C的Haskell导出函数

getGPS :: String -> IO (Double,Double)
getGPS ip = do
  html <- getHTML ip
  let ztags =Prelude.zip [0..] . filterStr . getTagText . getTags $html
  let nlat = Prelude.head $Prelude.map fst . Prelude.filter ((_,str) -> strEq str ("Latitude:" :: String)) $ztags
  let nlng = Prelude.head $Prelude.map fst . Prelude.filter ((_,str) -> strEq str ("Longitude:" :: String)) $ztags
  let lat = read (Prelude.head $Prelude.map snd . Prelude.filter ((n,_) -> n == nlat + 1) $ztags) :: Double
  let lng = read (Prelude.head $Prelude.map snd . Prelude.filter ((n,_) -> n == nlng + 1) $ztags) :: Double
  return (lat,lng)

工作正常.现在,我想通过FFI导出此功能,以便从C应用程序访问它.我做国外出口ccall getGPS :: CString – > IO(CDouble,CDouble)但这不起作用：

GPS.hs:45:1:
    Illegal foreign declaration: requires unregisterised,llvm (-fllvm) or native code generation (-fasm)
    When checking declaration:
      foreign export ccall "getGPS" getGPS
        :: CString -> IO (CDouble,CDouble)

GPS.hs:45:1:
    Unacceptable result type in foreign declaration:
      ‘(CDouble,CDouble)’ cannot be marshalled in a foreign call
    When checking declaration:
      foreign export ccall "getGPS" getGPS
        :: CString -> IO (CDouble,CDouble)

GPS.hs:45:1:
    Couldn't match type ‘Double’ with ‘CDouble’
    Expected type: CString -> IO (CDouble,CDouble)
      Actual type: String -> IO (Double,Double)
    In the expression: getGPS
    When checking declaration:
      foreign export ccall "getGPS" getGPS
        :: CString -> IO (CDouble,CDouble)
Failed,modules loaded: none.

如何正确导出此功能？