小编dan*_*131的帖子

此代码根据要求中的约束检查给定密码是否有效

import Data.Char

strong :: String -> Bool
strong password = all ($ password) requirements
  where requirements = [minLength 15, any isUpper, any isLower, any isDigit] 
        minLength n str = n <= length str  

我有一个代码片段,如下所示

displayAge :: Show a => Maybe a -> IO ()
displayAge maybeAge =
  case maybeAge of
    Nothing -> putStrLn "Invalid year"
    Just maybeAge -> putStrLn $ show maybeAge

该函数从控制台获取其参数,并通过readMay函数传递给它,如下所示;

main = do
  putStrLn "Your birth year"
  strYear <- getLine
  let maybeAge = do
        intYear <- readMay strYear
        return (processYear intYear)
  displayAge maybeAge

你能解释一下这个displayAge函数的类型声明吗？为什么Show a用？