我如何解释在 clojure 中采用 fn 的 merge-with？

Question

我目前正在学习 Clojure，我完全是个初学者，如果能帮助我理解，我将不胜感激。我今天浏览了一些代码并找到了这个。

(let [timepoints (merge-with (fn [mf swt] [mf swt]) timepoint-max timepoint-sum )])

其中 mf, swt, timepoint-max and timepoint-sum 看起来像

{"Timepoint1": 3, "Timepoint2": 2}

那么上面的代码是做什么的？

我知道我们将变量 timepoints 设置为两个映射之间的某种联合（？）。但我对 fn [mf swt] [mf swt] 部分特别困惑。

Answer 1

表达式 (fn [mf swt] [mf swt]) 是一个匿名 clojure 函数，它根据传递给它的两个参数构造一个向量，例如

((fn [mf swt] [mf swt]) :a :b)
;; => [:a :b]

表达式 (merge-with (fn [mf swt] [mf swt]) timepoint-max timepoint-sum ) 是对函数 merge-with 的调用，第一个参数 (fn [mf swt] [mf swt])、第二个参数 timepoint-max 和第三个参数 timepoint-sum。这是我们将 timepoint-max 和 timepoint-sum 绑定到一些示例值的示例：

(def timepoint-max {:x 0 :y 1})
(def timepoint-sum {:y 100 :z 200})

(merge-with (fn [mf swt] [mf swt]) timepoint-max timepoint-sum)
;; => {:x 0, :y [1 100], :z 200}

阅读 merge-with 的文档以了解它的作用：

Returns a map that consists of the rest of the maps conj-ed onto the first. If a key occurs in more than one map, the mapping(s) from the latter (left-to-right) will be combined with the mapping in the result by calling (f val-in-result val-in-latter).

在上面的例子中，我们实际计算的是一样的

{:x (:x timepoint-max)
 :y ((fn [mf swt] [mf swt]) (:y timepoint-max) (:y timepoint-sum))
 :z (:z timepoint-sum)}
;; => {:x 0, :y [1 100], :z 200}

其中函数 (fn [mf swt] [mf swt]) 用于在唯一重叠的键 :y 处组合两个值。

完整的表达式 (let [timepoints (merge-with (fn [mf swt] [mf swt]) timepoint-max timepoint-sum )]) 是一种 let 形式，它将值绑定到符号，但它不是很有用，因为它的 *exprs 部分是空的，所以它总是计算为 nil:

(let [timepoints (merge-with (fn [mf swt] [mf swt]) timepoint-max timepoint-sum )])
;; => nil

为了让它评估其他东西，例如timepoints，它必须被修改成类似

的东西

(let [timepoints (merge-with (fn [mf swt] [mf swt]) timepoint-max timepoint-sum) ] 
  timepoints)
;; => {:x 0, :y [1 100], :z 200}

我如何解释在 clojure 中采用 fn 的 merge-with？

How do I interpret merge-with that takes a fn in clojure?

clojure

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