nicer pattern matching

lib/structures/enum.js

@@ -1,4 +1,5 @@
 import { compareStrings } from "../compare/primitives.js";
+import { unit } from "../primitives/unit.js";
 import { capitalizeFirstLetter } from "../util/util.js";
 
 const eatParameters = (numParams, result) => {
@@ -33,6 +34,24 @@ const matchFn = (enumm, variantNames) => {
   };
 };
 
+export const symbolElse = Symbol('else');
+
+export const makeNiceMatchFn = (variantNames) => handlers => enumm => {
+  const incomplete = variantNames.filter(({l: variant}) => !handlers.hasOwnProperty(variant)).length > 0;
+  if (incomplete && !handlers.hasOwnProperty(symbolElse)) {
+    throw new Error("if not every variant is handled, must have 'else' handler");
+  }
+  if (!incomplete && handlers.hasOwnProperty(symbolElse)) {
+    console.warn("'else' handler will never be invoked because every variant is already handled");
+  }
+  for (const [variant, handler] of Object.entries(handlers)) {
+    if (enumm.variant === variant) {
+      return handler(enumm.value);
+    }
+  }
+  return handlers[symbolElse](unit);
+}
+
 export const makeConstructors = variantNames => {
   if (new Set(variantNames).size !== variantNames.length) {
     throw new Error("precondition failed: non-unique variant names");