encoding documentation

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

{-# LANGUAGE RankNTypes, BangPatterns #-}

-- | This module uses the stream decoding functions from
--  <http://hackage.haskell.org/package/streaming-commons streaming-commons> 
--  package to define decoding functions and lenses.  The exported names
--  conflict with names in @Data.Text.Encoding@ but not with the @Prelude@ 

module Pipes.Text.Encoding
    ( 
    -- * Decoding ByteStrings and Encoding Texts
    -- ** Simple usage
    -- $usage
    
    -- ** Lens usage
    -- $lenses
  
    
    -- * Basic lens operations
    Codec
    , decode
    , eof
    -- * Decoding lenses
    , utf8
    , utf8Pure
    , utf16LE
    , utf16BE
    , utf32LE
    , utf32BE
    -- * Non-lens decoding functions 
    -- $decoders
    , decodeUtf8
    , decodeUtf8Pure
    , decodeUtf16LE
    , decodeUtf16BE
    , decodeUtf32LE
    , decodeUtf32BE
    -- * Re-encoding functions
    -- $encoders
    , encodeUtf8
    , encodeUtf16LE
    , encodeUtf16BE
    , encodeUtf32LE
    , encodeUtf32BE
    -- * Functions for latin and ascii text
    -- $ascii
    , encodeAscii
    , decodeAscii
    , encodeIso8859_1
    , decodeIso8859_1
    ) 
    where

import Data.Functor.Constant (Constant(..))
import Data.Char (ord)
import Data.ByteString as B 
import Data.ByteString (ByteString)
import Data.ByteString.Char8 as B8
import Data.Text (Text)
import qualified Data.Text as T 
import qualified Data.Text.Encoding as TE 
import qualified Data.Streaming.Text as Stream
import Data.Streaming.Text (DecodeResult(..))
import Control.Monad (join, liftM)
import Data.Word (Word8)
import Pipes



{- $usage
    Given 

>   text :: Producer Text IO ()

    we can encode it with @Data.Text.Encoding@ and ordinary pipe operations:

>   text >-> P.map TE.encodeUtf8 :: Producer.ByteString IO ()

    or, using this module, with

>   for text encodeUtf8 :: Producer.ByteString IO ()

    Given 

>   bytes :: Producer ByteString Text IO ()

    we can apply a decoding function from this module:

>   decodeUtf8 bytes :: Producer Text IO (Producer ByteString IO ())

    The Text producer ends wherever decoding first fails. Thus we can re-encode
    as uft8 as much of our byte stream as is decodeUtf16BE decodable, with, e.g.

>   for (decodeUtf16BE bytes) encodeUtf8 :: Producer ByteString IO (Producer ByteString IO ())
    
    The bytestring producer that is returned begins with where utf16BE decoding
    failed; it it didn't fail the producer is empty. 

-}

{- $lenses
    We get a bit more flexibility, though, if we use a lens like @utf8@ or @utf16BE@ 
    that looks for text in an appropriately encoded byte stream.

>   type Lens' a b = forall f . Functor f => (b -> f b) -> (a -> f a)

    is just an alias for a Prelude type.   We abbreviate this further, for our use case, as

>   type Codec
>     =  forall m r .  Monad m => Lens' (Producer ByteString m r) (Producer Text m (Producer ByteString m r))

    and call the decoding lenses @utf8@, @utf16BE@ \"codecs\", since they can 
    re-encode what they have decoded.  Thus you use any particular codec with
    the @view@ / @(^.)@ , @zoom@ and @over@ functions from the standard lens libraries;
    we presuppose neither <http://hackage.haskell.org/package/lens lens> 
    nor  <http://hackage.haskell.org/package/lens-family lens-family> 
    since we already have access to the types they require.             

    Each decoding lens looks into a byte stream that is supposed to contain text.
    The particular lenses are named in accordance with the expected 
    encoding, 'utf8', 'utf16LE' etc. To turn a such a lens or @Codec@ 
    into an ordinary function, use @view@ / @(^.)@ -- here also called 'decode':

>   view utf8 :: Producer ByteString m r -> Producer Text m (Producer ByteString m r)
>   decode utf8 Byte.stdin :: Producer Text IO (Producer ByteString IO r)
>   Bytes.stdin ^. utf8 ::  Producer Text IO (Producer ByteString IO r)

    These simple uses of a codec with @view@ or @(^.)@ or 'decode' can always be replaced by 
    the specialized decoding functions exported here, e.g. 

