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]
1 Answers
Since there are only 26 characters in that interval, you can represent your type as a finite type of 26 values, using e.g. 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
Of course, you can interpret the same type into uppercase letters as well if you want to.
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