Question

我有这个代码，可以在我的txt中添加一个人。但在名称中，例如，我想只允许字母。和年龄一样，我只想允许数字

add = do
    putStrLn "Name:"
    name <- getLine
    putStrLn "Age:"
    age <- getLine
    let new =  (name ++ " "++ idade ++ "\n")
    appendFile "funcionarios.txt" new
    putStrLn "Success!"

Answer 1

来自Read班级来源：

class Read a where
  readsPrec :: Int -> ReadS a
  -- (...)

这个ReadS是什么？来自hoogle：

type ReadS a = String -> [(a, String)]

这允许我们编写可能失败的读取：

maybeRead :: Read a => String -> Maybe a
maybeRead str = case readsPrec 0 str of
    [(a, "")] -> Just a
--    ^   ^---- no remaining input string
--    |- output
    _         -> Nothing

然后是修改IO的简单案例。这是一个快速肮脏的例子：

main = do
  putStrLn "Enter an integer:"

  let loop = do
        str <- getLine
        maybe loop return (maybeRead str :: Maybe Int)

  num <- loop
  print $ num + 1

如何在Haskell中仅允许一种类型getLine

1 个答案: