Hmm cool. Binary does not appear in the GHC docs... Is it new? -Alex-
On Thu, 9 Dec 2004, Duncan Coutts wrote: > On Thu, 2004-12-09 at 11:04 -0500, S. Alexander Jacobson wrote: > > Or is there a better way to (de-)serialize > > FiniteMaps? > > There's a neat trick that we use in c2hs (at least the patched version > shipped with gtk2hs) to strictly read all the keys of the finite map but > read all the values lazily. This gives massive savings (from 10s of sec > to .1s of sec) since for this application the keys are small (an int or > two) and only a small number of items get looked up. > > This uses ghc's Binary (de)serialisation module. The trick is to lazy > reading/writing is to store offsets so that items can be skipped over > and later seek back to them if the value was demanded. The actual > details are already implemented in the Binary module as lazyPut/lazyGet. > > So for FiniteMap, what I do is: > > instance (Binary key, Ord key, Binary elem) => > Binary (FiniteMap key elem) where > > -- This would be the ordinary strict implementation > -- put_ bh fm = put_ bh (fmToList fm) > -- get bh = do list <- get bh > -- return (listToFM list) > > put_ bh fm = do let list = fmToList fm > put_ bh (length list) > mapM_ (\(key, val) -> do put_ bh key > lazyPut bh val) list > get bh = do len <- get bh > let getMany :: Int -> IO [(key,elem)] > getMany 0 = return [] > getMany n = do key <- get bh > val <- lazyGet bh > xs <- getMany (n-1) > return ((key,val):xs) > list <- getMany len > return (listToFM list) > > So as you can see, on deserialisation we build a perfectly ordinary > FiniteMap but all the values in the map are thunks that will on-demand > read and deserialise the value from disk. > > Actually, this is a rather nice example of using laziness. In a strict > language to do this lazy reading trick you would have to change every > user of FiniteMap. > > Of course all this relies on doing binary rather than textual > serialisation, otherwise byte offsets into the file make no sense and > you couldn't skip over stuff and come back to it later. > > Duncan > ______________________________________________________________ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com _______________________________________________ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
