[Haskell-cafe] Indentation Creep

13 Jul 2007

      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
drtomc@gmail.com

Silence is the perfectest herald of joy:
I were but little happy, if I could say how much.