如何使用管道对从文件中读取的行进行编号？

Question

我是一名Haskell初学者,试图将我的头包裹在导管库中.

我尝试过类似的东西,但它没有编译:

import Data.Conduit
import Data.Conduit.Binary as CB
import Data.ByteString.Char8 as BS

numberLine :: Monad m => Conduit BS.ByteString m BS.ByteString
numberLine = conduitState 0 push close
  where
    push lno input = return $ StateProducing (lno + 1) [BS.pack (show lno ++ BS.unpack input)]
    close state = return state

main = do
  runResourceT $ CB.sourceFile "wp.txt" $= CB.lines $= numberLine $$ CB.sinkFile "test.txt"

似乎conduitState中的状态必须与管道的输入类型具有相同的类型.或者至少这是我从错误信息中理解的:

$ ghc --make exp.hs
[1 of 1] Compiling Main             ( exp.hs, exp.o )

exp.hs:8:27:
    Could not deduce (Num [ByteString]) arising from the literal `0'
    from the context (Monad m)
      bound by the type signature for
                 numberLine :: Monad m => Conduit ByteString m ByteString
      at exp.hs:(8,1)-(11,30)
    Possible fix:
      add (Num [ByteString]) to the context of
        the type signature for
          numberLine :: Monad m => Conduit ByteString m ByteString
      or add an instance declaration for (Num [ByteString])
    In the first argument of `conduitState', namely `0'
    In the expression: conduitState 0 push close
    In an equation for `numberLine':
        numberLine
          = conduitState 0 push close
          where
              push lno input
                = return
                  $ StateProducing (lno + 1) [pack (show lno ++ unpack input)]
              close state = return state

如何使用导管完成这项工作？我想从文件中读取行并在每行附加行号.

Answer 1

是的,可以做到.我更喜欢使用辅助函数,Data.Conduit.List并且Data.ByteString.Char8尽可能避免使用.我假设你的文件是UTF-8编码的.

import Data.Conduit
import Data.Conduit.Binary as CB
import Data.Conduit.List as Cl
import Data.Conduit.Text as Ct
import Data.Monoid ((<>))
import Data.Text as T

numberLine :: Monad m => Conduit Text m Text
numberLine = Cl.concatMapAccum step 0 where
  format input lno = T.pack (show lno) <> T.pack " " <> input <> T.pack "\n"
  step input lno = (lno+1, [format input lno])

main :: IO ()
main =
  runResourceT
     $ CB.sourceFile "wp.txt"
    $$ Ct.decode Ct.utf8
    =$ Ct.lines
    =$ numberLine
    =$ Ct.encode Ct.utf8
    =$ CB.sinkFile "test.txt"