capSentences.hs

{- Capitalize sentences in a string -}

-- represents work to be processed
data Work s d = Work {
    input :: Maybe d, -- Nothing indicates work completed
    output :: d,
    state :: s -- use-case specific work state
} deriving (Show)

-- processes work over f until input is Nothing
doWork _ (Work Nothing o _) = o
doWork f w@(Work (Just i) o s)=
    f w & doWork f

type StringWork = Work Bool String

capSentences' :: StringWork -> StringWork
capSentences' w@(Work Nothing _ _) = w
capSentences' w@(Work (Just "") o s) = Work Nothing (reverse o) s
capSentences' (Work (Just ('.':i')) o s) =
    Work (Just i') ('.':o) True
capSentences' (Work (Just (c:i')) o False) =
    Work (Just i') (c:o) False
capSentences' (Work (Just (c:i')) o True) =
    if isAlpha c then
        Work (Just i') (toUpper c : o) False
    else
        Work (Just i') (c:o) True
        
capSentences s = doWork capSentences' (Work (Just s) "" True)

main = do
  print $ capSentences "blah. woot ha."

capSentences :: String -> String
capSentences = result
  where
    (_finalState, result) = mapAccumL capSentences' True

capSentences' :: Bool -> Char -> (Bool, Char)
capSentences' s     '.' = (True , '.')
capSentences' False c   = (False, c  )
capSentences' True  c   =
    if isAlpha c
      then (False, toUpper c)
      else (True , c        )