Add CSV File Read/Write Example using incremental API

Data/Csv.hs

@@ -124,6 +124,50 @@ import Data.Csv.Types
 --
 -- In practice, the return type of 'decode' rarely needs to be given,
 -- as it can often be inferred from the context.
+-- 
+-- Demonstration of reading from a CSV file/ writing to a CSV file
+-- using the incremental API:
+--
+-- > {-#LANGUAGE DeriveGeneric#-}
+-- > {-#LANGUAGE OverloadedStrings#-}
+-- > {-#LANGUAGE BangPatterns#-}
+-- > 
+-- > import Data.ByteString (ByteString, hGetSome, empty)
+-- > import qualified Data.ByteString.Lazy as BL
+-- > import GHC.Generics
+-- > import Data.Csv.Incremental
+-- > import Data.Csv (FromRecord, ToRecord)    
+-- > import Data.Monoid ((<>), mempty)
+-- > import System.IO
+-- > import System.Exit (exitFailure)
+-- > 
+-- > data Person = Person {
+-- >       name :: ByteString,
+-- >       age :: Int
+-- >     } deriving (Show, Eq, Generic)
+-- > 
+-- > instance FromRecord Person
+-- > instance ToRecord Person
+-- > 
+-- > persons = [Person "John Doe" 19, Person "Smith" 20]
+-- > 
+-- > writeToFile :: IO ()
+-- > writeToFile = BL.writeFile "persons.csv" $ encode $ foldr (<>) mempty (map encodeRecord persons)
+-- > 
+-- > feed :: (ByteString -> Parser Person) -> Handle -> IO (Parser Person)
+-- > feed k csvFile = do
+-- >   isEof <- hIsEOF csvFile
+-- >   if isEof
+-- >   then return $ k empty
+-- >   else k `fmap` hGetSome csvFile 4096
+-- > 
+-- > readFromFile :: IO ()
+-- > readFromFile = withFile "persons.csv" ReadMode $ \csvFile -> do
+-- >                  let loop !_ (Fail _ errMsg) = putStrLn errMsg >> exitFailure
+-- >                      loop acc (Many rs k)   = loop (acc <> rs) =<< feed k csvFile
+-- >                      loop acc (Done rs)     = print (acc <> rs)
+-- >                  loop [] (decode NoHeader)
+
 
 -- $example-instance
 --