I just realized that I mixed up data bases and functional programming and apologize for this. Of course, I meant "referential transparency"! Sven Biedermann Am 02.11.2006 um 16:43 schrieb Sven Biedermann:
Dear Haskellers,
The OI Comonad in Richard Kieburtz' paper does break referential integrity, but he worte, that the implementation is just something "...to experiment with". In this papers he states, that a real OI needs special properties. For instance enableOI needs "...to have the effect of copying pointers to currently accessible IO resources, in effect duplicating the current IO environment."
So, I wrote a simple OI-Comonad for stdin/stdout only, that preserves referential integrity. At least, my reasoning and the tests I made didn't reveal anything different. The code and some explanations are presented at the end of this mail.
Haskellers: Could you review the solution, please? Does it really preserves referential integrity? Or am I completely wrong? Do you have any suggestions on generalizing the idea?
I would be glad to hear from you!
Best regards
Sven Biedermann
----------------------------------------------------------------
The implementation is based on two ideas:
1) The OI outside world is modeled an infinite structure (OIReality) that keeps track effects created in the outside world. All "futures" of a reality at a point of "time" are remembered in a lazy fashion. So, if one probes on referential integrity, the structure will be re-iterated through, playing back already computed results. If a reality at some point of "time" isn't referenced any more, that reality will be reclaimed by the garbage collector.
The following statement is an example, that no space leaks occur as long as a specific reality isn't remembered (stdPutStr builds on basic stdPutChar using the standard operators .>> and =>>):
extract $ mkOI 'x' .>> (cycle ['A'..'z']) =>> oiPutStr
Amazingly,
extract $ mkOI 'x' .>> (cycle ['A'..'z']) =>> oiPutStrLn
produces a space leak, because the implementation isn't lazy enough, yet.
2) The OI-Comonad is a pointer to the current reality, together with the value, that can be extracted from OI. Whenever an OI ist rememberable by the user, the pointer will be duplicated. This is crucial for referential integrity.
Comain, as defined by Andrew Bromage in :
comain :: OI a -> () comain w = extract (w .>> show (a,b) =>> oiPutStrLn) where a = extract (w .>> () =>> oiGetChar) b = extract (w .>> () =>> oiGetChar)
now works as intended and delivers, if 'x' is typed in on standard input:
('x','x')
"a = ..." runs on reality w, yielding 'x' . "b = ..." is just a play back of "a = ...". "extract (w.>> show (a,b) =>> stdPutStrLn)" is another future of w, which just prints ('x','x').
module SimpleOI where
import Control.Monad.Instances import System.IO.Unsafe import Data.Char import Data.IORef
-------------------------------------- -- copied from http://www.eyrie.org/~zednenem/2004/hsce/
class Functor w => Comonad w where extract :: w a -> a duplicate :: w a -> w (w a) extend :: (w a -> b) -> (w a -> w b)
extend f = fmap f . duplicate duplicate = extend id
-- | 'fmap' defined in terms of 'extend' liftW :: Comonad w => (a -> b) -> (w a -> w b) liftW f = extend (f . extract)
-- | 'extend' with the arguments swapped. Dual to '>>=' for monads. (=>>) :: Comonad w => w a -> (w a -> b) -> w b (=>>) = flip extend
-- | Injects a value into the comonad. (.>>) :: Comonad w => w a -> b -> w b w .>> b = extend (\_ -> b) w
-- end of copy -------------------------------------
data OIReality = OIReality GetChar [PutChar] Char data GetChar = GetChar Char OIReality data PutChar = PutChar () OIReality
constructReality :: Char -> OIReality constructReality c = let r = OIReality (constructGet r) (constructPuts r) c in r
constructGet :: OIReality -> GetChar constructGet r = let c = unsafePerformIO $ seq r getChar in GetChar c (constructReality c)
constructPut :: OIReality -> Char -> PutChar constructPut r c = let v = unsafePerformIO $ seq r $ putChar c in PutChar v (constructReality c)
constructPuts :: OIReality -> [PutChar] constructPuts r = map (constructPut r.chr) [0..255]
nextGetCharReality :: OIReality -> OIReality nextGetCharReality (OIReality (GetChar c r) _ _) = r
nextPutCharReality :: Char -> OIReality -> OIReality nextPutCharReality c (OIReality _ puts _) = let PutChar v r = puts !! (ord c) in seq v r
data OIComonad a = OI (IORef OIReality) a
-- extract OI _ a = a instance Functor OIComonad where fmap f (OI ref a) = seq a $ OI (unsafeDuplicateRef ref) (f a) -- too many seq's at the moment
instance Comonad OIComonad where duplicate w = fmap (const w) w extract (OI _ a) = a
mkOI :: Char -> OIComonad Char mkOI c = OI (unsafePerformIO $ newIORef $ constructReality c) c
oiGetChar :: OIComonad a -> Char oiGetChar (OI ref _) = unsafePerformIO $ do { modifyIORef ref nextGetCharReality; OIReality _ _ c <- readIORef ref; return (c) }
oiPutChar :: OIComonad Char -> () oiPutChar (OI ref c) = unsafePerformIO $ do { modifyIORef ref (nextPutCharReality c); OIReality _ _ d <- readIORef ref; -- not!? modified without further read return $ seq d ()}
unsafeDuplicateRef :: IORef OIReality -> IORef OIReality unsafeDuplicateRef ref = unsafePerformIO $ do { r <- readIORef ref; newIORef r; }
oiPutStrLn w = extract $ w =>> oiPutStr .>> '\n' =>> oiPutChar -- space leak if input string is suffiently long -- too many seqs in code
oiPutStr w = extract $ oiPutStrS w .>> ()
oiPutStrS :: OIComonad String -> OIComonad String oiPutStrS w = case (extract w) of [] -> w (c:cs) -> oiPutStrS (w .>> c =>> oiPutChar .>> cs)
oiGetLine = "t.b.d. using oiGetChar"
-- adapted from http://www.mail-archive.com/haskell- cafe@haskell.org/msg02408.html comain w = extract (w .>> show (a,b) =>> oiPutStr) where a = extract (w .>> () =>> oiGetChar) b = extract (w .>> () =>> oiGetChar)
{- Original from http://www.mail-archive.com/haskell- cafe@haskell.org/msg02408.html
-- Bootstrap into the OI comonad main :: IO () main = return $! comain stdOI
-- The following are the OI functions which we use. -- stdGetChar :: OI () -> Char -- stdPutStrLn :: OI String -> ()
comain :: OI a -> () comain w = coeval (w .>> show (a,b) =>> stdPutStrLn) where a = coeval (w .>> () =>> stdGetChar) b = coeval (w .>> () =>> stdGetChar)
-}
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