IO documentation wibbles

[github/fretlink/text-pipes.git] / Pipes / Text / IO.hs

{-#LANGUAGE RankNTypes#-}


module Pipes.Text.IO 
   ( 
   -- * Text IO
   -- $textio
   
   -- * Caveats
   -- $caveats
   
   -- * Producers
   fromHandle
   , stdin
   , readFile
   -- * Consumers
   , toHandle
   , stdout
   , writeFile
   ) where

import qualified System.IO as IO
import Control.Exception (throwIO, try)
import Foreign.C.Error (Errno(Errno), ePIPE)
import qualified GHC.IO.Exception as G
import Data.Text (Text)
import qualified Data.Text as T
import qualified Data.Text.IO as T
import Pipes
import qualified Pipes.Safe.Prelude as Safe
import qualified Pipes.Safe as Safe
import Pipes.Safe (MonadSafe(..), Base(..))
import Prelude hiding (readFile, writeFile)

{- $textio
    Where pipes IO replaces lazy IO, @Producer Text m r@ replaces lazy 'Text'. 
    This module exports some convenient functions for producing and consuming 
    pipes 'Text' in IO, namely, 'readFile', 'writeFile', 'fromHandle', 'toHandle', 
    'stdin' and 'stdout'.  Some caveats described below. 
    
    The main points are as in 
    <https://hackage.haskell.org/package/pipes-bytestring-1.0.0/docs/Pipes-ByteString.html Pipes.ByteString>
    
    An 'IO.Handle' can be associated with a 'Producer' or 'Consumer' according 
    as it is read or written to.
    
> import Pipes
> import qualified Pipes.Text as Text
> import qualified Pipes.Text.IO as Text
> import System.IO
>
> main =
>     withFile "inFile.txt"  ReadMode  $ \hIn  ->
>     withFile "outFile.txt" WriteMode $ \hOut ->
>     runEffect $ Text.fromHandle hIn >-> Text.toHandle hOut

To stream from files, the following is perhaps more Prelude-like (note that it uses Pipes.Safe):

> import Pipes
> import qualified Pipes.Text as Text
> import qualified Pipes.Text.IO as Text
> import Pipes.Safe
>
> main = runSafeT $ runEffect $ Text.readFile "inFile.txt" >-> Text.writeFile "outFile.txt"

    Finally, you can stream to and from 'stdin' and 'stdout' using the predefined 'stdin'
    and 'stdout' pipes, as with the following \"echo\" program:

> main = runEffect $ Text.stdin >-> Text.stdout


-}


{- $caveats

    The operations exported here are a convenience, like the similar operations in 
    @Data.Text.IO@  (or rather, @Data.Text.Lazy.IO@, since, again, @Producer Text m r@ is
    'effectful text' and something like the pipes equivalent of lazy Text.)

    * Like the functions in @Data.Text.IO@, they attempt to work with the system encoding. 
  
    * Like the functions in @Data.Text.IO@, they significantly slower than ByteString operations. Where
       you know what encoding you are working with, use @Pipes.ByteString@ and @Pipes.Text.Encoding@ instead,
       e.g. @view utf8 Bytes.stdin@ instead of @Text.stdin@
  
    * Like the functions in  @Data.Text.IO@ , they use Text exceptions, not the standard Pipes protocols. 

   Something like 
 
>  view utf8 . Bytes.fromHandle :: Handle -> Producer Text IO (Producer ByteString m ()) 

   yields a stream of Text, and follows
   standard pipes protocols by reverting to (i.e. returning) the underlying byte stream
   upon reaching any decoding error. (See especially the pipes-binary package.) 

  By contrast, something like 

> Text.fromHandle :: Handle -> Producer Text IO () 

  supplies a stream of text returning '()', which is convenient for many tasks, 
  but violates the pipes @pipes-binary@ approach to decoding errors and 
  throws an exception of the kind characteristic of the @text@ library instead.


-}

{-| Convert a 'IO.Handle' into a text stream using a text size 
    determined by the good sense of the text library. Note with the remarks 
    at the head of this module that this
    is  slower than @view utf8 (Pipes.ByteString.fromHandle h)@
    but uses the system encoding and has other nice @Data.Text.IO@ features
-}

fromHandle :: MonadIO m => IO.Handle -> Producer Text m ()
fromHandle h =  go where
      go = do txt <- liftIO (T.hGetChunk h)
              if T.null txt then return ()
                            else do yield txt
                                    go 
{-# INLINABLE fromHandle#-}

-- | Stream text from 'stdin'
stdin :: MonadIO m => Producer Text m ()
stdin = fromHandle IO.stdin
{-# INLINE stdin #-}


{-| Stream text from a file in the simple fashion of @Data.Text.IO@ 

>>> runSafeT $ runEffect $ Text.readFile "hello.hs" >-> Text.map toUpper >-> hoist lift Text.stdout
MAIN = PUTSTRLN "HELLO WORLD"
-}

readFile :: MonadSafe m => FilePath -> Producer Text m ()
readFile file = Safe.withFile file IO.ReadMode fromHandle
{-# INLINE readFile #-}


{-| Stream text to 'stdout'

    Unlike 'toHandle', 'stdout' gracefully terminates on a broken output pipe.

    Note: For best performance, it might be best just to use @(for source (liftIO . putStr))@ 
    instead of @(source >-> stdout)@ .
-}
stdout :: MonadIO m => Consumer' Text m ()
stdout = go
  where
    go = do
        txt <- await
        x  <- liftIO $ try (T.putStr txt)
        case x of
            Left (G.IOError { G.ioe_type  = G.ResourceVanished
                            , G.ioe_errno = Just ioe })
                 | Errno ioe == ePIPE
                     -> return ()
            Left  e  -> liftIO (throwIO e)
            Right () -> go
{-# INLINABLE stdout #-}


{-| Convert a text stream into a 'Handle'

    Note: again, for best performance, where possible use 
    @(for source (liftIO . hPutStr handle))@ instead of @(source >-> toHandle handle)@.
-}
toHandle :: MonadIO m => IO.Handle -> Consumer' Text m r
toHandle h = for cat (liftIO . T.hPutStr h)
{-# INLINABLE toHandle #-}

{-# RULES "p >-> toHandle h" forall p h .
        p >-> toHandle h = for p (\txt -> liftIO (T.hPutStr h txt))
  #-}


-- | Stream text into a file. Uses @pipes-safe@.
writeFile :: (MonadSafe m) => FilePath -> Consumer' Text m ()
writeFile file = Safe.withFile file IO.WriteMode toHandle
{-# INLINE writeFile #-}