Getting the Local AppData folder in Haskell

Question

Hi, I'm trying to get the location of Window's Local AppData folder in a version-agnostic manner using Haskell, and I'm having a bit of trouble doing so. I've tried using the System.Win32.Registry library, and I was able to get the code below (after some trial and error), but I wasn't able to figure out how to use the regQueryValueEx or any other function to get the value I need.

import System.Win32.Types
import System.Win32.Registry

userShellFolders :: IO HKEY
userShellFolders = regOpenKeyEx hKEY_CURRENT_USER "Software\\Microsoft\\Windows\\CurrentVersion\\Explorer\\User Shell Folders\\" kEY_QUERY_VALUE

I also tried looking at the source code for the getAppUserDataDirectory function in the System.Directory module, but that didn't help me either.

Maybe there's an easier way to do this that I'm just missing.

Answer 1

To read the values from the registry in a useful format quite some code is necessary to convert between Haskell and C types. And that the values in question are usually of type REG_EXPAND_SZ also doesn't help. So it's not pretty, but this works for me:

{-# LANGUAGE ForeignFunctionInterface #-}

import System.Win32.Types
import System.Win32.Registry
import Foreign.Ptr (castPtr)
import Foreign.Marshal.Alloc (allocaBytes)
import Foreign.C.String (peekCWString, withCWString)
import Control.Exception (bracket, throwIO)

-- // parse a string from a registry value of certain type
parseRegString :: RegValueType -> LPBYTE -> IO String
parseRegString ty mem
   | ty == rEG_SZ        = peekCWString (castPtr mem)
   | ty == rEG_EXPAND_SZ = peekCWString (castPtr mem) >>=
                              expandEnvironmentStrings
   | otherwise           = ioError (userError "Invalid registry value type")

-- // FFI import of the ExpandEnvironmentStrings function needed
-- // to make use of the registry values
expandEnvironmentStrings :: String -> IO String
expandEnvironmentStrings toexpand =
   withCWString toexpand $ \input ->
   allocaBytes 512 $ \output ->
   do c_ExpandEnvironmentStrings input output 256
      peekCWString output
foreign import stdcall unsafe "windows.h ExpandEnvironmentStringsW"
  c_ExpandEnvironmentStrings :: LPCTSTR -> LPTSTR -> DWORD -> IO DWORD

-- // open the registry key
userShellFolders :: IO HKEY
userShellFolders = regOpenKeyEx hKEY_CURRENT_USER
   "Software\\Microsoft\\Windows\\CurrentVersion\\Explorer\\User Shell Folders"
   kEY_QUERY_VALUE

-- // read the actual value
localAppData :: IO String
localAppData =
   bracket userShellFolders regCloseKey $ \usfkey ->
   allocaBytes 512 $ \mem ->
   do ty <- regQueryValueEx usfkey "Local AppData" mem 512
      parseRegString ty mem

main = localAppData >>= print

I'm not sure if all the error cases are handled correctly (like if the passed buffer was to small), so you might want to check the windows docs to see what happens in these cases.

Answer 2

If you want portability, you shouldn't access registry directly. There is an API function to get special folders: SHGetFolderPath. You can call it thus:

{-# LANGUAGE ForeignFunctionInterface #-}
import System.Win32.Types
import Graphics.Win32.GDI.Types
import Foreign.C.String
import Foreign.Marshal.Array

foreign import stdcall unsafe "SHGetFolderPathW"
    cSHGetFolderPathW :: HWND -> INT -> HANDLE -> DWORD -> CWString -> IO LONG

maxPath = 260
cSIDL_LOCAL_APPDATA = 0x001c -- //see file ShlObj.h in MS Platform SDK for other CSIDL constants

getShellFolder :: INT -> IO String
getShellFolder csidl = allocaArray0 maxPath $ \path -> do
    cSHGetFolderPathW nullHANDLE csidl nullHANDLE 0 path
    peekCWString path

main = getShellFolder cSIDL_LOCAL_APPDATA >>= putStrLn