haskell - 如何将带参数的函数传递给 Haskell 中的另一个函数？

Question

我正在制作一个简单的 Haskell 待办事项列表，我想递归调用“提示”函数并根据用户输入显示不同的菜单选项。问题在于，在初始调用时，prompt 应该期望接收一个函数作为参数之一，它本身不期望任何参数。 “prompt”的任何后续调用可能需要调用一个函数，该函数确实需要传递给传递给 prompt 的函数的参数。

这是我的代码:

mainMenuOptions :: IO ()
mainMenuOptions = do
    putStrLn ""
    putStrLn "What would you like to do?"
    putStrLn ""
    putStrLn "OPTIONS"
    putStrLn ""
    putStrLn "'+' : add items    |   '-' : remove items"

subMenuOptions ::  [String] -> String -> IO ()
subMenuOptions todos operation = do
    putStrLn ""
    if operation == "add"
        then do putStrLn ("Type in the TASK you'd like to " ++ operation ++ ", then hit ENTER")
                addListItemOptions todos
    else do
        putStrLn ("Type in the NUMBER of the task you'd like to " ++ operation ++ ", then hit ENTER")
        putStrLn "('r' : return to the main menu)"

prompt :: [String] -> IO () -> IO ()
prompt todos showOptions = do
    showTasks todos
    showOptions
    input <- getLine
    interpret input todos


interpret :: String -> [String] -> IO ()
interpret input todos
    | input == "r" = prompt todos mainMenuOptions
    | input == "+" = prompt todos subMenuOptions "add"
    | input == "-" = prompt todos subMenuOptions "remove"
    | otherwise = do
                    putStrLn ""
                    putStrLn "SORRY, did not get that! Please enter a valid option."
                    prompt todos mainMenuOptions


main :: IO ()
main = do
    putStrLn "Haskell ToDo List"
    prompt [] mainMenuOptions

我尝试执行此操作时遇到的错误是:

Couldn't match expected type ‘[Char] -> IO ()’ with actual type ‘IO ()’ • The function ‘prompt’ is applied to three arguments, but its type ‘[String] -> IO () -> IO ()’ has only two

Answer 1

在这条线上

| input == "+" = prompt todos subMenuOptions "add"

首先，prompt 只需要 2 个参数，而我们传递了三个参数，就像错误所说的那样。对于 prompt 的第二个参数，我们希望使用 subMenuOptions

将其传递给 IO ()

subMenuOptions :: [String] -> String -> IO ()

它说如果我们给它一个字符串列表和一个字符串，它就会给我们我们要找的IO()。所以:

| input == "+" = prompt todos (subMenuOptions todos "add")

我们需要括号因为

prompt todos subMenuOptions todos "add"

这意味着我们将 4 个参数传递给 prompt。