th32SnapEnumProcesses in System.Win32.Process prints debugging information to stdout
TSIA, pretty much. I suspect that when the author was developing this function he put in some putStrLn calls for his own use and simply forgot to remove them. For my application I just took the code out of the library and ripped out the offending statements, but I don't think they're really supposed to be there and ought to be either removed or #ifdef'd out somehow. Here's my cleaned up version (I also removed some extraneous parenthesis, FWIW).
th32SnapEnumProcesses :: Th32SnapHandle -> IO [ProcessEntry32]
th32SnapEnumProcesses h = allocaBytes 556 $ \pe -> do
pokeByteOff pe 0 (556 :: DWORD)
ok <- c_Process32First h pe
readAndNext ok pe []
where
errStr = "th32SnapEnumProcesses: Process32First/Process32Next"
readAndNext ok pe res
| not ok = do
err <- getLastError
if err == 18
then return $ reverse res
else failWith errStr err
| otherwise = do
entry <- peekProcessEntry32 pe
ok' <- c_Process32Next h pe
readAndNext ok' pe (entry:res)