Clojure的懒惰如何与Java / impure代码的调用交互？

问题描述

Clojure评估不纯的代码（或调用Java）

似乎副作用+延迟序列可能导致奇怪的行为。

---

这是我们知道的导致这个问题：

Clojure有延迟序列：

  user => （取5（范围））; （范围）返回无限列表
（0 1 2 3 4）
  

  user => （def value（println 5））
 5; 5打印输出到屏幕
 user => value 
 nil; 'value'被赋值nil 
  

此外，Clojure可以调用Java对象，效果。
但是，副作用可能与延迟评估交互不佳：

  user => （def my-seq（map＃（do（println％）％）（range）））
＃'user / my-seq 
 user => （取5 my-seq）
（0 
 1 
 2 
 3 
 4 
 5 
 6 
 7 
 8 
 9 
 10 
 11 
 12 
 13 
 14 
 15 
 16 
 17 
 18 
 19 
 20 
 21 
 22 
 23 
 24 
 25 
 26 
 27 
 28 
 29 
 30 
 31 
 0 1 2 3 4）
  

所以它返回前5个元素，但打印第一个31！

我假设如果调用side-effect Java对象的方法。

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附带问题：

• 是否由程序员注意并防止此类情况？（是？）


• 除了序列，Clojure是否执行严格的评估？（是？）

解决方案

Clojure的惰性seqs块大约30项，所以小的开销进一步减少。这不是纯粹主义者的选择，而是实用的选择。请咨询Clojure的欢乐为一个普通的解决方案来实现一个元素在时间。

懒惰seqs不是一个完美的匹配不纯的函数，因为你遇到的原因。

Clojure也会严格评估，但是使用宏有些不同。 如果会自然保留评估。

We stumbled upon an issue in our code today, and couldn't answer this Clojure question:

Does Clojure evaluate impure code (or calls to Java code) strictly or lazily?

It seems that side-effects + lazy sequences can lead to strange behavior.

---

Here's what we know that led to the question:

Clojure has lazy sequences:

user=> (take 5 (range)) ; (range) returns an infinite list
(0 1 2 3 4)

And Clojure has side-effects and impure functions:

user=> (def value (println 5))
5                               ; 5 is printed out to screen
user=> value
nil                             ; 'value' is assigned nil

Also, Clojure can make calls to Java objects, which may include side-effects.However, side-effects may interact poorly with lazy evaluation:

user=> (def my-seq (map #(do (println %) %) (range)))
#'user/my-seq
user=> (take 5 my-seq)
(0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
0 1 2 3 4)

So it returned the first 5 elements, but printed the first 31!

I assume the same kinds of problems could occur if calling side-effecting methods on Java objects. This could make it really hard to reason about code and figure out what's going to happen.

---

Ancillary questions:

• Is it up to the programmer to watch out for and prevent such situations? (Yes?)
• Besides sequences, does Clojure perform strict evaluation? (Yes?)

解决方案

Clojure's lazy seqs chunk about 30 items so the little overhead is further reduced. It's not the purist's choice but a practical one. Consult "The Joy of Clojure" for an ordinary solution to realize one element at time.

Lazy seqs aren't a perfect match for impure functions for the reason you encountered.

Clojure will also evaluate strictly, but with macros things are a bit different. Builtins such as if will naturally hold evaluating.

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