cannot import Control.Proxy.Trans.Either

Question

I'm trying to learn how to use pipes together with attoparsec by following the tutorial https://hackage.haskell.org/package/pipes-attoparsec-0.1.0.1/docs/Control-Proxy-Attoparsec-Tutorial.html . But I was not able to import Control.Proxy.Trans.Either . In which lib is this module located?

Answer 1

You hit on an old version of pipes-attoparsec corresponding to an old version of pipes. With recent versions, something like the first example would be written without a pipe. We would use the parsed function, which just applies a parser repeatedly until it fails, streaming good parses as they come.

{-# LANGUAGE OverloadedStrings #-}
import Pipes
import qualified Pipes.Prelude as P
import Pipes.Attoparsec
import Data.Attoparsec.Text
import Data.Text (Text)

data Name = Name Text deriving (Show)

hello :: Parser Name
hello = fmap Name $ "Hello " *> takeWhile1 (/='.') <* "."

helloparses :: Monad m =>  Producer Text m r -> Producer Name m (Either (ParsingError, Producer Text m r) r)
helloparses = parsed hello 

process txt = do 
  e <- runEffect $ helloparses txt >-> P.print
  case e of 
    Left (err,rest) -> print err >> runEffect (rest >-> P.print)
    Right ()        -> return ()

input1, input2 :: Monad m => Producer Text m ()
input1 = each
  [ "Hello Kate."
  , "Hello Mary.Hello Jef"
  , "f."
  , "Hel"
  , "lo Tom."
  ]
input2 = input1 >> yield "garbage"

Then we see

-- >>> process input1
-- Name "Kate"
-- Name "Mary"
-- Name "Jeff"
-- Name "Tom"

-- >>> process input2
-- Name "Kate"
-- Name "Mary"
-- Name "Jeff"
-- Name "Tom"
-- ParsingError {peContexts = [], peMessage = "string"}
-- "garbage"

The other principle function pipes-attoparsec defined is just parse. This converts an attoparsec parser into a pipes StateT parser to parse an initial segment of a producer that matches the parser. You can read about them here http://www.haskellforall.com/2014/02/pipes-parse-30-lens-based-parsing.html