Have you seen the OOHaskell paper (the follow up to the HList paper)... It looks like you do much the same thing - with some differences... Would be interesting to get your comments on the paper: http://homepages.cwi.nl/~ralf/OOHaskell/ Keean. Einar Karttunen wrote:
Hello
I recently ended up hacking a quite concise implementation of mutable open (extensible) records in Haskell. Most of the ideas came from the HList-paper, but this seems like a very simple way of doing things.
Run with ghci -fglasgow-exts -fallow-overlapping-instances.
Import some stuff we are going to need later:
import Control.Monad.Reader import Data.IORef import System
Monad for mutable record calculations - to get implisit this/self in the OO sense.
newtype OO t r = OO (ReaderT t IO r) deriving(Monad, MonadReader t, MonadIO)
with :: s -> OO s a -> OO b a with this (OO c) = liftIO (runReaderT c this)
ooToIO :: OO s a -> IO a ooToIO (OO c) = runReaderT c undefined
Records
First the record constructor - followed by the terminator.
data a :.: r = RC !a !r infixr :.: data END = END
Next we define a field access method.
class Select r f t | r f -> t where (!) :: r -> f -> Ref t instance Select (Field f t :.: r) f t where (!) (RC (F x) _) _ = x instance Select r f t => Select (a :.: r) f t where (!) (RC _ t) = (!) t
And finally the type of mutable fields.
type Ref a = IORef a newtype Field name rtype = F (Ref rtype)
Next we define a way to construct record values.
infixr ## (##) :: v -> OO s r -> OO s ((Field f v) :.: r) (##) v r = do { h <- liftIO (newIORef v); t <- r; return (RC (F h) t) } end = return END :: OO s END
Get the value of a field.
value :: Select s f t => f -> OO s t value a = do x <- asks (\s -> s!a) liftIO (readIORef x)
Or set the value of a field.
(<-:) :: Select s f t => f -> t -> OO s () a <-: b = do x <- asks (\s -> s!a) liftIO (writeIORef x b)
And as a convenience add value to an int field.
(+=) :: Select s f Int => f -> Int -> OO s Int a += b = do x <- asks (\s -> s!a) val <- liftIO (readIORef x) let z = val+b z `seq` liftIO (writeIORef x z) return z
Now implement the classic ocaml OO tutorial:
data X = X type Point = Field X Int :.: END
newPoint :: OO s Point newPoint = 0 ## end
getX :: Select s X t => OO s t getX = value X
move d = X += d
data Color = Color type ColoredPoint = Field Color String :.: Point
newColoredPoint :: String -> OO s ColoredPoint newColoredPoint c = c ## 0 ## end
color :: Select s Color t => OO s t color = value Color
The code looks in more complex examples like this: ((~=) is prepending into list fields.)
newArrival :: Patient -> OO Hospital () newArrival patient = do with patient (HospitalVisits += 1) staff <- value FreeStaff if staff > 0 then do FreeStaff += (-1) Examination ~= patient with patient (do HospitalTime += 3 RemainingTime <-: 3) else do Triage ~= patient
main = ooToIO (do c1 <- newPoint c2 <- newColoredPoint "green" with c1 $ move 7 with c2 $ move 4 let p x = liftIO (print x) p =<< with c1 getX p =<< with c2 getX)
- Einar Karttunen _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe