ircparser.hs

import Text.Parsec
import Text.Parsec.String
import Control.Monad

-- Below is the Augmented BNF description of the IRC protocol,
-- according to RFC 2812, along with parsers written using Parsec
-- in Haskell

-- Let's start with something simple
-- letter     =  %x41-5A / %x61-7A       ; A-Z / a-z
-- Letter is either A-Z or a-z
letterP :: Parser Char
letterP = oneOf $ ['A' .. 'Z'] ++ ['a' .. 'z']
-- digit      =  %x30-39                 ; 0-9
-- Digit is just 0-9
digitP :: Parser Char
digitP = oneOf ['0' .. '9']
-- command    =  1*letter / 3digit
-- A command however is one or more letter or 3 digits.
commandP :: Parser String
commandP = (try (many1 letter) -- Try the letter parser. If it fails, backtrack.
           <|> (count 3 digit)) <* eof  -- If it fails, this parser is used instead.
-- The eof is there to specify that there may not be anything left in the input
-- Output from running below:
-- λ> main
-- 363
-- PRIVMSG
-- privmsg
-- PING
-- 123
-- Could not parse:
-- 	as123123
-- Could not parse:
-- 	123123


main :: IO ()
main = do
    forM_ commands $ \cmd -> do
         let result = parse commandP cmd cmd
         case result of
           Left _ -> do
             putStrLn "Could not parse:"
             putStrLn $ "\t" ++ cmd
           Right res' ->
             putStrLn res'
    return ()
  where
    commands =
       [ "363"
       , "PRIVMSG"
       , "privmsg"
       , "PING"
       , "123"
       , "as123123" -- Bad
       , "123123" -- Bad
       ]