Haskell-Do while循环

Question

我是哈斯克尔的新手，如果有人愿意帮助我，我会很高兴的！我正在尝试让这个程序使用do-while循环。

第二个getLine命令的结果被放入变量goGlenn中，如果goGlenn不等于“start ”,那么程序将返回起点

    start = do
    loop $ do lift performAction
        putStrLn "Hello, what is your name?"
        name <- getLine
        putStrLn ("Welcome to our personality test " ++ name ++ ", inspired by the Big Five Theory.") 
        putStrLn "You will receive fifty questions in total to which you can reply with Yes or No."
        putStrLn "Whenever you feel ready to begin please write Start"
        goGlenn <- getLine
        putStrLn goGlenn
    while (goGlenn /= "start")

Answer 1

不确定，也许这个版本可以帮助您：

import Control.Monad

loop action = do
    condition <- action
    when condition (loop action)

while = return

start =
    let action = do {
        putStrLn "Hello, what is your name?";
        name <- getLine;
        putStrLn ("Welcome to our personality test " ++ name ++ ", inspired by the Big Five Theory.");
        putStrLn "You will receive fifty questions in total to which you can reply with Yes or No.";
        putStrLn "Whenever you feel ready to begin please write Start";
        goGlenn <- getLine;
        putStrLn goGlenn;
        while (goGlenn /= "start");
    }
    in loop action

（编辑）或者也可以：

start =
    loop (do {
        putStrLn "Hello, what is your name?";
        name <- getLine;
        putStrLn ("Welcome to our personality test " ++ name ++ ", inspired by the Big Five Theory.");
        putStrLn "You will receive fifty questions in total to which you can reply with Yes or No.";
        putStrLn "Whenever you feel ready to begin please write Start";
        goGlenn <- getLine;
        putStrLn goGlenn;
        while (goGlenn /= "start");
    })        

Answer 2

在Haskell中，大多数情况下，您递归地编写“循环”。

import Control.Monad

-- ....

start = do
    putStrLn "Before the loop!"
    -- we define "loop" as a recursive IO action
    let loop = do
            putStrLn "Hello, what is your name?"
            name <- getLine
            putStrLn $ "Welcome to our personality test " ++ name 
                     ++ ", inspired by the Big Five Theory."
            putStrLn "You will receive fifty questions in total to which you can reply with Yes or No."
            putStrLn "Whenever you feel ready to begin please write Start"
            goGlenn <- getLine
            putStrLn goGlenn
            -- if we did not finish, start another loop
            when (goGlenn /= "start") loop
    loop  -- start the first iteration 
    putStrLn "After the loop!"