Pickling a finite map (Binary + zlib) [was: [Haskell-cafe] Data.Binary poor read performance]

Felipe Lessa felipe.lessa at gmail.com
Tue Feb 24 03:24:19 EST 2009

On Tue, Feb 24, 2009 at 4:59 AM, Don Stewart <dons at galois.com> wrote:
> Looks like the Map reading/showing via association lists could do with
> further work.
> Anyone want to dig around in the Map instance? (There's also some patches for
> an alternative lazy Map serialisation, if people are keen to load maps -- happstack devs?).

>From binary-0.5:

instance (Ord k, Binary k, Binary e) => Binary (Map.Map k e) where
    put m = put (Map.size m) >> mapM_ put (Map.toAscList m)
    get   = liftM Map.fromDistinctAscList get

instance Binary a => Binary [a] where
    put l  = put (length l) >> mapM_ put l
    get    = do n <- get :: Get Int
                replicateM n get

Can't get better, I think. Now, from containers-

fromDistinctAscList :: [(k,a)] -> Map k a
fromDistinctAscList xs
  = build const (length xs) xs
    -- 1) use continutations so that we use heap space instead of stack space.
    -- 2) special case for n==5 to build bushier trees.
    build c 0 xs'  = c Tip xs'
    build c 5 xs'  = case xs' of
                            -> c (bin k4 x4 (bin k2 x2 (singleton k1
x1) (singleton k3 x3)) (singleton k5 x5)) xx
                       _ -> error "fromDistinctAscList build"
    build c n xs'  = seq nr $ build (buildR nr c) nl xs'
                     nl = n `div` 2
                     nr = n - nl - 1

    buildR n c l ((k,x):ys) = build (buildB l k x c) n ys
    buildR _ _ _ []         = error "fromDistinctAscList buildR []"
    buildB l k x c r zs     = c (bin k x l r) zs

The builds seem fine, but we spot a (length xs) on the beginning.
Maybe this is the culprit? We already know the size of the map (it was
serialized), so it is just a matter of exporting

fromDistinctAscSizedList :: Int -> [(k, a)] -> Map k a

Too bad 'Map' is exported as an abstract data type and it's not
straighforward to test this conjecture. Any ideas?


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