[Haskell-cafe] Open mutable records

22 May 2005

      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