Pattern Matching in Haskell

pattern_matching.hs

module Main where

import Control.Monad.IO.Class (liftIO)
import Control.Monad.Trans.Resource (runResourceT)
import Data.Conduit (($$+-), ($=+), runConduit)
import Data.Conduit.List (mapM_, map, filter, catMaybes)
import Data.Text (unpack)
import Data.Maybe (fromJust)
import Web.Twitter.Types 
  (StreamingAPI(SStatus, SRetweetedStatus)
  , Status(Status), statusText, statusLang 
  , RetweetedStatus(RetweetedStatus), rsRetweetedStatus
  )
import Web.Twitter.Conduit.Stream (stream)


-- Filters Twitter tweets that are written only in English
filterEnglishTweets :: StreamingAPI -> Bool
filterEnglishTweets tweet = 
  let
    langIsEnglish (Status {statusLang=language}) = case language of 
      Just "en" -> True
      _ -> False
  in case tweet of 
    SStatus statusObj -> langIsEnglish statusObj
    SRetweetedStatus (RetweetedStatus {rsRetweetedStatus=statusObj}) -> 
      langIsEnglish statusObj
    _ -> False

 -- Filters Twitter tweets that are original posts
tweetParser :: StreamingAPI -> Maybe String
tweetParser tweet = case tweet of 
  SStatus (Status {statusText=status}) -> Just $ unpack status
  SRetweetedStatus (RetweetedStatus {rsRetweetedStatus=rstatus}) -> 
    Just $ unpack $ statusText rstatus
  _ -> Nothing


main :: IO ()
main = do  
    -- a bunch of connection setup details to Twitter
    {-
      Imagine a stream/production line of continually incoming Twitter tweets 
      out of this stream, non-English tweets are removed 
      each remaining tweet then gets packaged into one of two forms 
        - one for original tweets
        - one for non-original tweets (retweets and whatnot)
      We then only grab packaged forms of original tweets and display them!
    -}
    in runResourceT $ do 
      stream <- stream twitterInfo connectionManager apiRequest
      stream $=+ 
        Data.Conduit.List.filter filterEnglishTweets $=+
        Data.Conduit.List.map tweetParser $=+ 
        Data.Conduit.List.catMaybes $$+- 
        Data.Conduit.List.mapM_ (liftIO . putStrLn)