Support for multiproject builds with nix

- Closes #28

backend/src/Devel/Main.hs

@@ -0,0 +1,60 @@
+{-# Language TypeApplications #-}
+{-# Language DataKinds #-}
+{-# Language FlexibleContexts #-}
+module Devel.Main where
+
+import Prelude
+import Control.Monad.Trans.Reader (runReaderT)
+import Main (withApp, defaultMain)
+import Control.Concurrent
+import Control.Monad (void)
+import Data.IORef (IORef, newIORef, readIORef, writeIORef)
+import Foreign.Store (Store(..), lookupStore, readStore, storeAction, withStore)
+import GHC.Word (Word32)
+import Dhall (input, auto)
+
+import Database.Schema
+import Database.Selda (tryCreateTable)
+import Database
+
+update :: IO ()
+update = do
+  lookupStore tidStoreNum >>= maybe setupNew restart
+  where
+    doneStore :: Store (MVar ())
+    doneStore = Store 0
+    setupNew :: IO ()
+    setupNew = do
+      done <- storeAction doneStore newEmptyMVar
+      tid <- start done
+      void $ storeAction (Store tidStoreNum) (newIORef tid)
+    restart tidStore = modifyStoredIORef tidStore $ \tid -> do
+      killThread tid
+      withStore doneStore takeMVar
+      readStore doneStore >>= start
+    start :: MVar () -> IO ThreadId
+    start done = forkFinally develMain (\_ -> putMVar done ())
+
+develMain :: IO ()
+develMain = do
+  conf <- input auto "../config/devel.dhall"
+  withApp conf $ \app -> do
+    void $ runReaderT (runDB migrate) app
+    defaultMain app
+  where
+    migrate = do
+      tryCreateTable (gen users)
+      tryCreateTable (gen books)
+      tryCreateTable (gen tags)
+      tryCreateTable (gen channels)
+      tryCreateTable (gen bookTags)
+      tryCreateTable (gen bookChannels)
+
+modifyStoredIORef :: Store (IORef a) -> (a -> IO a) -> IO ()
+modifyStoredIORef store f = withStore store $ \ref -> do
+  v <- readIORef ref
+  f v >>= writeIORef ref
+
+tidStoreNum :: Word32
+tidStoreNum = 1
+