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

问题描述

我知道 cons 返回一个 seq 而 conj 返回一个集合.我也知道 conj 将项目添加"到集合的最佳末尾，而 cons 总是添加"项目到前面.这个例子说明了这两点:

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)

对于向量、地图和集合，这些差异对我来说很有意义.但是，对于列表，它们似乎完全相同.

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，或者它们真的可以互换?换种说法，有没有列表和序列不能等价使用的例子?

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以插入到集合中，而 cons 接受只有一个:

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

另一个区别在于返回值的类:

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，这些实际上不能互换；特别是clojure.lang.Cons没有实现clojure.lang.Counted，所以对它的count不再是一个常数时间操作(在这种情况下，它可能会减少到 1 + 3——1 来自第一个元素的线性遍历，3 来自 (next (cons 4 '(1 2 3)) 是一个PersistentList 因此Counted).

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).

名称背后的意图，我相信，cons 的意思是 cons(truct a seq)¹，而 conj 的意思是conj(将项目添加到集合上).由 cons 构造的 seq 以作为其第一个参数传递的元素开始，并具有作为它的 next/rest将 seq 应用到第二个参数所产生的结果分开；如上所示，整个内容属于 clojure.lang.Cons 类.相比之下，conj 总是返回一个与传递给它的集合类型大致相同的集合.(粗略地说，因为 PersistentArrayMap 一旦超过 9 个条目就会变成 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(tructs a pair)，所以 Clojure 在拥有它的 cons 函数构造一个没有传统 cdr 的 seq.cons 的一般用法表示构建某种类型的记录以将多个值保存在一起"目前在编程语言及其实现的研究中无处不在.这就是提到避免 consing"时的意思.

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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