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    <p>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 <code>REG_EXPAND_SZ</code> also doesn't help. So it's not pretty, but this works for me:</p> <pre><code>{-# 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 -&gt; LPBYTE -&gt; IO String parseRegString ty mem | ty == rEG_SZ = peekCWString (castPtr mem) | ty == rEG_EXPAND_SZ = peekCWString (castPtr mem) &gt;&gt;= expandEnvironmentStrings | otherwise = ioError (userError "Invalid registry value type") -- // FFI import of the ExpandEnvironmentStrings function needed -- // to make use of the registry values expandEnvironmentStrings :: String -&gt; IO String expandEnvironmentStrings toexpand = withCWString toexpand $ \input -&gt; allocaBytes 512 $ \output -&gt; do c_ExpandEnvironmentStrings input output 256 peekCWString output foreign import stdcall unsafe "windows.h ExpandEnvironmentStringsW" c_ExpandEnvironmentStrings :: LPCTSTR -&gt; LPTSTR -&gt; DWORD -&gt; 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 -&gt; allocaBytes 512 $ \mem -&gt; do ty &lt;- regQueryValueEx usfkey "Local AppData" mem 512 parseRegString ty mem main = localAppData &gt;&gt;= print </code></pre> <p>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. </p>
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    1. POGetting the Local AppData folder in Haskell
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    1. COThank you very much, this does exactly what I need.
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      1. USAlasdair
    2. CONot the documented way to do this, and unclear on which Windows it does work. It may be the closest to what Alasdairn had already, but the API solution is better.
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    3. COI did just go by what Alasdair already had and didn't look for any other Api calls. Anyway, I won't delete this since it might be useful for other people that are trying to read different registry values that don't have a separate Api to access.
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