Hi,
Sigbjorn Finne (Intl Vendor) wrote:
>> PS: I ever thought that such file operations are quite usual.
>> Why don't they appear in the standard prelude?
> Who knows, but a concrete proposal of what operations you'd like
> to see supported here would be more than welcome.
Just an idea:
class Storable s where
hWrite :: Handle -> s -> IO ()
hRead :: Handle -> IO s
...
some examples:
1.
instance Storable (ByteArray ix) where
hWrite h ba = do
(size,ba') <- toByteArray ba -- convertion to ByteArray
hPutStrLn h (show size) -- first write the size of the ByteArray
hPutBufBA h ba' size -- does this operation append the
information
-- to a file?
hRead h = do
size <- liftM read (hGetLine h)
mba <- stToIO $ newCharArray (0,size-1)
ba <- stToIO $ freezeCharArray mba -- though, somehow strange ;^)
s' <- hFillBufBA ba size
when (s'<size) (error "...")
return $ unsafeCoerce# ba -- Is this coercion ok?
2.
instance (Storable a, Storable b) => Storable (a,b) where
hWrite h (x,y) = do
hWrite h x
hWrite h y
hRead h = do
x <- hRead h
y <- hRead h
return (x,y)
3.
data Tree a = Empty | Node a (Tree a) (Tree a)
Assuming that String is an instance of Storable, we can define (with 2.)
instance (Storable [a]) => Storable (Tree a) where
hWrite h t = hWrite (structure t "", nodeValues t [])
where structure :: Tree a -> String -> String -- describing the Tree
structure Empty s = 'e':s
structure (Node _ tl tr) = structure tl 'n':(structure tr)
nodeValues :: Tree a -> [a] -- the Values of the
Tree
...
hRead h = do
(st,vs) <- hRead h
(t,st',vs') <- return $ getTree st vals
when ((st'/="")||(vs'/=[])) (error "...")
return t
where getTree :: String -> [a] -> (Tree a,String,[a])
getTree ('e':st) vs = (Empty,st,v)
getTree ('n':st) (v:vs) = (Node v tl tr,st',vs')
where ...
Providing instances of some basic data structures (like lists, tupels) in the
Prelude, one could easily define hWrite and hRead for arbitrary algebraic data
types like Tree.
Martin Stein
