哈斯克尔名单冻结 [英] Haskell list frozen

问题描述

我是Haskell的新手，并试图了解一些事情。如果我做了以下，我收到一个问题：

  list1 = [1 ..] 
 list2 = [x | x<  -  list1，x< = 4] 
 print list2 
  

code> [1,2,3,4- 。它没有末端括号，所以就好像列表正在加载或冻结一样。这是它的样子：

  Prelude> print list2 
 [1,2,3,4 
  

这是怎么回事？ / p> 你知道列表单调递增，但Haskell不知道。使用 takeWhile ，而不是列表理解，以便在找到大于4的值时可以停止评估 list1 。

 > list1 = [1 ..] 
> list2 = takeWhile（< = 4）list1 
> print list2 
 [1,2,3,4] 
  

I'm new to Haskell and playing around trying to understand a few things. If I do the following I receive a problem:

list1 = [1..]
list2 = [x | x <- list1, x <= 4]
print list2

which returns [1,2,3,4. There isn't an end bracket on it, so it is as if the list is loading or frozen. Here is how it looks:

Prelude> print list2
[1,2,3,4

What is going on here?

解决方案

You know that the list is monotonically increasing, but Haskell does not. Use takeWhile, instead of a list comprehension, so that list1 can stop being evaluated once you find a value greater than 4.

> list1 = [1..]
> list2 = takeWhile (<= 4) list1
> print list2
[1,2,3,4]

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