{-# LANGUAGE OverloadedStrings #-}
module Main where

-- library: text, http-conduit, xml-conduit
import           Control.Applicative
import           Control.Arrow        ((&&&))
import           Control.Exception
import           Control.Monad
import qualified Data.Text            as T
import qualified Data.Text.IO         as TIO
import           Network.HTTP.Conduit (simpleHttp)
import           System.Environment   (getArgs)
import qualified Text.XML             as XML
import           Text.XML.Cursor      (($//), (&/))
import qualified Text.XML.Cursor      as XML

commentsUrl :: String
commentsUrl = "https://www.reddit.com/r/programming_jp/comments/.rss"

main :: IO ()
main = do
    args <- map T.pack <$> getArgs
    case args of
        (target:_) -> do
            items <- getItems commentsUrl `catch` (\(SomeException e) -> print e >> return [])
            forM_ items $ \(t, d) -> when (any (T.isInfixOf target) [t, d]) $ TIO.putStr $ T.unlines [t, d, "---"]
        _ -> putStrLn "検索語句が指定されていません"

type Item = (T.Text, T.Text)

getItems :: String -> IO [Item]
getItems url = either throwIO (return . constructItems) . XML.parseLBS XML.def =<< simpleHttp url

constructItems :: XML.Document -> [Item]
constructItems doc = map (content "title" &&& content "description") (XML.fromDocument doc $// XML.element "item")
  where
    content name c = T.concat $ c $// XML.element name &/ XML.content

Error occurred
ERROR "t.hs" - Can't find imported module "Text.XML.Cursor"