reading and writing UTF-8 in Haskell

Posted on 2 February 2007
Tags: haskell

We were discussing on the Haskell fr what it would take to work with UTF-8. Here is an example I cobbled together. Do whatever you want; it's going in the public domain. Note that I've also posted it to the wiki, so please make corrections there if you can.

Note that I don't really know what the best practices are wrt to reading and writing UTF-8, but here's what works for me.


> module Main where

> import Control.Monad (mapM_)
> import Data.Word (Word8)
> import Foreign.Marshal.Array (allocaArray, peekArray, pokeArray)
> import System.Environment (getArgs)
> import System.IO (hFileSize, Handle, hGetBuf, hPutBuf, openBinaryFile,
> IOMode(ReadMode, WriteMode))


We're going to be using the 2002 UTF-8 implementation by Sven Moritz Hallberg. I don't know if this is the best one, but it's what darcs uses. (I do wonder though, if it's worth turning it into a little library, something like Data.Char.UTF8)


> import UTF8


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 lines, and writing it back out with the extension '.reversed'


> main :: IO ()
> main =
> do args <- getArgs
> mapM_ reverseUTF8File args

> reverseUTF8File :: FilePath -> IO ()
> reverseUTF8File f =
> do fb <- readFileBytes f
> case decode fb of
> (cs, []) -> writeFileBytes (f ++ ".reverse") $ encode $ reverseLines cs
> (_, xs) -> fail $ show xs
> where
> reverseLines = unlines . map reverse . lines


For this to work, we need to have some helper functions for reading and writing [Word8]. I don't know if this is the right way to go about it.


> readFileBytes :: FilePath -> IO [Word8]
> readFileBytes f =
> do h <- openBinaryFile f ReadMode
> hsize <- fromIntegral `fmap` hFileSize h
> hGetBytes h hsize
>
> writeFileBytes :: FilePath -> [Word8] -> IO ()
> writeFileBytes f ws =
> do h <- openBinaryFile f WriteMode
> hPutBytes h (length ws) ws

> hGetBytes :: Handle -> Int -> IO [Word8]
> hGetBytes h c = allocaArray c $ \p ->
> do c' <- hGetBuf h p c
> peekArray c' p
>
> hPutBytes :: Handle -> Int -> [Word8] -> IO ()
> hPutBytes h c ws = allocaArray c $ \p ->
> do pokeArray p ws
> hPutBuf h p c

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