Read env variables using envy library

Using my dear friend's, dmjio's, excellent library, envy -- to read and parse
variables from the system environment.

I added and git-ignored the .envrc file that contains API secrets. I'm using
Envy to read these values, so that I don't hard-code these values into the
source code.

.gitignore

@@ -1,3 +1,4 @@
+.envrc
 *.db
 *.sqlite3
 !populate.sqlite3

shell.nix

@@ -10,6 +10,7 @@ in pkgs.mkShell {
       hpkgs.warp
       hpkgs.cryptonite
       hpkgs.uuid
+      hpkgs.envy
     ]))
   ];
 }

src/App.hs

@@ -37,8 +37,8 @@ err429 = ServerError
   , errHeaders = []
   }
 
-server :: FilePath -> Server API
-server dbFile = createAccount
+server :: T.Config -> Server API
+server T.Config{..} = createAccount
                  :<|> deleteAccount
                  :<|> listAccounts
                  :<|> createTrip
@@ -124,6 +124,6 @@ server dbFile = createAccount
           liftIO $ Sessions.delete dbFile uuid
           pure $ addHeader Auth.emptyCookie NoContent
 
-run :: FilePath -> IO ()
-run dbFile =
-  Warp.run 3000 (serve (Proxy @ API) $ server dbFile)
+run :: T.Config -> IO ()
+run config =
+  Warp.run 3000 (serve (Proxy @ API) $ server config)

src/Main.hs

@@ -1,7 +1,13 @@
+--------------------------------------------------------------------------------
 module Main where
 --------------------------------------------------------------------------------
 import qualified App
+import qualified System.Envy as Envy
 --------------------------------------------------------------------------------
 
 main :: IO ()
-main = App.run "../db.sqlite3"
+main = do
+  mEnv <- Envy.decodeEnv
+  case mEnv of
+    Left err -> putStrLn err
+    Right env -> App.run env

src/Types.hs

@@ -16,6 +16,7 @@ import Database.SQLite.Simple.ToField
 import GHC.Generics
 import Web.Cookie
 import Servant.API
+import System.Envy (FromEnv, fromEnv, env)
 import Crypto.Random.Types (MonadRandom)
 
 import qualified Crypto.KDF.BCrypt as BC
@@ -26,6 +27,17 @@ import qualified Data.Text.Encoding as TE
 import qualified Data.UUID as UUID
 --------------------------------------------------------------------------------
 
+-- | Top-level application configuration.
+data Config = Config
+  { mailgunAPIKey :: Text
+  , dbFile :: FilePath
+  } deriving (Eq, Show)
+
+instance FromEnv Config where
+  fromEnv _ =
+    Config <$> env "MAILGUN_API_KEY"
+           <*> env "DB_FILE"
+
 -- TODO(wpcarro): Properly handle NULL for columns like profilePicture.
 forNewtype :: (Typeable b) => (Text -> b) -> FieldParser b
 forNewtype wrapper field =