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Current revision (02:22, 22 July 2008) (edit) (undo)
(question - what about other string encodings?)
 
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[[Category:Code]]
[[Category:Code]]
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A small example showing how to read and write UTF-8 in Haskell.
 
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Do whatever you want; it's going in the public domain (Eric Kow on 2007-02-02 says so, anyway)
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The simplest solution seems to be to use the [http://hackage.haskell.org/cgi-bin/hackage-scripts/package/utf8-string utf8-string package] from Galois. It
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provides a drop-in replacement for System.IO
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<haskell>
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''What about other string encodings?''
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> module Main where
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> import Control.Monad (mapM_)
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> import Data.Word (Word8)
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> import Foreign.Marshal.Array (allocaArray, peekArray, pokeArray)
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> import System.Environment (getArgs)
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> import System.IO (hFileSize, Handle, hGetBuf, hPutBuf, openBinaryFile,
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> IOMode(ReadMode, WriteMode))
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</haskell>
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We're going to be using the 2002 UTF-8 implementation by Sven Moritz Hallberg. It happens to be the one that darcs uses ( http://abridgegame.org/repos/darcs/UTF8.lhs ). Note that Pugs also has a UTF-8 library of its own, which if I believe to handle ByteStrings.
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== Example ==
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If we use a function from System.IO.UTF8, we should also hide the equivalent one from the Prelude. (Alternatively, we could import the UTF8 module qualified)
<haskell>
<haskell>
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> import UTF8
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> import System.IO.UTF8
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> import Prelude hiding (readFile, writeFile)
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> import System.Environment (getArgs)
</haskell>
</haskell>
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We perform the demonstration on a list of files, specified as command line arguments. What we want to show is that we can both read and write UTF-8, so the demonstration will be of reading a file in, reverse every one of its
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The readFile and writeFile functions are the same as before...
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lines, and writing it back out with the extension '.reversed'
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<haskell>
<haskell>
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> reverseUTF8File :: FilePath -> IO ()
> reverseUTF8File :: FilePath -> IO ()
> reverseUTF8File f =
> reverseUTF8File f =
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> do fb <- readFileBytes f
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> do c <- readFile f
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> case decode fb of
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> writeFile (f ++ ".rev") $ reverseLines c
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> (cs, []) -> writeFileBytes (f ++ ".reverse") $ encode $ reverseLines cs
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> (_, xs) -> fail $ show xs
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> where
> where
> reverseLines = unlines . map reverse . lines
> reverseLines = unlines . map reverse . lines
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</haskell>
 
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For this to work, we need to have some helper functions for reading and
 
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writing [Word8]. It would be nice is if there were some standard functions for reading and writing [Word8] in files.
 
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<haskell>
 
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> readFileBytes :: FilePath -> IO [Word8]
 
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> readFileBytes f =
 
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> do h <- openBinaryFile f ReadMode
 
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> hsize <- fromIntegral `fmap` hFileSize h
 
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> hGetBytes h hsize
 
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>
 
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> writeFileBytes :: FilePath -> [Word8] -> IO ()
 
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> writeFileBytes f ws =
 
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> do h <- openBinaryFile f WriteMode
 
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> hPutBytes h (length ws) ws
 
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> hGetBytes :: Handle -> Int -> IO [Word8]
 
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> hGetBytes h c = allocaArray c $ \p ->
 
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> do c' <- hGetBuf h p c
 
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> peekArray c' p
 
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>
 
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> hPutBytes :: Handle -> Int -> [Word8] -> IO ()
 
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> hPutBytes h c ws = allocaArray c $ \p ->
 
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> do pokeArray p ws
 
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> hPutBuf h p c
 
</haskell>
</haskell>

Current revision


The simplest solution seems to be to use the utf8-string package from Galois. It provides a drop-in replacement for System.IO

What about other string encodings?

Example

If we use a function from System.IO.UTF8, we should also hide the equivalent one from the Prelude. (Alternatively, we could import the UTF8 module qualified) 

> import System.IO.UTF8
> import Prelude hiding (readFile, writeFile)
> import System.Environment (getArgs)

The readFile and writeFile functions are the same as before... 

> main :: IO ()
> main =
>  do args <- getArgs
>     mapM_ reverseUTF8File args
 
> reverseUTF8File :: FilePath -> IO ()
> reverseUTF8File f =
>   do c <- readFile f
>      writeFile (f ++ ".rev") $ reverseLines c
>   where
>     reverseLines = unlines . map reverse . lines
Retrieved from "http://www.haskell.org/haskellwiki/UTF-8"

Category: Code