为什么`putStrLn getLine`不起作用？

Question

我在Haskell上完成了新手.我的Haskell脚本GHCi,

Prelude> let a = putStrLn getLine

发生这样的错误.

<interactive>:1:17:
    Couldn't match expected type `String'
           against inferred type `IO String'
    In the first argument of `putStrLn', namely `getLine'
    In the expression: putStrLn getLine
    In the definition of `a': a = putStrLn getLine
Prelude> 

为什么它不起作用,我如何打印输入的东西stdin？

Answer 1

putStrLn :: String -> IO ()
getLine :: IO String

类型不匹配. getLine是一个IO动作,并putStrLn采用一个普通的字符串.

您需要做的是绑定IOmonad中的行以将其传递给putStrLn.以下是等效的:

a = do line <- getLine
       putStrLn line

a = getLine >>= \line -> putStrLn line

a = getLine >>= putStrLn