Hi !
I’ve been trying to spend some time with Haskell for long but college.. assignments … etc..were keeping me away. Finally got time + kick + excitement last night and decided to code something meaningful.
Thought to solve this challenge one more time > http://punetech.com/apply
Its quite simple. Read from input.txt [which is actually a twitter feed] > remove duplicate id entries > sort in descending order of Timestamp and save to output.txt.
Took pretty long time though. I was also reading relevant pages from RWH. had to read some things like Monads twice/thrice to understand. Then talking to folks in IRC #Haskell.. Still have to go through alot.
but it was fun. yeah! my first Haskell program that works as expected 😀
here’s the code. It still requires alot of cleaning and will do that after posting this. but this is the final v1.0 that works. So wanted to share this one only 😀
--pune.hs
-- to solve challenge program given here > http://punetech.com/apply/
import System.IO
import List
main :: IO()
main = do
-- openFile returns a Handle
inh <- openFile "pInput.txt" ReadMode
-- get all the contents from input file
inputData <- hGetContents inh
-- get file contents split into array
let pData = lines inputData
-- store all lines in a list of Tweet data type
let tweets = getTweets pData
-- Remove duplicate entries
-- nubBy function removes duplicate entries
-- first argument is a 'comparer function'
-- second argument is list on which it works
let nDup = List.nubBy tweetEq tweets
-- Sort in descending order of timestamp
-- first argument is ordering function
-- second argument is list to work on
let sDup = List.sortBy tweetOrd nDup
-- open file for writing. openFile returns a Handle
outh <- openFile "pOutput.txt" WriteMode
-- send Handle and data to write to file
writeToFile outh sDup
--close both Handles
hClose inh
hClose outh
-- Custom data type to store a tweet
data Tweet = Tweet {
tid :: Int,
name :: String,
time :: Double,
message :: String
}deriving(Eq,Show)
-- function that equates two Tweets
-- passed to nubBy
tweetEq :: Tweet -> Tweet -> Bool
tweetEq t1 t2 = (tid t1) == (tid t2)
-- function that checks order
-- passed to sortBy
tweetOrd :: Tweet -> Tweet -> Ordering
tweetOrd t1 t2 = compare (time t2) (time t1)
-- convert Tweet Lines into List of Tweets
getTweets :: [String] -> [Tweet]
getTweets [] = []
getTweets (x:xs) = let parts = split x '|' in
let temp = Tweet{
tid = read (parts !! 0),
time = read (parts !! 1),
name = (parts !! 2),
message = (parts !! 3)
}
in [temp] ++ (getTweets xs)
-- no comments :-P
split :: String -> Char -> [String]
split [] delim = [""]
split (c:cs) delim
| c == delim = "" : rest
| otherwise = (c : head rest) : tail rest
where
rest = split cs delim
-- no comments
writeToFile :: Handle -> [Tweet] -> IO()
writeToFile outh [] = hPutStrLn outh ""
writeToFile outh (x:xs) = do
let line = (show(tid x)) ++ "|" ++ (show(time x)) ++ "|" ++ (name x) ++ "|" ++ (message x)
hPutStrLn outh line
writeToFile outh xs
sweet !