reorganize directory and file structure

lib/structures/sum.js

@@ -0,0 +1,11 @@
+// Sum-type (also called: tagged union, disjoint union, variant type)
+// A Sum-type always has only two variants, called "left" and "right".
+// Sum-types of more variants (called Enums) can be constructed by nesting Sum-types.
+
+export const newLeft  = left  => ({t: "L", v: left });
+export const newRight = right => ({t: "R", v: right});
+
+export const match = sum => leftHandler => rightHandler =>
+  sum.t === "L"
+    ? leftHandler(sum.v)
+    : rightHandler(sum.v);