Add product and sum types

type_registry.js

@@ -0,0 +1,62 @@
+// This module ensures that we never accidentally create a Symbol for the same type twice.
+// Maybe we shouldn't use Symbols for types, but we also cannot use primitive values for types because they may accidentally overlap with 'real' values (that are not types)
+// We also cannot use objects for types because similar to Symbols, objects are only equal to themselves if we use them as Map keys.
+
+import { DefaultMap } from "./util.js";
+
+// Global store of function types:
+const fnTypes = new DefaultMap(() => new Map());
+export const fnType = ({in: inType, out: outType}) => {
+  const m2 = fnTypes.getdefault(inType, true);
+  if (m2.has(outType)) {
+    return m2.get(outType);
+  }
+  else {
+    const fnType = {in: inType, out: outType};
+    m2.set(outType, fnType);
+    return fnType;
+  }
+};
+
+// Global store of list types:
+const listTypes = new Map();
+export function getListType(elementType) {
+  if (listTypes.has(elementType)) {
+    // only generate each list type once
+    // this would not be necessary if we could define our own equality and hash functions on objects in JavaScript.
+    return listTypes.get(elementType);
+  }
+  else {
+    const type = Symbol('ListOf:' + elementType.toString());
+    listTypes.set(elementType, type);
+    return type;
+  }
+}
+
+// Global store of product types:
+const productTypes = new DefaultMap(() => new Map());
+export const prodType = (leftType, rightType) => {
+  const m2 = productTypes.getdefault(leftType, true);
+  if (m2.has(rightType)) {
+    return m2.get(rightType);
+  }
+  else {
+    const pt = Symbol(`${leftType.toString()} × ${rightType.toString()}`);
+    m2.set(rightType, pt);
+    return pt;
+  }
+}
+
+// Global store of sum types:
+const sumTypes = new DefaultMap(() => new Map());
+export const sumType = (leftType, rightType) => {
+  const m2 = sumTypes.getdefault(leftType, true);
+  if (m2.has(rightType)) {
+    return m2.get(rightType);
+  }
+  else {
+    const st = Symbol(`${leftType.toString()} + ${rightType.toString()}`);
+    m2.set(rightType, st);
+    return st;
+  }
+}