>   decodeUtf8 ::  Producer ByteString m r -> Producer Text m (Producer ByteString m r)
>   decodeUtf8 Byte.stdin :: Producer Text IO (Producer ByteString IO r)

    As with these functions, the stream of text that a @Codec@ \'sees\' 
    in the stream of bytes begins at its head. 
    At any point of decoding failure, the stream of text ends and reverts to (returns) 
    the original byte stream. Thus if the first bytes are already
    un-decodable, the whole ByteString producer will be returned, i.e.

>   view utf8 bad_bytestream 

    will just come to the same as 

>   return bad_bytestream

    Where there is no decoding failure, the return value of the text stream will be
    an empty byte stream followed by its own return value.  In all cases you must
    deal with the fact that it is a /ByteString producer/ that is returned, even if
    it can be thrown away with @Control.Monad.void@

>   void (Bytes.stdin ^. utf8) :: Producer Text IO ()

    The @eof@ lens permits you to pattern match: if there is a Right value,
    it is the leftover bytestring producer, if there is a Right value, it 
    is the return value of the original bytestring producer:

>   Bytes.stdin ^. utf8 . eof :: Producer Text IO (Either (Producer ByteString IO IO) ())
    
    Thus for the stream of un-decodable bytes mentioned above,

>   view (utf8 . eof) bad_bytestream

    will be the same as 

>   return (Left bad_bytestream)

    @zoom@ converts a Text parser into a ByteString parser:

>   zoom utf8 drawChar :: Monad m => StateT (Producer ByteString m r) m (Maybe Char)

    or, using the type synonymn from @Pipes.Parse@:
    
>   zoom utf8 drawChar :: Monad m => Parser ByteString m (Maybe Char)

    Thus we can define a ByteString parser (in the pipes-parse sense) like this:
    
>   charPlusByte :: Parser ByteString m (Maybe Char, Maybe Word8))) 
>   charPlusByte = do char_ <- zoom utf8 Text.drawChar
>                     byte_ <- Bytes.peekByte
>                     return (char_, byte_)

     Though @charPlusByte@ is partly defined with a Text parser 'drawChar'; 
     but it is a ByteString parser; it will return the first valid utf8-encoded 
     Char in a ByteString, whatever its byte-length, 
     and the first byte following, if both exist. Because 
     we \'draw\' one and \'peek\' at the other, the parser as a whole only 
     advances one Char's length along the bytestring, whatever that length may be.
     See the slightly more complex example \'decode.hs\' in the 
     <http://www.haskellforall.com/2014/02/pipes-parse-30-lens-based-parsing.html#batteries-included haskellforall blog> 
     discussion of this type of byte stream parsing. 
    -}

type Lens' a b = forall f . Functor f => (b -> f b) -> (a -> f a)

type Codec
    =  forall m r
    .  Monad m
    => Lens' (Producer ByteString m r)
             (Producer Text m (Producer ByteString m r))


{- | @decode@ is just the ordinary @view@ or @(^.)@ of the lens libraries;
   exported here under a name appropriate to the material. Thus

>    decode utf8 bytes :: Producer Text IO (Producer ByteString IO ())

    All of these are thus the same:

>    decode utf8 bytes = view utf8 bytes = bytes ^. utf8 = decodeUtf8 bytes


-}

decode :: ((b -> Constant b b) -> (a -> Constant b a)) -> a -> b
decode codec a = getConstant (codec Constant a)

{- | @eof@ tells you explicitly when decoding stops due to bad bytes or 
    instead reaches end-of-file happily. (Without it one just makes an explicit 
    test for emptiness of the resulting bytestring production using next) Thus

>    decode (utf8 . eof) bytes :: Producer T.Text IO (Either (Producer B.ByteString IO ()) ())

    If we hit undecodable bytes, the remaining bytestring producer will be 
    returned as a Left value; in the happy case, a Right value is returned 
    with the anticipated return value for the original bytestring producer.

