll))
#'user/eager-filter
user> (eager-filter even? (range 10))
[0 2 4 6 8]

user> (defn eager-reverse [coll]
         (reduce conj () coll))
#'user/eager-reverse
user> (eager-reverse (range 10))
(9 8 7 6 5 4 3 2 1 0)

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