Haskell做语法和I / O.(Haskell do syntax and I/O)

 我正在玩Haskell中的一个简单程序： 
 
hello :: String -> String
hello s = "Hello, " ++ (trim s) ++ "!\n"

trim :: String -> String
trim [] = []
trim s = head $ words s

main :: IO()
main = do putStr "\nPlease enter your name: "
          name <- getLine
          hstring <- return $ hello name
          putStr hstring
 
 这是我期待的输出： 
 
Please enter your name: John Doe
Hello, John!
 
 当我将程序加载到ghci时，这可以正常工作。 但是当我使用编译程序时 
 
ghc -o hello.exe hello.hs
 
 它启动，等待输入，然后同时打印两个提示： 
 
John Doe
Please enter your name: Hello, John!
 
 为什么交互式环境和编译器之间的行为有所不同，我如何让编译器做我想做的事情？ 
 
 先谢谢您的帮助！ 

I was playing around with a simple program in Haskell:
 
hello :: String -> String
hello s = "Hello, " ++ (trim s) ++ "!\n"

trim :: String -> String
trim [] = []
trim s = head $ words s

main :: IO()
main = do putStr "\nPlease enter your name: "
          name <- getLine
          hstring <- return $ hello name
          putStr hstring
 
This is the output I am expecting:
 
Please enter your name: John Doe
Hello, John!
 
This works as expected when I load the program into ghci. However when I compile the program using
 
ghc -o hello.exe hello.hs
 
it starts, waits for input, and then prints both prompts at the same time:
 
John Doe
Please enter your name: Hello, John!
 
Why is the behavior different between the interactive environment and compiler, and how can I make the compiler do what I want?
 
Thanks in advance for the help!

原文：https://stackoverflow.com/questions/6222521