Data type for only lowercase chars

Question

I'm trying to make a data type in Haskell that will only match lowercase chars, but not uppercase chars. I'm looking for something like this:

data LowerChar = [a..z]

Answer 1

由于该间隔中只有26个字符，因此可以使用例如26个值的有限类型来表示类型Data.Finite：

{-# LANGUAGE DataKinds #-}
import Data.Finite
import Data.Char

newtype Letter = Letter{ getLetterIndex :: Finite 26 }

toLowerChar :: Letter -> Char
toLowerChar = chr . (+ ord 'a') . fromIntegral . getFinite . getLetterIndex

fromLowerChar :: Char -> Maybe Letter
fromLowerChar = fmap Letter . packFinite . fromIntegral . subtract (ord 'a') . ord

当然，如果需要，您也可以将同一类型解释为大写字母。