HASKELL *** 异常：Prelude.read：没有解析（可能重复）

Question

我有这个代码

import Data.Char (isDigit)

eval :: [Int] -> IO()
eval liste = do
    putStrLn "Please enter a positive integer or an operater ( + / - / * ): "
    input <- getLine
    let
        ord = words input
        cmd = read (head ord) :: Char
        in
            if isDigit cmd then
                let nyliste = (read [cmd] :: Int) : liste in do
                print nyliste
                eval nyliste
            else if isOperator cmd then if null liste || length liste == 1 then do
                    putStrLn "Invalid input! Start by adding at least two positive integers"
                    eval liste
                else let
                    fst = head liste
                    snd = head $ tail liste
                    in case cmd of
                        '+' -> let
                            newValue = fst + snd
                            oppliste = newValue : drop 2 liste
                            in do
                                print oppliste
                                eval oppliste
                        '-' -> let
                            newValue = fst - snd
                            oppliste = newValue : drop 2 liste
                            in do
                                print oppliste
                                eval oppliste
                        '*' -> let
                            newValue = fst * snd
                            oppliste = newValue : drop 2 liste
                            in do
                                print oppliste
                                eval oppliste
                        _ -> do
                            putStrLn "Invalid input! Start by adding at least two positive integers"
                            eval liste
            else do
                putStrLn "Invalid input! Start by adding at least two positive integers"
                eval liste


isOperator :: Char -> Bool
isOperator c = c == '*' || c == '+' || c == '-'

main :: IO()
main = eval []

其中，当我尝试运行它时，它给我这个错误：

[1 of 1] Compiling Main             ( test.hs, interpreted )
Ok, one module loaded.
ghci> main
Please enter a positive integer or an operater ( + / - / * ):
1
*** Exception: Prelude.read: no parse
ghci>

我看过类似的问题，我知道错误与我使用read有关，但我不明白更多。我在这里做错了什么？

Answer 1

head ord 就是 "1"。要使 read 成为 Char，则必须改为 "'1'"。因为 String 只是一个 [Char]，你可以只取第一个元素来得到你想要的，而不用在 read 那里费心。

HASKELL *** 异常：Prelude.read：没有解析（可能重复）

HASKELL *** Exception: Prelude.read: no parse (possible duplicate)

parsing

haskell

parse-error