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");

tests/enum.js

@@ -1,6 +1,7 @@
 import { makeTypeConstructor } from "../lib/meta/type_constructor.js";
 import { Char, Double, Unit } from "../lib/primitives/primitive_types.js";
 import { unit } from "../lib/primitives/unit.js";
+import { makeNiceMatchFn, symbolElse } from "../lib/structures/enum.js";
 import { makeModuleEnum } from "../lib/structures/enum.types.js";
 import { newProduct } from "../lib/structures/product.js";
 import { lsType } from "../lib/structures/type_constructors.types.js";
@@ -44,3 +45,13 @@ const description = symptom => matchSymptom(symptom)
 assert.strictEqual("Fever (38.5 degrees)", description(fever));
 assert.strictEqual("Confused"            , description(confused));
 assert.strictEqual("Other: sweating"     , description(other));
+
+// looks almost the same, but no currying, and the ability to have 'else'
+const description2 = makeNiceMatchFn(variants)({
+  fever: fever => `Fever (${fever} degrees)`,
+  [symbolElse]: _ => `Something else`,
+});
+
+assert.strictEqual("Fever (38.5 degrees)", description2(fever));
+assert.strictEqual("Something else"      , description2(confused));
+assert.strictEqual("Something else"      , description2(other));