Hi All,
In the best spirit of Haskelling, I thought I'd try dropping in a
completely different data structure in a spot where I thought the
existing one was (1) ugly (2) leaking memory. In particular, I wrote a
Trie implementation. Now the point is actually not much to do with the
data structure itself, but code layout. I mention this particular data
structure only because it is the one I was working on, but it seems to
come up quite often.
Consider the following function:
data Trie t = Empty | Trie (TriePtr t) (MaybePtr t) (TriePtr t)
type TriePtr t = TVar (Trie t)
type MaybePtr t = TVar (Maybe t)
data Bit = Zero | One
deriving Show
dmin p = do
mv <- dmin' p
case mv of
Nothing -> error "dmin: no values"
Just (v,_) -> return v
dmin' p = do
t <- readTVar p
case t of
Empty -> return Nothing
Trie l m r -> do
mv <- dmin' l
case mv of
Nothing -> do
mv <- readTVar m
case mv of
Nothing -> do
mv <- dmin' r
case mv of
Nothing -> error "emit nasal daemons"
Just (v,e) -> do
if e
then writeTVar p Empty
else return ()
return mv
Just v -> do
re <- null r
case re of
False -> writeTVar m Nothing
True -> writeTVar p Empty
return (Just (v,re))
Just (v,e) -> do
case e of
True -> do
me <- empty m
re <- null r
case me && re of
False -> writeTVar m Nothing
True -> writeTVar p Empty
return (Just (v,me && re))
False -> return mv
where
empty m = do
v <- readTVar m
case v of
Nothing -> return True
Just _ -> return False
All that case analysis causes indentation to creep, and lots of
vertical space "feels" wasted. Is that just a fact of life, or is
there Haskellmagic that I still need to learn?
cheers,
T.
--
Dr Thomas Conway
[EMAIL PROTECTED]
Silence is the perfectest herald of joy:
I were but little happy, if I could say how much.
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