Haskell 如何将getLine设置为仅接收可转换为数字的字符串？ main=do putStrLn“你好，请问姓名？” 首先，这里有一个快速的想法可以帮助你： main = do putStrLn "Hello,Name please?" first <-

如何将getLine设置为仅接收可转换为数字的字符串？

main=do
putStrLn“你好，请问姓名？”
首先，这里有一个快速的想法可以帮助你：

main = do 
putStrLn "Hello,Name please?"
first <- getLine
second <- getLine
third <- getLine
if (second == "divide") then putStrLn (show (read first ::Double )/ (read third :: Double))
    else putStrLn "Cannot do"

如果用户输入一个随机字符串，会发生什么？没有考虑到这一点，可能会抛出一个错误？与其定义自己的验证器，为什么不使用
readMaybe
from
Text.Read
@雷恩里奇：是的，这样做是绝对有效的（甚至更好）。
import Control.Applicative
import Data.Char

getNum :: (Num a, Read a) => IO (Maybe a)
getNum = do
  (x, xs) <- break (not . isDigit ) <$> getLine
  case xs of
    [] -> return $ Just $ read x
    _ -> return Nothing

main :: IO ()
main = do
  x <- getNum :: IO (Maybe Double)
  case x of
    Nothing -> do
      putStrLn "Not a number, try again"
      main
    Just x' ->
      putStrLn $ "Your number: " ++ show x'
  return ()

import Control.Applicative
import Data.Char
import Data.Maybe

getNum :: (Num a, Read a) => IO (Maybe a)
getNum = do
  (x, xs) <- break (not . isDigit ) <$> getLine
  case xs of
    [] -> return $ Just $ read x
    _ -> return Nothing

main :: IO ()
main = do
  putStr "First: "
  x <- getNum :: IO (Maybe Double)

  putStr "Operation: "
  op <- getLine

  putStr "Second: "
  y <- getNum :: IO (Maybe Double)

  if isJust x && isJust y then do
    let
      x' = fromJust x
      y' = fromJust y
    putStr "Result: "
    case op of
      "divide" -> putStrLn $ show $ x' / y'
      "sum" -> putStrLn $ show $ x' + y'
      _ -> putStrLn "No operation"
  else
    putStrLn "Invalid numbers"

  return ()