1 files changed, 41 insertions, 0 deletions
diff --git a/fig-frontend/src/Fig/Frontend/State.hs b/fig-frontend/src/Fig/Frontend/State.hs
new file mode 100644
index 0000000..6053105
--- /dev/null
+++ b/fig-frontend/src/Fig/Frontend/State.hs
@@ -0,0 +1,41 @@
+{-# Language TemplateHaskell #-}
+
+module Fig.Frontend.State where
+
+import Control.Lens.TH (makeLensesFor)
+import Control.Lens ((<>=))
+import Control.Monad.State (runStateT)
+
+import Fig.Prelude
+
+import qualified Data.IORef as IORef
+
+newtype State = State
+  { buffer :: Text
+  }
+makeLensesFor [("buffer", "buffer")] ''State
+
+defaultState :: State
+defaultState = State
+  { buffer = ""
+  }
+
+type StateRef = IORef.IORef State
+
+stateRef :: IO StateRef
+stateRef = IORef.newIORef defaultState
+
+withState ::
+  MonadIO m' =>
+  StateRef ->
+  (forall m. (MonadIO m, MonadState State m) => m a) ->
+  m' a
+withState ref f = do
+  s <- liftIO $ IORef.readIORef ref
+  (res, s') <- liftIO $ runStateT f s
+  liftIO $ IORef.writeIORef ref s'
+  pure res
+
+sayHi :: StateRef -> IO ()
+sayHi ref = withState ref do
+  buffer <>= "hi"