How to replace double tabs with single tabs using pipes?

Question

I need to replace all consecutive tabs in a bytestring with single tabs, like so:

"___\t___\t\t___\t\t\t___"

becomes

"___\t___\t___\t___"

I have no idea how to do it.

After half an hour of figuring stuff out I managed to replace the first occurrence of double tabs, like so (and even this is actually wrong – it adds a tab to an empty string):

import qualified Pipes.ByteString as PB
import qualified Data.ByteString as B

removeConsecutiveTabs =
  PB.break (== tab) . mapped %~ \p -> do
    yield (B.singleton tab)
    PB.dropWhile (== tab) p

However, I still don't know how to replace all occurrences of consecutive tabs.

Answer 1

Try this:

{-# LANGUAGE OverloadedStrings #-}

import Pipes
import qualified Pipes.Prelude as P
import qualified Pipes.ByteString as PB
import           Data.ByteString (ByteString)
import Control.Lens hiding (each)

cleanTabs p = do
  p1 <- view (PB.span (/= 9)) p
  x <- lift $ next p1
  case x of
    Left r -> return r
    Right (a, p2) -> do
      yield "\t"
      let p3 = PB.dropWhile (== 9) (yield a >> p2)
      cleanTabs p3

source :: Monad m => Producer ByteString m ()
source = each [ "this", "is\t an", "\t\texample\t", "\t.", "\t\tmiddle\t", "\there"]

example = do
  putStrLn $ "input: " ++ (show $ P.toList source)
  putStrLn $ "output:" ++ (show $ P.toList (cleanTabs source))