小编Maf*_*afi的帖子

我想让程序跳回选项部分,不要为变量"expression"赋值

我试着做嵌套的do函数:

main = do 
...
option = do

然后我会在某些情况下只调用"选项",但它似乎没有这样工作,我找不到任何地方任何不同的解决方案.请问有人给出建议吗？

main = do
putStrLn "Enter expression:"
expression <- getLine

putStrLn "Enter a number from 1 to 5:"
putStrLn "1. Convert to Postfix"
putStrLn "2. Convert to Prefix"
putStrLn "3. Convert to Binary Expression Tree"
putStrLn "4. Substitution"
putStrLn "5. Evaluate Expression"
choice <- getLine

case (choice) of
    "1" -> putStrLn $ toPostfix expression
    "2" -> putStrLn $ toPrefix expression
    "3" -> print (parse expression)
    "4" -> do { putStrLn "Enter …