为什么Haskell不能正确排序这些IO操作？ [英] Why doesn't Haskell sequence these IO actions properly?

问题描述

我的一个朋友问我为什么要学习Haskell。为了演示哈斯克尔的力量，我编写了一个小程序，其中显示了一个素数列表：

pre $ code main = do
putStr输入要显示的素数的数量：
number< - fmap read getLine :: IO Int
print。以号码。过滤器isPrime $ [2 ..]

isPrime :: Integer - > Bool
isPrime n =不是。任何（（== 0）。mod n）$ [2..floor。 sqrt。 fromInteger $ n]

程序按预期工作，保存了一个小异常。它会在输入用户输入号码后输出提示消息，从而产生如下输出：

  12 
输入要显示的素数的数量：[2,3,5,7,11,13,17,19,23,29,31,37] 
  

为什么Haskell没有对IO操作进行正确排序？我在哪里出错？

解决方案

这看起来更像是一种缓冲而不是排序问题。你在哪个平台上？您尝试强制unbuffered输出？

  hSetBuffering标准输出NoBuffering  - 从System.IO 
  

A friend of mine asked me why was I learning Haskell. To demonstrate the power of Haskell I wrote a small program which displayed a list of prime numbers:

main = do
    putStr "Enter the number of prime numbers to display: "
    number <- fmap read getLine :: IO Int
    print . take number . filter isPrime $ [2..]

isPrime :: Integer -> Bool
isPrime n = not . any ((== 0) . mod n) $ [2..floor . sqrt . fromInteger $ n]

The program works as expected save a minor anomaly. It prints the prompt message after taking an input number from the user resulting in an output like:

12
Enter the number of prime numbers to display: [2,3,5,7,11,13,17,19,23,29,31,37]

Why is Haskell not sequencing the IO actions correctly? Where am I going wrong?

解决方案

This looks more like a buffering than a sequencing problem. What platform are you on? Have you tried forcing unbuffered output?

hSetBuffering stdout NoBuffering -- from System.IO

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