[Haskell-cafe] Is StateT what I need?

Andre Nathan andre at digirati.com.br
Mon Dec 17 14:33:20 EST 2007

Hello (Newbie question ahead :) 

I'm trying to write a program which will build a tree (here represented
as a Map) of unix processes. The tree should be built by reading the
process information stored in /proc/PID/status. There is also another
Map which will be used for faster insertions on the process tree, which
I'd like to handle as my program state. So far I have the functions to
get a list of entries from /proc, filter the ones that represent
processes and get the information from their status file.

Now I need an "insertProc" function, which should get the information
for a given process ID, update the state and return that information.
This is where I think I need StateT, but I couldn't find out how to use
it (never used StateT before...).

This is what I have so far:

> type Pid = FilePath
> type Uid = String
> type PsData     = Map String Uid
> type PsChildren = Map Pid PsInfo
> data PsInfo = PsInfo PsData PsChildren
> type PsMap  = Map Pid PsInfo
> type PsTree = Map Pid PsInfo
> parent :: PsData -> Pid
> parent psData = fromJust $ Map.lookup "PPid" psData
> getProcInfo :: PsData -> String -> IO PsData
> getProcInfo psData line = do
>   case matchRegex (mkRegex "^([a-z]+):[[:space:]]+(.*)$") line of
>     Nothing           -> return (psData)
>     Just [key, value] -> return (Map.insert key value psData)
> procInfo :: Pid -> IO PsInfo
> procInfo pid = do
>   procData <- readFile $ "/proc/" ++ pid ++ "/status"
>   psData <- foldM getProcInfo Map.empty (lines procData)
>   let [rUid, eUid, _] = words $ fromJust (Map.lookup "Uid" psData)
>   let [rGid, eGid, _] = words $ fromJust (Map.lookup "Gid" psData)
>   let uids = Map.fromList [("RUid", rUid), ("EUid", eUid),
>                            ("RGid", rGid), ("EGid", eGid)]
>   let psData' = Map.union psData uids
>   return (PsInfo psData' Map.empty)

I tried this for insertProc, but it obviously doesn't work... what would
be the correct way to do this?

> insertProc :: Pid -> StateT PsMap IO PsInfo
> insertProc pid = do
>   proc <- procInfo pid -- XXX this is obviously wrong...
>   psMap <- get
>   put (Map.insert pid proc psMap)
>   return (proc)

A second question: is it possible to make getProcInfo's type to be
PsData -> String -> PsData and use some operation similar to lift so
that it can be used with foldM, instead of making its return type to be
IO PsData explicitely?

Thanks in advance,

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