Playing around with the monad State. The program finds the number of sequences (defined as repetitions of a character) in a string.

state_sequences.hs

import Control.Monad.State

data Sequence = Sequence { sequenceChar     :: Char
                         , sequenceCount    :: Integer } deriving (Eq, Show)

-- Without State monad.
-- Unused.
getNextSequence :: String -> (Sequence, String)
getNextSequence (h : []) = (Sequence h 1, [])
getNextSequence (h : ss) = 
    if h == (head ss) then
        let (ns, rs)    = getNextSequence ss
            cns         = sequenceCount ns
        in (Sequence h (1 + cns), rs)
    else (Sequence h 1, ss)

nextSequence :: State String Sequence
nextSequence = do
    (h : ss) <- get
    case ss of
        [] -> do
            put ss
            return (Sequence h 1)
        otherwise -> 
            if h == (head ss) then do
                let (ns, rs) = runState nextSequence ss
                let cns = sequenceCount ns
                put rs
                return $ Sequence h (1 + cns)
            else do
                put $ ss
                return $ Sequence h 1

sequences :: String -> [Sequence]
sequences s = 
    let (seq, rest) = runState nextSequence s
    in 
        if rest == "" then return seq
        else seq : (sequences rest)

main = mapM_ (putStrLn . show) (sequences "Jefffrey")
-- Sequence {sequenceChar = 'J', sequenceCount = 1}
-- Sequence {sequenceChar = 'e', sequenceCount = 1}
-- Sequence {sequenceChar = 'f', sequenceCount = 3}
-- Sequence {sequenceChar = 'r', sequenceCount = 1}
-- Sequence {sequenceChar = 'e', sequenceCount = 1}
-- Sequence {sequenceChar = 'y', sequenceCount = 1}