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Serialising data structures with sharing (i.e. after "tying the knot")
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| {-# LANGUAGE ExistentialQuantification #-} | |
| module SerialiseKnots where | |
| import System.Mem.StableName | |
| import Data.Map (Map) | |
| import qualified Data.Map as Map | |
| import Control.Monad.Trans.State | |
| import Control.Monad.Trans.Class | |
| import Control.Monad (when) | |
| data Ref = forall a. Ref (StableName a) | |
| instance Eq Ref where | |
| (==) (Ref a) (Ref b) = eqStableName a b | |
| instance Ord Ref where | |
| -- HACK. Hashing can give false positives | |
| compare (Ref a) (Ref b) = compare (hashStableName a) (hashStableName b) | |
| type Ser a = StateT (Map Ref String) IO a | |
| class SerialiseKnots a where | |
| serialiseKnots :: a -> Ser String | |
| instance SerialiseKnots Int where | |
| serialiseKnots a = pure (show a) | |
| data List a = Empty | Node a (List a) | |
| instance SerialiseKnots a => SerialiseKnots (List a) where | |
| serialiseKnots Empty = pure "Empty" | |
| serialiseKnots (Node x xs) = do | |
| ref <- fmap Ref (lift $ makeStableName x) | |
| refs <- get | |
| when (not $ Map.member ref refs) $ do | |
| rep <- serialiseKnots x | |
| refs <- get | |
| put $ Map.insert ref rep refs | |
| rest <- serialiseKnots xs | |
| pure $ makeVar ref ++ ":" ++ rest | |
| makeVar :: Ref -> String | |
| makeVar (Ref s) = "v" ++ show (hashStableName s) | |
| serialise :: SerialiseKnots a => a -> IO String | |
| serialise a = do | |
| (rep, refs) <- runStateT (serialiseKnots a) Map.empty | |
| pure $ "let\n" ++ unlines (serialiseIndex refs) ++ "\nin\n" ++ rep | |
| serialiseIndex :: Map Ref String -> [String] | |
| serialiseIndex = Map.foldrWithKey f [] | |
| where | |
| f ref v s = (makeVar ref ++ " = " ++ v):s | |
| sample :: List (List Int) | |
| sample = Node x (Node x (Node x Empty)) | |
| where | |
| x = Node 12 Empty | |
| main :: IO () | |
| main = do | |
| v <- serialise sample | |
| putStrLn v | |
| -- Sample run: | |
| -- | |
| -- λ stack run | |
| -- let | |
| -- v1 = v2:Empty | |
| -- v2 = 12 | |
| -- | |
| -- in | |
| -- v1:v1:v1:Empty |
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