Clojure：缺点（seq）与缺点（清单） [英] Clojure: cons (seq) vs. conj (list)

问题描述

我知道 cons 返回一个序列，而 conj 返回一个集合。我也知道 conj 将项目添加到集合的最佳末端，而 cons 总是添加该项目在最前面。此示例说明了这两点：

I know that cons returns a seq and conj returns a collection. I also know that conj "adds" the item to the optimal end of the collection, and cons always "adds" the item to the front. This example illustrates both of these points:

user=> (conj [1 2 3] 4) ; returns a collection
[1 2 3 4]
user=> (cons 4 [1 2 3]) ; returns a seq
(4 1 2 3)

对于矢量，地图并设置这些差异对我来说很有意义。但是，对于列表而言，它们似乎是相同的。

For vectors, maps, and sets these differences make sense to me. However, for lists they seem identical.

user=> (conj (list 3 2 1) 4) ; returns a list
(4 3 2 1)
user=> (cons 4 (list 3 2 1)) ; returns a seq
(4 3 2 1)

是否有使用列表的示例，其中 conj 与 cons 表现出不同的行为，还是它们真正可互换？措辞不同，是否有一个示例，其中列表和seq不能等效使用？

Are there any examples using lists where conj vs. cons exhibit different behaviors, or are they truly interchangeable? Phrased differently, is there an example where a list and a seq cannot be used equivalently?

推荐答案

一个区别是 conj 接受任意数量的参数插入集合，而 cons 只接受一个参数：

One difference is that conj accepts any number of arguments to insert into a collection, while cons takes just one:

(conj '(1 2 3) 4 5 6)
; => (6 5 4 1 2 3)

(cons 4 5 6 '(1 2 3))
; => IllegalArgumentException due to wrong arity

另一个区别是返回值的类别：

Another difference is in the class of the return value:

(class (conj '(1 2 3) 4))
; => clojure.lang.PersistentList

(class (cons 4 '(1 2 3))
; => clojure.lang.Cons

请注意，它们并不是真正可以互换的；特别是 clojure.lang.Cons 不能实现 clojure.lang.Counted ，因此对其上的 count 不再是固定时间的操作（在这种情况下，减少到1 + 3-1来自第一个元素的线性遍历，3来自（next（cons 4'（1 2 3））是 PersistentList ，因此已计数）。

Note that these are not really interchangeable; in particular, clojure.lang.Cons does not implement clojure.lang.Counted, so a count on it is no longer a constant time operation (in this case it would probably reduce to 1 + 3 -- the 1 comes from linear traversal over the first element, the 3 comes from (next (cons 4 '(1 2 3)) being a PersistentList and thus Counted).

我相信，名字是 cons 的意思是构造（构造一个序列） ¹ ，而 conj 的意思是conj（将一个项目放入集合中）。 cons 正在构建的 seq 以元素作为第一个参数传递，并具有 next / rest 部分作为应用<$ c的结果$ c>序列到第二个参数；如上所示，整个内容属于 clojure.lang.Cons 类。相反， conj 总是返回与传递给它的集合大致相同类型的集合。 （大致来说，因为 PersistentArrayMap 一旦增长到超过9个条目，就会被转换为 PersistentHashMap 。）

The intention behind the names is, I believe, that cons means to cons(truct a seq)¹, whereas conj means to conj(oin an item onto a collection). The seq being constructed by cons starts with the element passed as its first argument and has as its next / rest part the thing resulting from the application of seq to the second argument; as displayed above, the whole thing is of class clojure.lang.Cons. In contrast, conj always returns a collection of roughly the same type as the collection passed to it. (Roughly, because a PersistentArrayMap will be turned into a PersistentHashMap as soon as it grows beyond 9 entries.)

¹ 传统上，在Lisp世界中， cons cons（构成一对），因此Clojure偏离了Lisp的传统，因为它的 cons 函数构造了一个没有传统的 cdr 。在编程语言及其实现的研究中， cons 的普遍用法是指构造某种类型的记录以将多个值组合在一起。这就是提到避免打扰的意思。

¹ Traditionally, in the Lisp world, cons cons(tructs a pair), so Clojure departs from the Lisp tradition in having its cons function construct a seq which doesn't have a traditional cdr. The generalised usage of cons to mean "construct a record of some type or other to hold a number of values together" is currently ubiquitous in the study of programming languages and their implementation; that's what's meant when "avoiding consing" is mentioned.

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