From 1f93dc17459189dbeaebe564eede5baed7c4a00d Mon Sep 17 00:00:00 2001
From: Danila Fedorin <danila.fedorin@gmail.com>
Date: Sat, 4 Mar 2023 14:24:52 -0800
Subject: [PATCH] Add MonadUnify instance for the state monad.

---
 src/Control/Monad/Unify/Class.purs | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/Control/Monad/Unify/Class.purs b/src/Control/Monad/Unify/Class.purs
index 3dc8103..b4def73 100644
--- a/src/Control/Monad/Unify/Class.purs
+++ b/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