Parsing input is best achieved through larger parser functions that are composed of smaller, single purpose ones.

Let's say we wished to parse the following text which represents working hours:

Monday: 0800 1600.

We could split these into two "tokens": the day name -- "Monday" -- and a time portion "0800" to "1600".

To parse a day name, we could write the following:

data Day = Day String

day :: Parser Day
day = do
  name <- takeWhile1 (/= ':')
  skipMany1 (char ':')
  skipSpace
  return $ Day name

To parse the time portion we could write:

data TimePortion = TimePortion String String

time = do
    start <- takeWhile1 isDigit
    skipSpace
    end <- takeWhile1 isDigit
    return $ TimePortion start end

Now we have two parsers for our individual parts of the text, we can combine these in a "larger" parser to read an entire day's working hours:

data WorkPeriod = WorkPeriod Day TimePortion

work = do
    d <- day
    t <- time
    return $ WorkPeriod d t
    

and then run the parser:

parseOnly work "Monday: 0800 1600"

Attoparsec makes parsing binary data trivial. Assuming these definitions:

import           Data.Attoparsec.ByteString (Parser, eitherResult, parse, take)
import           Data.Binary.Get            (getWord32le, runGet)
import           Data.ByteString            (ByteString, readFile)
import           Data.ByteString.Char8      (unpack)
import           Data.ByteString.Lazy       (fromStrict)
import           Prelude                    hiding (readFile, take)

-- The DIB section from a bitmap header
data DIB = BM | BA | CI | CP | IC | PT
           deriving (Show, Read)

type Reserved = ByteString

-- The entire bitmap header
data Header = Header DIB Int Reserved Reserved Int
              deriving (Show)

We can parse the header from a bitmap file easily. Here, we have 4 parser functions that represent the header section from a bitmap file:

Firstly, the DIB section can be read by taking the first 2 bytes

dibP :: Parser DIB
dibP = read . unpack <$> take 2

Similarly, the size of the bitmap, the reserved sections and the pixel offset can be read easily too:

sizeP :: Parser Int
sizeP = fromIntegral . runGet getWord32le . fromStrict <$> take 4

reservedP :: Parser Reserved
reservedP = take 2

addressP :: Parser Int
addressP = fromIntegral . runGet getWord32le . fromStrict <$> take 4

which can then be combined into a larger parser function for the entire header:

bitmapHeader :: Parser Header
bitmapHeader = do
    dib <- dibP
    sz <- sizeP
    reservedP
    reservedP
    offset <- addressP
    return $ Header dib sz "" "" offset