Programs to transform text strings

fullwidth.hs

import StrConv

fullWidth c
    | c >= 0x21 && c <= 0x7E = Just $ c + 0xFEE0 -- ASCII printable
    | c == 0x20 = Just 0x3000 -- ASCII space -> ideographic space
    | otherwise = Nothing

main = strConv (charify fullWidth)

regional.hs

import StrConv

regional c
    | c >= 0x41 && c <= 0x5A = Just $ c + 0x1F1A5 -- Uppercase
    | c >= 0x61 && c <= 0x7A = Just $ c + 0x1F185 -- Lowercase
    | otherwise = Nothing

main = strConv (charify regional)

StrConv.hs

module StrConv (strConv, charify) where

import System.Environment
import System.Exit
import Data.Char

convertStr convertChar = map $ \c -> case convertChar c of
    Just converted -> converted
    _ -> c

processArgs args
    | length args == 0 = putStrLn "Not enough arguments" >> return Nothing
    | length args == 1 = return (Just $ args !! 0)
    | otherwise =
        putStrLn "Warning: Only the first argument will be converted"
            >> return (Just $ args !! 0)

writeArg convertChar argResult = case argResult of
    Just arg -> writeConverted >> exitSuccess
        where writeConverted = putStrLn $ convertStr convertChar arg
    Nothing -> exitFailure

strConv :: (Char -> Maybe Char) -> IO ()
strConv convertChar = getArgs >>= processArgs >>= writeArg convertChar

charify :: (Int -> Maybe Int) -> Char -> Maybe Char
charify convert = (fmap chr) . convert . ord