Question

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

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