How to properly add IO to attoparsec Parser?

Question

I want to do some tracing/debugging in my attoparsec parser. Here's minimal [not] working example:

import Data.Text as T
import Data.Attoparsec.Text
import Data.Attoparsec.Combinator
import Control.Applicative ((<*), (*>))

parseSentences :: Parser [T.Text]
parseSentences = many1 $ takeWhile1 (/= '.') <* char '.' <* skipSpace

parser :: Parser [T.Text] 
parser = do
    stuff <- parseSentences
--    putStrLn $ "Got stuff: " ++ show stuff

    tail <- takeText
--    putStrLn $ "Got tail: " ++ show tail

    return $ stuff ++ [tail, T.pack "more stuff"]

main = do
    let input = T.pack "sample. example. bang"
    print $ parseOnly parser input

What i have to do in order to use IO actions in my parser?

Answer 1

If you had used the Parsec library, you would have had the possibility of using the Parsec monad transformer for mixing IO and parser commands in your code.

Attoparsec, however, is a pure parser, so you will have to use the Debug.Trace.trace function to output messages to the terminal for debugging purposes.

parser = do
  stuff <- parseSentences
  tail <- takeText
  return .
    trace ("Got stuff: " + show stuff) .
    trace ("Got tail: "  + show tail) $
    stuff ++ [tail, T.pack "more stuff"]

The messages will be printed when the associated value (here the result of the expression stuff ++ ...) is evaluated.