String 将字符串转换为数字并打印到命令行

我正在练习第1章的练习。对于问题2，我想使用

read

函数将字符串转换为数字，但下面的代码不起作用

main = do
    args <- getArgs
    myNum <- read $ args !! 0
    putStrLn myNum

main=do
args这里有几件事不对

首先，要在此函数中保存变量，需要使用
let variable=“something”
语句类型，而不是

ex2.hs:7:12:
    No instance for (Read (IO t0)) arising from a use of ‘read’
    In the expression: read
    In a stmt of a 'do' block: one <- read $ (args !! 0)
    In the expression:
      do { args <- getArgs;
           myNum <- read $ (args !! 0);
           putStrLn myNum }

main = do
    -- get command line arguments
    args <- getArgs

    -- get the first indexed element; convert it from string to float
    let myNum = read (args !! 0) :: Float

    -- print this number to the command line (as a string)
    putStrLn (show myNum)