Question

试图找出比较两个具有相似值但不同类型的元素的最佳方法。

例如，如果用户为其他任何内容输入“A”，“B”或“C”和“否”，则此功能应打印“是”。

data Letter = A | B | C deriving (Read, Show)
type LetterList = [Letter]

main = do
   putStrLn "Enter a capital letter, and we'll see if it's in the Letter type!"
   myLetter <- getLine
   if myLetter `elem` myLetters -- here's the rub
       then do
           print "Yes"
       else do
           print "No" 
   where
      myLetters :: LetterList
      myLetters = [A,B,C]

Answer 1

对于您的具体示例，您可能只是尝试使用reads解析字符串：

case reads myLetter :: [(Letter, String)] of
    [(_, "")] -> putStrLn "yes"
    otherwise -> putStrLn "no"

比较Haskell中的数据类型

1 个答案: