From 6763ee3304ec28f152283933ab617bf7f95f58fa Mon Sep 17 00:00:00 2001
From: Matthew Leon <ml@matthewleon.com>
Date: Thu, 25 Jan 2018 20:58:56 -0500
Subject: [PATCH] TypeEqualsBool class can assert type inequality.

Depends on Instance Chains.
---
 bower.json             |  3 +--
 src/Type/Equality.purs | 41 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+), 2 deletions(-)
 create mode 100644 src/Type/Equality.purs

diff --git a/bower.json b/bower.json
index f737bf1..e60e1b2 100644
--- a/bower.json
+++ b/bower.json
@@ -17,7 +17,6 @@
   ],
   "dependencies": {
     "purescript-proxy": "^2.0.0",
-    "purescript-symbols": "^3.0.0",
-    "purescript-type-equality": "^2.0.0"
+    "purescript-symbols": "^3.0.0"
   }
 }
diff --git a/src/Type/Equality.purs b/src/Type/Equality.purs
new file mode 100644
index 0000000..fbf6a89
--- /dev/null
+++ b/src/Type/Equality.purs
@@ -0,0 +1,41 @@
+module Type.Equality
+  ( class TypeEquals
+  , to
+  , from
+  , class TypeEqualsBool
+  ) where
+
+import Type.Data.Boolean (kind Boolean, True, False)
+
+-- | This type class asserts that types `a` and `b`
+-- | are equal.
+-- |
+-- | The functional dependencies and the single
+-- | instance below will force the two type arguments
+-- | to unify when either one is known.
+-- |
+-- | Note: any instance will necessarily overlap with
+-- | `refl` below, so instances of this class should
+-- | not be defined in libraries.
+class TypeEquals a b | a -> b, b -> a where
+  to :: a -> b
+  from :: b -> a
+
+instance refl :: TypeEquals a a where
+  to a = a
+  from a = a
+
+-- | This type class asserts that types `a` and `b`
+-- | are or are not equal, depending on the type of `o`.
+-- |
+-- | `o` will be either `True`, if `a` is `b`,
+-- | or `False` otherwise.
+-- |
+-- | When `o` is specified to be `True`/`False` then this type class
+-- | acts as an assertion that `a` and `b` are equal/not equal, respectively.
+-- |
+-- | Instances should not be defined in libraries.
+class TypeEqualsBool a b (o :: Boolean) | a b -> o
+
+instance reflTypeEqualsBool :: TypeEqualsBool a a True
+else instance notTypeEqualsBool :: TypeEqualsBool a b False