    Again, all of these are the same

>    decode (utf8 . eof) bytes = view (utf8 . eof) p = p^.utf8.eof

-}

eof :: Monad m => Lens' (Producer Text m (Producer ByteString m r))
                       (Producer Text m (Either (Producer ByteString m r) r))
eof k p = fmap fromEither (k (toEither p)) where

 fromEither = liftM (either id return)

 toEither pp = do p <- pp
                  check p

 check p = do e <- lift (next p)
              case e of 
                Left r -> return (Right r)
                Right (bs,pb) ->  if B.null bs 
                                    then check pb
                                    else return (Left (do yield bs
                                                          pb))

utf8 :: Codec
utf8 = mkCodec decodeUtf8 TE.encodeUtf8

utf8Pure :: Codec
utf8Pure = mkCodec decodeUtf8Pure TE.encodeUtf8

utf16LE :: Codec
utf16LE = mkCodec decodeUtf16LE TE.encodeUtf16LE

utf16BE :: Codec
utf16BE = mkCodec decodeUtf16BE TE.encodeUtf16BE

utf32LE :: Codec
utf32LE = mkCodec decodeUtf32LE TE.encodeUtf32LE

utf32BE :: Codec
utf32BE = mkCodec decodeUtf32BE TE.encodeUtf32BE

decodeStream :: Monad m 
       => (B.ByteString -> DecodeResult) 
       -> Producer ByteString m r -> Producer Text m (Producer ByteString m r)
decodeStream = loop where
  loop dec0 p = 
    do x <- lift (next p) 
       case x of Left r -> return (return r)
                 Right (chunk, p') -> case dec0 chunk of 
                    DecodeResultSuccess text dec -> do yield text
                                                       loop dec p'
                    DecodeResultFailure text bs -> do yield text 
                                                      return (do yield bs 
                                                                 p')
{-# INLINABLE decodeStream#-}


{- $decoders
   These are functions with the simple type:
   
>   decodeUtf8 :: Monad m => Producer ByteString m r -> Producer Text m (Producer ByteString m r)

   Thus in general 

>     decodeUtf8 = view utf8
>     decodeUtf16LE = view utf16LE

   and so forth, but these forms
   may be more convenient (and give better type errors!) where lenses are
   not desired.
-}


decodeUtf8 :: Monad m => Producer ByteString m r -> Producer Text m (Producer ByteString m r)
decodeUtf8 = decodeStream Stream.decodeUtf8
{-# INLINE decodeUtf8 #-}

decodeUtf8Pure :: Monad m => Producer ByteString m r -> Producer Text m (Producer ByteString m r)
decodeUtf8Pure = decodeStream Stream.decodeUtf8Pure
{-# INLINE decodeUtf8Pure #-}

decodeUtf16LE :: Monad m => Producer ByteString m r -> Producer Text m (Producer ByteString m r)
decodeUtf16LE = decodeStream Stream.decodeUtf16LE
{-# INLINE decodeUtf16LE #-}

decodeUtf16BE :: Monad m => Producer ByteString m r -> Producer Text m (Producer ByteString m r)
decodeUtf16BE = decodeStream Stream.decodeUtf16BE
{-# INLINE decodeUtf16BE #-}

decodeUtf32LE :: Monad m => Producer ByteString m r -> Producer Text m (Producer ByteString m r)
decodeUtf32LE = decodeStream Stream.decodeUtf32LE
{-# INLINE decodeUtf32LE #-}

decodeUtf32BE :: Monad m => Producer ByteString m r -> Producer Text m (Producer ByteString m r)
decodeUtf32BE = decodeStream Stream.decodeUtf32BE
{-# INLINE decodeUtf32BE #-}


{- $encoders
   These are simply defined 
   
>      encodeUtf8 = yield . TE.encodeUtf8
   
   They are intended for use with 'for'
   
