[Haskell-cafe] Trying to serialize HList

Matthias Fischmann fis at wiwi.hu-berlin.de
Sun Oct 8 15:04:34 EDT 2006

... and here is the code I am giving up on for today: Serialization of
HLists.  Questions below.

{-# OPTIONS -fglasgow-exts #-}
{-# OPTIONS -fallow-undecidable-instances #-}
{-# OPTIONS -fallow-overlapping-instances #-}

module Foo where
import Char
import List
import Monad
import Permutation
import HListPrelude  -- [1] http://web.engr.oregonstate.edu/~erwig/pfp/

-- Serializable is like Show, but
--   (a) it carries explicit type information, and
--   (b) it allows for serializing IORefs etc.

class Serializable s
    serialize :: s -> IO String
    deSerialize :: String -> IO s

instance Serializable Int where
    serialize = return . ("Int::" ++) . show
    deSerialize s | isPrefixOf "Int::" s = return . (read :: String -> Int) . drop 5 $ s

instance Serializable Char where
    serialize = return . ("Char::" ++) . show
    deSerialize s | isPrefixOf "Char::" s = return . (read :: String -> Char) . drop 6 $ s

-- SList is a list of heterogenous serializable elements...
class HList l => SList l
instance SList HNil
instance (Serializable s, SList ss) => SList (HCons s ss)

-- ... so it should be possible to write instantiate Serializable, right?:
instance (SList s, HMapOut Serialize s (IO String)) => Serializable s
    serialize = liftM show . (sequence :: [IO String] -> IO [String]) . hMapOut Serialize
    deSerialize = error "Not yet.  (I am not even done with serialize yet.)"

-- Seems we need the trick from the paper that oleg pointed out to me earlier in this thread:
data Serialize = Serialize
instance (Serializable s) => Apply Serialize s (IO String) where apply _ = serialize

-- Example:
slist = HCons (1 :: Int) (HCons ('c' :: Char) HNil)

test1 = serialize slist
-- (This is where -fallow-overlapping-instances helps.  There is a
-- section in [1] on how to get rid of it, which I haven't read yet.)

test2 :: IO (HCons Int (HCons Char HNil))
test2 = test1 >>= deSerialize

Two questions:

 (1) Do you see any reasons why it should be impossible in principle
     to write deSerialize for the SList instance of Serializable?  (I
     think the answer is "it's possible to write it, but you need to
     add quite some type information by hand".)

 (2) The problem with test2 is that I need to know its precise
     object-level type, ie which types occur at which positions in the
     SList.  I am pretty sure this is a restriction I have to live
     with.  Please tell me I am wrong.  (-:  (I think my application
     will make it possible for ghc to infer the type, which is fixed
     at compile time anyways, so it's not a severe restriction.)

 (3) (bonus question :) Who wants to write deSerialize for SLists for

And another one: Why do I need to list the HMapOut instance in the
context of the instance declaration of Serializable for SLists?  (It's
not a big deal, but I can't see why it can't be inferred automatically
from the rest of the code.)

Possibly back with another issue of my HList diary tomorrow.  (Please
tell me if you find this interesting or if you would like me to stop
being so verbose.)

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