src/ConnectionString.hs

   1 {-# LANGUAGE DeriveDataTypeable #-}
   2 {-# LANGUAGE OverloadedStrings #-}
   3
   4 -- | Definition of and instances for the ConnectionString type. This
   5 --   type is simply a wrapper around a 'String', but the newtype
   6 --   allows us to give it a separate 'Default' instance.
   7 --
   8 module ConnectionString (
   9   ConnectionString(..) )
  10 where
  11
  12 -- System imports.
  13 -- DC is needed only for the DCT.Configured instance of String.
  14 import qualified Data.Configurator as DC()
  15 import qualified Data.Configurator.Types as DCT (
  16   Configured,
  17   Value( String ),
  18   convert )
  19 import Data.Data (Data)
  20 import System.Console.CmdArgs.Default (Default(..))
  21 import Data.Typeable (Typeable)
  22
  23
  24 -- | A newtype around a string that allows us to give a more
  25 --   appropriate default value for a connection string.
  26 --
  27 newtype ConnectionString =
  28   ConnectionString { get_connection_string :: String }
  29   deriving (Data, Show, Typeable)
  30
  31
  32 instance Default ConnectionString where
  33   -- | This default is appropriate for SQLite databases which require
  34   --   no authentication and live entirely in a file (or in this case,
  35   --   memory).
  36   def = ConnectionString ":memory:"
  37
  38
  39 instance DCT.Configured ConnectionString where
  40   -- | This allows us to read a ConnectionString out of a Configurator
  41   --   config file. By default Configurator wouldn't know what to do,
  42   --   so we have to tell it that we expect a DCT.String, and if one
  43   --   exists, to apply the ConnectionString constructor to it.
  44   convert s@(DCT.String _) =
  45     fmap ConnectionString (convert_string s)
  46     where
  47       convert_string :: DCT.Value -> Maybe String
  48       convert_string = DCT.convert
  49
  50   -- If we read anything other than a DCT.String out of the file, fail.
  51   convert _ = Nothing