>      for Text.stdin encodeUtf8 :: Producer ByteString IO ()

   which would have the effect of 
   
>      Text.stdin >-> Pipes.Prelude.map (TE.encodeUtf8)

   using the encoding functions from Data.Text.Encoding 
-}

encodeUtf8 :: Monad m => Text -> Producer' ByteString m ()
encodeUtf8 = yield . TE.encodeUtf8
encodeUtf16LE :: Monad m => Text -> Producer' ByteString m ()
encodeUtf16LE = yield . TE.encodeUtf16LE
encodeUtf16BE :: Monad m => Text -> Producer' ByteString m ()
encodeUtf16BE = yield . TE.encodeUtf16BE
encodeUtf32LE :: Monad m => Text -> Producer' ByteString m ()
encodeUtf32LE = yield . TE.encodeUtf32LE
encodeUtf32BE :: Monad m => Text -> Producer' ByteString m ()
encodeUtf32BE = yield . TE.encodeUtf32BE

mkCodec :: (forall r m . Monad m => 
           Producer ByteString m r -> Producer Text m (Producer ByteString m r ))
        -> (Text -> ByteString)
        -> Codec
mkCodec dec enc = \k p0 -> fmap (\p -> join (for p (yield . enc)))  (k (dec p0))



{- $ascii
   ascii and latin encodings only use a small number of the characters 'Text'
     recognizes; thus we cannot use the pipes @Lens@ style to work with them. 
     Rather we simply define functions each way. 
-}


-- | 'encodeAscii' reduces as much of your stream of 'Text' actually is ascii to a byte stream,
--   returning the rest of the 'Text' at the first non-ascii 'Char'

encodeAscii :: Monad m => Producer Text m r -> Producer ByteString m (Producer Text m r)
encodeAscii = go where
  go p = do e <- lift (next p)
            case e of 
              Left r -> return (return r)
              Right (chunk, p') -> 
                 if T.null chunk 
                   then go p'
                   else let (safe, unsafe)  = T.span (\c -> ord c <= 0x7F) chunk
                        in do yield (B8.pack (T.unpack safe))
                              if T.null unsafe
                                then go p'
                                else return $ do yield unsafe 
                                                 p'
                                                 
{- | Reduce as much of your stream of 'Text' actually is iso8859 or latin1 to a byte stream,
     returning the rest of the 'Text' upon hitting any non-latin 'Char'
   -}
encodeIso8859_1 :: Monad m => Producer Text m r -> Producer ByteString m (Producer Text m r)
encodeIso8859_1 = go where
  go p = do e <- lift (next p)
            case e of 
              Left r -> return (return r)
              Right (txt, p') -> 
                 if T.null txt 
                   then go p'
                   else let (safe, unsafe)  = T.span (\c -> ord c <= 0xFF) txt
                        in do yield (B8.pack (T.unpack safe))
                              if T.null unsafe
                                then go p'
                                else return $ do yield unsafe 
                                                 p'

{- | Reduce a byte stream to a corresponding stream of ascii chars, returning the
     unused 'ByteString' upon hitting an un-ascii byte.
   -}
decodeAscii :: Monad m => Producer ByteString m r -> Producer Text m (Producer ByteString m r)
decodeAscii = go where
  go p = do e <- lift (next p)
            case e of 
              Left r -> return (return r)
              Right (chunk, p') -> 
                 if B.null chunk 
                   then go p'
                   else let (safe, unsafe) = B.span (<= 0x7F) chunk
                        in do yield (T.pack (B8.unpack safe))
                              if B.null unsafe
                                then go p'
                                else return (do yield unsafe 
                                                p')

{- | Reduce a byte stream to a corresponding stream of ascii chars, returning the
     unused 'ByteString' upon hitting the rare un-latinizable byte.
     -}
decodeIso8859_1 :: Monad m => Producer ByteString m r -> Producer Text m (Producer ByteString m r)
decodeIso8859_1 = go where
  go p = do e <- lift (next p)
            case e of 
              Left r -> return (return r)
              Right (chunk, p') -> 
                 if B.null chunk 
                    then go p'
                    else do let (safe, unsafe) = B.span (<= 0xFF) chunk
                            yield (T.pack (B8.unpack safe))
                            if B.null unsafe 
                               then go p'
                               else return (do yield unsafe 
                                               p')