Add instance for MonadUnify for ReaderT

src/Control/Monad/Unify/Class.purs

@@ -6,6 +6,7 @@ import Control.Applicative (pure)
 import Control.Bind (bind)
 import Control.MonadPlus (class MonadPlus, empty)
 import Control.Monad.State.Trans (StateT)
+import Control.Monad.Reader.Trans (ReaderT)
 import Control.Monad.Trans.Class (lift)
 import Data.Traversable (class Traversable, traverse)
 import Data.Tuple (Tuple(..))
@@ -51,3 +52,9 @@ instance MonadUnify k f m => MonadUnify k f (StateT s m) where
     merge m k = lift $ merge m k
     store k f = lift $ store k f
     reify f = lift $ reify f
+
+instance MonadUnify k f m => MonadUnify k f (ReaderT r m) where
+    fresh = lift fresh
+    merge m k = lift $ merge m k
+    store k f = lift $ store k f
+    reify f = lift $ reify f