On 10 Jan 2008, at 10:21 AM, Andre Nathan wrote:
Hi Jonathan
On Wed, 2008-01-09 at 21:32 -0800, Jonathan Cast wrote:
An actual coding question, abuse? We should be so lucky.
:) Your comments are much appreciated.
You're welcome.
This function is fairly complicated, simply because of the number of
separate definitions involved; I would be looking for opportunities
to inline definitions here, so it's clearer what the definitions
are. (Also, I would try to build a single, self-recursive function
at the top level, put the call to procInfo there, and make everything
else pure).
I rewrote insertInTree like below. Now it is the only function that
has
a StateT return type, and I also got rid of addProc, insertPid and
insertParent :)
insertInTree :: Pid -> StateT PsTree IO ()
insertInTree pid = do
tree <- get
if Map.member pid tree
then return ()
else do
info <- lift $ procInfo pid
modify (Map.insert pid info)
let ppid = parentPid info
if ppid /= "0"
then do
insertInTree ppid
modify (appendChild ppid pid)
else return ()
I also rewrote createTree like this:
createTree :: IO PsTree
createTree = do
entries <- getDirectoryContents "/proc"
let procs = filter (=~ "^[0-9]+$") entries
execStateT (mapM_ insertInTree procs) Map.empty
Is that a bad way to do it? If haskell wasn't lazy this would be 3 O
(n)
operations, and I could write it using readDirStream to process all
entries in one pass. I'm not sure if that's really necessary when
laziness is present though.
It might be faster; laziness usually has higher constants than direct
implementations. But I doubt the difference is critical in this
case, and I would definitely time a re-writing and throw it away
unless it was significantly faster. But I don't think this is a case
where laziness actually alters either the time or the space
asymptotics of the algorithm (you end up creating an ~ O(n) tree
anyway, so I'd figure O(n) space was OK for the loop, too).
jcc
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