Begin throwing real code together.

[mailbox-count.git] / src / OptionalConfiguration.hs

diff --git a/src/OptionalConfiguration.hs b/src/OptionalConfiguration.hs
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+{-# LANGUAGE DeriveDataTypeable #-}
+{-# LANGUAGE OverloadedStrings #-}
+
+-- | An OptionalConfiguration is just like a 'Configuration', except
+--   all of its fields are optional. The user can set options in two
+--   places: the command-line, and a configuration file. Obviously if
+--   a parameter is set in one place, it doesn't need to be set in the
+--   other. Thus, the latter needs to be optional.
+--
+
+module OptionalConfiguration (
+  OptionalConfiguration(..),
+  from_rc )
+where
+
+import qualified Data.Configurator as DC (
+  Worth(Optional),
+  load,
+  lookup )
+import Data.Data ( Data )
+import Data.Typeable ( Typeable )
+import Data.Monoid ( Monoid(..) )
+import Paths_mailbox_count ( getSysconfDir )
+import System.Directory ( getHomeDirectory )
+import System.FilePath ( (</>) )
+import System.IO.Error ( catchIOError )
+
+
+-- | The same as Configuration, except everything is optional. It's easy to
+--   merge two of these by simply dropping the Nothings in favor of
+--   the Justs. The 'feed_hosts' are left un-maybed so that cmdargs
+--   can parse more than one of them.
+--
+data OptionalConfiguration =
+  OptionalConfiguration {
+    both     :: Maybe Bool,
+    database :: Maybe String,
+    detail   :: Maybe Bool,
+    host     :: Maybe String,
+    password :: Maybe String,
+    port     :: Maybe Int,
+    username :: Maybe String }
+  deriving (Show, Data, Typeable)
+
+
+-- | Combine two Maybes into one, essentially mashing them
+--   together. We give precedence to the second argument when both are
+--   Justs.
+merge_maybes :: (Maybe a) -> (Maybe a) -> (Maybe a)
+merge_maybes Nothing Nothing   = Nothing
+merge_maybes (Just x) Nothing  = Just x
+merge_maybes Nothing (Just x)  = Just x
+merge_maybes (Just _) (Just y) = Just y
+
+
+-- | The Monoid instance for these lets us "combine" two
+--   OptionalConfigurations. The "combine" operation that we'd like to
+--   perform is, essentially, to mash them together. So if we have two
+--   OptionalConfigurations, each half full, we could combine them
+--   into one big one.
+--
+--   This is used to merge command-line and config-file settings.
+--
+instance Monoid OptionalConfiguration where
+  -- | An empty OptionalConfiguration.
+  mempty = OptionalConfiguration
+             Nothing
+             Nothing
+             Nothing
+             Nothing
+             Nothing
+             Nothing
+             Nothing
+
+  -- | Combine @cfg1@ and @cfg2@, giving precedence to @cfg2@.
+  cfg1 `mappend` cfg2 =
+    OptionalConfiguration
+      (merge_maybes (both cfg1) (both cfg2))
+      (merge_maybes (database cfg1) (database cfg2))
+      (merge_maybes (detail cfg1) (detail cfg2))
+      (merge_maybes (host cfg1) (host cfg2))
+      (merge_maybes (password cfg1) (password cfg2))
+      (merge_maybes (port cfg1) (port cfg2))
+      (merge_maybes (username cfg1) (username cfg2))
+
+
+-- | Obtain an OptionalConfiguration from mailbox-countrc in either
+--   the global configuration directory or the user's home
+--   directory. The one in $HOME is prefixed by a dot so that it is
+--   hidden.
+--
+--   We make an attempt at cross-platform compatibility; we will try
+--   to find the correct directory even on Windows. But if the calls
+--   to getHomeDirectory/getSysconfDir fail for whatever reason, we
+--   fall back to using the Unix-specific /etc and $HOME.
+--
+from_rc :: IO OptionalConfiguration
+from_rc = do
+  etc  <- catchIOError getSysconfDir (\e -> do
+                                        putStrLn $ "ERROR: " ++ (show e)
+                                        return "/etc")
+  home <- catchIOError getHomeDirectory (\e -> do
+                                           putStrLn $ "ERROR: " ++ (show e)
+                                           return "$(HOME)")
+  let global_config_path = etc </> "mailbox-countrc"
+  let user_config_path = home </> ".mailbox-countrc"
+  cfg <- DC.load [ DC.Optional global_config_path,
+                   DC.Optional user_config_path ]
+  cfg_both <- DC.lookup cfg "both"
+  cfg_database <- DC.lookup cfg "database"
+  cfg_detail <- DC.lookup cfg "detail"
+  cfg_host <- DC.lookup cfg "host"
+  cfg_password <- DC.lookup cfg "password"
+  cfg_port <- DC.lookup cfg "port"
+  cfg_username <- DC.lookup cfg "username"
+
+  return $ OptionalConfiguration
+             cfg_both
+             cfg_database
+             cfg_detail
+             cfg_host
+             cfg_password
+             cfg_port
+             cfg_username