1 files changed, 42 insertions, 0 deletions
diff --git a/stdlib/source/lux/abstract/equivalence.lux b/stdlib/source/lux/abstract/equivalence.lux
new file mode 100644
index 000000000..b773505de
--- /dev/null
+++ b/stdlib/source/lux/abstract/equivalence.lux
@@ -0,0 +1,42 @@
+(.module:
+  [lux #*]
+  [//
+   [functor (#+ Contravariant)]])
+
+(signature: #export (Equivalence a)
+  {#.doc "Equivalence for a type's instances."}
+  (: (-> a a Bit)
+     =))
+
+(def: #export (product left right)
+  (All [l r] (-> (Equivalence l) (Equivalence r) (Equivalence [l r])))
+  (structure
+   (def: (= [a b] [x y])
+     (and (:: left = a x)
+          (:: right = b y)))))
+
+(def: #export (sum left right)
+  (All [l r] (-> (Equivalence l) (Equivalence r) (Equivalence (| l r))))
+  (structure
+   (def: (= a|b x|y)
+     (case [a|b x|y]
+       [(0 a) (0 x)]
+       (:: left = a x)
+
+       [(1 b) (1 y)]
+       (:: right = b y)
+
+       _
+       #0))))
+
+(def: #export (rec sub)
+  (All [a] (-> (-> (Equivalence a) (Equivalence a)) (Equivalence a)))
+  (structure
+   (def: (= left right)
+     (sub (rec sub) left right))))
+
+(structure: #export contravariant (Contravariant Equivalence)
+  (def: (map-1 f equivalence)
+    (structure
+     (def: (= reference sample)
+       (:: equivalence = (f reference) (f sample))))))