Re: [Haskell-cafe] Is StateT what I need?

18 Dec 2007

      Hello

On Mon, 2007-12-17 at 21:22 -0200, Andre Nathan wrote:
...
Thanks everyone for the great suggestions. The code is much cleaner now
(not to mention it works :)
I'm trying to finish the process tree construction but I guess I'll need
some help again.

My idea is to have a function that would return a map representing the
process tree
...
createTree :: IO PsTree
createTree = do
  entries <- getDirectoryContents "/proc"
  return $ foldr buildTree Map.empty entries
The "return $ foldr ..." part is missing something, because buildTree
would have be something like:
...
buildTree :: String -> PsTree -> StateT PsMap IO PsTree
buildTree entry tree = do
  case matchRegex (mkRegex "^[0-9]+$") entry of
    Nothing -> return tree -- skip this entry
    Just _  -> do
      psMap <- get
      if Map.member dir psMap
        then return tree -- alread inserted
        else return $ insertInTree dir tree
so the types don't match. insertInTree would be something like (in
pseudo-code):
...
insertInTree pid tree = do
  procInfo <- insertProc pid -- this inserts pid in the state map
                             -- and returns a PsInfo, so its type is
                             -- Pid -> StateT PsMap IO PsInfo.
                             -- Can I use it here though?
  psMap <- get
  if pid == "1" -- init is the root of the tree
    then do modify (Map.insert "1" procInfo psMap)
            return $ Map.insert "1" procInfo tree
    else do
      let pPid = parentPid procInfo
      if Map.member pPid psMap
        then do psMap' <- new psMap with pid appended pPid's children
                return tree
        else do tree' <- insert pPid in the process tree
                modify (new psMap with pid appended pPid's children)
                return tree'
insertProc was in my first message, and it's like this:
...
insertProc :: Pid -> StateT PsMap IO PsInfo
insertProc pid = do
  process <- lift $ procInfo pid
  psMap <- get
  modify (Map.insert pid process)
  return (process)
At this point I'm not sure if this design is good or even correct. I'm
mixing (StateT PsMap IO PsInfo) with (StateT PsMap IO PsTree), which I'm
not sure I can do. There is probably a much cleaner way to do this but I
cannot see through the types right now :/

Anyone has any hints on how to make that scheme work?

Thanks,
Andre