Add product and sum types

structures/product.js

@@ -0,0 +1,29 @@
+import { fnType, prodType } from "../type_registry.js";
+import { Function } from "../metacircular.js";
+
+// In JS, all products are encoded in the same way:
+const constructor = left => right => ({left, right});
+const left        = product => product.left;
+const right       = product => product.right;
+
+// Given two types A and B, create the type (A × B).
+export const makeProductType = (leftType, rightType) => {
+  const productType = prodType(leftType, rightType);
+
+  const leftFnType  = fnType({in: productType, out: leftType});
+  const rightFnType = fnType({in: productType, out: rightType});
+
+  const constructorBoundType = fnType({in: rightType, out: productType});
+  const constructorType      = fnType({in: leftType, out: constructorBoundType});
+
+  return [
+    {i: left       , t: leftFnType},
+    {i: right      , t: rightFnType},
+    {i: leftFnType , t: Function},
+    {i: rightFnType, t: Function},
+
+    {i: constructor         , t: constructorType},
+    {i: constructorType     , t: Function},
+    {i: constructorBoundType, t: Function},
+  ];
+};