Personal tools

Binary IO

From HaskellWiki

Revision as of 14:49, 3 November 2006 by KetilMalde (Talk | contribs)

Jump to: navigation, search

There are a number of binary I/O libraries available for Haskell.

* JeremyShaw's update of HalDaume's NewBinary package (Cabalized): http://www.n-heptane.com/nhlab/repos/NewBinary
* Data.ByteString (Cabalised) also provides byte level operations, and is used in some applications for binary IO
* SerTH, the TH version (sort of) of NewBinary.
* PeterSimons's BlockIO package (Cabalized): http://cryp.to/blockio/
* JohnGoerzen's MissingH package (Cabalized): http://quux.org/devel/missingh
* SimonMarlow's experimental NewIO package: http://www.haskell.org/~simonmar/new-io.tar.gz (documentation at http://www.haskell.org/~simonmar/io/)

If you have simple binary IO requirements, then FPS might be easiest -- you get a List-like interface to packed byte arrays (interface documented here). For more complex serialisation, NewBinary would be preferred. Lambdabot follows this rule: when serialising lists and maps, it uses Data.ByteStrings. For complex algebraic datatypes, NewBinary is used (see Plugin/Seen.hs in Lambdabot).

NewBinary is based on Binary.hs from GHC, and traces back to NHC's binary library, described in "The Bits Between The Lambdas". It is simple to use: for each value you wish to serialise/unserialise, you write an instance of Binary for its type. This can be automated with the DrIFT tool, which will derive binary for you (as is done in GHC's .hi files). You can rip out your own copy of Binary.hs from NewBinary -- just take Binary.hs and FastMutInt.lhs.

Lamdabot, hmp3 and hs-plugins are applications I've written using NewBinary, and have found it simple and effective. Here, for example, is the binary serialisation code for the hmp3 database type, an Array, and a user defined type: File. Easy!

    instance Binary a => Binary (Array Int a) where
        put_ bh arr = do
            put_ bh (bounds arr)
            mapM_ (put_ bh) (elems arr)
        get bh      = do
            ((x,y) :: (Int,Int)) <- get bh
            (els   :: [a])       <- sequence $ take (y+1) $ repeat (get bh)
            return $! listArray (x,y) els
 
    instance Binary File where
        put_ bh (File nm i) = do
            put_ bh nm
            put_ bh i
        get bh = do
            nm <- get bh
            i  <- get bh
            return (File nm i)

As an aside, SerTH lets you derive instance of a Binary-alike class automagically using TH, rather than requiring hand-written instances, or DrIFT.

So, in summary. There are a number of ways to do binary IO in Haskell, efficiently and simply. It's really not that hard at all.