recursive types (and operations on them, like pretty-printing, comparison and unification) seem to be working.

big part of the code base still needs to be 'ported' to the updated type constructors.

util/util.js

@@ -1,3 +1,5 @@
+import { lsType, setType } from "../structures/types.js";
+import { pretty } from "./pretty.js";
 
 // re-inventing the wheel:
 export function deepEqual(a, b) {
@@ -46,4 +48,54 @@ export function capitalizeFirstLetter(val) {
   return String(val).charAt(0).toUpperCase() + String(val).slice(1);
 }
 
+const _mapRecursiveStructure = mapping => transform => root => {
+  const found = mapping.get(root);
+  if (found) {
+    // already mapped
+    // return existing result to prevent endless recursion
+    return found;
+  }
+  // note the indirection (wrapped in lamda), this allows the user to recursively map the children (which may refer to the root) without yet having finished mapping the root.
+  let memo;
+  const result = () => {
+    // memoization is necessary for correctness
+    return memo || (memo = transform(root, _mapRecursiveStructure(mapping)(transform)));
+  };
+  mapping.set(root, result);
+  return result;
+};
 
+export const mapRecursiveStructure = _mapRecursiveStructure(new Map());
+
+const _transformType = mapping => transform => type => {
+  const found = mapping.get(type);
+  if (found) {
+    return found;
+  }
+  const mapped = transform(type, _transformType(mapping)(transform));
+  mapping.set(type, mapped);
+  return mapped;
+}
+
+export const transformType = _transformType(new Map());
+
+const __memo = () => {
+  let memo;
+  return fn => memo || (memo = fn());
+}
+export const memo = fn => {
+  return __memo()(fn);
+}
+
+// let infiniteSet = mapRecursiveStructure((type, map) => {
+//   const ps = [];
+//   for (const p of type.params) {
+//     ps.push(map(p()));
+//   }
+//   return setType(ps[0]);
+// })(infiniteList)();
+// console.log(infiniteSet);
+// // while (true) {
+// //   console.log(infiniteSet);
+// //   infiniteSet = infiniteSet.params[0]();
+// // }