Haskell implementation of text processing DSL

DSLFSM.hs

module DSLFSM where
    
import Data.Char (toLower, toUpper)

-- a DSL for basic text processing
-- and a FSM for implementing that DSL.
-- adapted to Haskell from https://www.youtube.com/watch?v=7D9GE3-o54o

data MachineState = Normal | Comment | Upper | Lower

machineCycle :: MachineState -> Char -> (Maybe Char, MachineState)

machineCycle Normal '#'  = (Nothing, Comment)
machineCycle Normal '^'  = (Nothing, Upper)
machineCycle Normal '_'  = (Nothing, Lower)
machineCycle Normal ch   = (Just ch, Normal)

machineCycle Comment '#' = (Nothing, Normal)
machineCycle Comment ch  = (Nothing, Comment)

machineCycle Upper '^'   = (Nothing, Normal)
machineCycle Upper ch    = (Just $ toUpper ch, Upper)

machineCycle Lower '_'   = (Nothing, Normal)
machineCycle Lower ch    = (Just $ toLower ch, Lower)

processText :: String -> String
processText str = outString
  where (outString, _) = foldr step ("", Normal) str

step :: Char -> (String, MachineState) -> (String, MachineState)
step ch (str, state) = (newStr, newState)
  where (maybeChar, newState) = machineCycle state ch
        newStr = appendChar maybeChar str
        appendChar Nothing str = str
        appendChar (Just c) str = c : str