Nice transactions. No more gratuitous runDB in model. Monad stacks yay

Handler/Root.hs

@@ -73,7 +73,7 @@ postTasksR = maybeAuthId >>= postTasksR' where
     ((result, taskWidget), _) <- runFormPost taskForm
     case result of
       FormSuccess task -> do
-        createTaskAtBottom userId task
+        runDB $ createTaskAtBottom userId task
         redirectTemporary TasksR
       _ -> undefined -- TODO
 

Model.hs

@@ -1,9 +1,10 @@
-{-# LANGUAGE QuasiQuotes, TypeFamilies, GeneralizedNewtypeDeriving, TemplateHaskell, GADTs, OverloadedStrings #-}
+{-# LANGUAGE QuasiQuotes, TypeFamilies, GeneralizedNewtypeDeriving, TemplateHaskell, GADTs, OverloadedStrings, FlexibleContexts #-}
 module Model where
 
 import Yesod
 import Data.String (IsString)
 import Data.Text (Text)
+import Database.Persist.GenericSql.Raw (SqlPersist)
 import Text.Blaze (ToHtml)
 
 
@@ -23,11 +24,11 @@ data NewTask = NewTask { newTaskTitle :: Text } deriving (Show)
 newTask :: UserId -> Int -> NewTask -> Task
 newTask uid order (NewTask title) = Task uid title 0 False order
 
-
+createTaskAtBottom :: PersistBackend SqlPersist m => UserId -> NewTask -> SqlPersist m TaskId
 createTaskAtBottom userId task = do
-  maybeLastTask <- runDB $ selectFirst [TaskUser ==. userId] [Desc TaskOrder]
+  maybeLastTask <- selectFirst [TaskUser ==. userId] [Desc TaskOrder]
   let lastOrder = maybe 0 (taskOrder . snd) maybeLastTask
-  runDB $ insert $ newTask userId (succ lastOrder) task
+  insert $ newTask userId (succ lastOrder) task
 
 taskState :: Task -> TaskState
 taskState task = if taskDone task then "done" else "pending"