Add MonadUnify instance for the state monad.

src/Control/Monad/Unify/Class.purs

@@ -5,6 +5,8 @@ import Prelude (Unit, unit, ($))
 import Control.Applicative (pure)
 import Control.Bind (bind)
 import Control.MonadPlus (class MonadPlus, empty)
+import Control.Monad.State.Trans (StateT)
+import Control.Monad.Trans.Class (lift)
 import Data.Traversable (class Traversable, traverse)
 import Data.Tuple (Tuple(..))
 import Data.Lazy (Lazy, force)
@@ -43,3 +45,9 @@ unify f1 f2 =
       process (Merge k1 k2) = merge k1 k2
       process (Store k f) = store k f
       process Fail = empty
+
+instance MonadUnify k f m => MonadUnify k f (StateT s m) where
+    fresh = lift fresh
+    merge m k = lift $ merge m k
+    store k f = lift $ store k f
+    reify f = lift $ reify f