CompositionT

high-sql.cabal

@@ -39,6 +39,7 @@ library
     library
   other-modules:
     HighSQL.Prelude
+    HighSQL.CompositionT
   exposed-modules:
     HighSQL
     HighSQL.Backend

library/HighSQL/CompositionT.hs

@@ -0,0 +1,40 @@
+module HighSQL.CompositionT where
+
+import HighSQL.Prelude
+
+
+-- |
+-- A monad transformer, 
+-- which serves a purpose of detecting composition of the inner monad.
+data T m r =
+  T Bool (m r)
+
+-- |
+-- Unwrap into a boolean signifying whether the base monad is a composition and
+-- the base monad itself.
+run :: T m r -> (Bool, m r)
+run (T c m) = (c, m)
+
+instance Monad m => Monad (T m) where
+  return a =
+    T False (return a)
+  (>>=) (T _ m) k =
+    T True (m >>= \a -> case k a of T _ m' -> m')
+
+instance Functor f => Functor (T f) where
+  fmap f (T c m) = T c (fmap f m)
+
+instance Applicative f => Applicative (T f) where
+  pure a =
+    T False (pure a) 
+  (<*>) (T _ a) (T _ b) = 
+    T True (a <*> b)
+
+instance MonadTrans T where
+  lift m =
+    T False m
+
+instance MonadIO m => MonadIO (T m) where
+  liftIO io =
+    T False (liftIO io)
+