Fully support login, logout

Refactor my handlers to use the `Handler a` type instead of `IO a`; this allows
me to throwError inside of handlers that Servant properly handles. Previously I
was creating 500 errors unnecessarily.

src/Auth.hs

@@ -0,0 +1,54 @@
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE RecordWildCards #-}
+--------------------------------------------------------------------------------
+module Auth where
+--------------------------------------------------------------------------------
+import Database.SQLite.Simple
+import Utils
+import Web.Cookie
+
+import qualified Data.UUID as UUID
+import qualified Web.Cookie as WC
+import qualified Sessions as Sessions
+import qualified Accounts as Accounts
+import qualified Types as T
+import qualified Data.ByteString.Lazy as LBS
+--------------------------------------------------------------------------------
+
+-- | Return the UUID from a Session cookie.
+uuidFromCookie :: T.SessionCookie -> Maybe T.SessionUUID
+uuidFromCookie (T.SessionCookie cookies) = do
+  auth <- lookup "auth" cookies
+  uuid <- UUID.fromASCIIBytes auth
+  pure $ T.SessionUUID uuid
+
+-- | Attempt to return the user role associated with the `cookie`.
+roleFromCookie :: FilePath -> T.SessionCookie -> IO (Maybe T.Role)
+roleFromCookie dbFile cookie = withConnection dbFile $ \conn -> do
+  case uuidFromCookie cookie of
+    Nothing -> pure Nothing
+    Just uuid -> do
+      mSession <- Sessions.get dbFile uuid
+      case mSession of
+        Nothing -> pure Nothing
+        Just T.StoredSession{..} -> do
+          mAccount <- Accounts.lookup dbFile storedSessionUsername
+          case mAccount of
+            Nothing -> pure Nothing
+            Just T.Account{..} -> pure (Just accountRole)
+
+-- | Create a new session cookie.
+mkCookie :: T.SessionUUID -> SetCookie
+mkCookie (T.SessionUUID uuid) =
+  defaultSetCookie
+    { setCookieName = "auth"
+    , setCookieValue = UUID.toASCIIBytes uuid
+    }
+
+-- | Use this to clear out the session cookie.
+emptyCookie :: SetCookie
+emptyCookie =
+  defaultSetCookie
+    { setCookieName = "auth"
+    , setCookieValue = ""
+    }