[Haskell-cafe] Is StateT what I need?

[Brent Yorgey]

> 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?
>

Yes, and in fact, you don't even need foldM.  The only thing that actually
uses IO is the readFile, so ideally you should just have a small function
that just does the readFile and then processes the result using some (pure)
functions.  Something like this:

> procInfo :: Pid -> IO PsInfo
> procInfo pid = do
>   procData <- readFile $ "/proc/" ++ pid ++ "/status"
>   return $ processData procData
>
> processData :: String -> PsInfo
> ... and so on ...

and so on.  Now instead of using foldM you can just use foldr.  IO is a
cancer, best to keep it confined to as little of your program as possible!
=)

-Brent
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