Merge pull request #37 from lambdalisue/better-default-message

👍 Improve the assertion error message

util.ts

@@ -1,5 +1,19 @@
 import type { Predicate } from "./is.ts";
 
+export type AssertMessageFactory = (
+  x: unknown,
+  pred: Predicate<unknown>,
+) => string;
+
+export const defaultAssertMessageFactory: AssertMessageFactory = (x, pred) => {
+  const p = pred.name || "anonymous predicate";
+  const t = typeof x;
+  const v = JSON.stringify(x, null, 2);
+  return `Expected a value that satisfies the predicate ${p}, got ${t}: ${v}`;
+};
+
+let assertMessageFactory = defaultAssertMessageFactory;
+
 /**
  * Represents an error that occurs when an assertion fails.
  */
@@ -19,6 +33,31 @@ export class AssertError extends Error {
   }
 }
 
+/**
+ * Sets the factory function used to generate assertion error messages.
+ * @param factory The factory function.
+ * @example
+ * ```ts
+ * import { setAssertMessageFactory } from "./util.ts";
+ * import is from "./is.ts";
+ *
+ * setAssertMessageFactory((x, pred) => {
+ *   if (pred === is.String) {
+ *     return `Expected a string, got ${typeof x}`;
+ *   } else if (pred === is.Number) {
+ *     return `Expected a number, got ${typeof x}`;
+ *   } else if (pred === is.Boolean) {
+ *     return `Expected a boolean, got ${typeof x}`;
+ *   } else {
+ *     return `Expected a value that satisfies the predicate, got ${typeof x}`;
+ *   }
+ * });
+ * ```
+ */
+export function setAssertMessageFactory(factory: AssertMessageFactory): void {
+  assertMessageFactory = factory;
+}
+
 /**
  * Asserts that the given value satisfies the provided predicate.
  *
@@ -44,7 +83,7 @@ export function assert<T>(
 ): asserts x is T {
   if (!pred(x)) {
     throw new AssertError(
-      options.message ?? "The value is not the expected type",
+      options.message ?? assertMessageFactory(x, pred),
     );
   }
 }

util_test.ts

@@ -2,24 +2,51 @@ import {
   assertStrictEquals,
   assertThrows,
 } from "https://deno.land/[email protected]/testing/asserts.ts";
-import { assert, AssertError, ensure, maybe } from "./util.ts";
+import {
+  assert,
+  AssertError,
+  defaultAssertMessageFactory,
+  ensure,
+  maybe,
+  setAssertMessageFactory,
+} from "./util.ts";
 
 const x: unknown = Symbol("x");
 
+function truePredicate(_x: unknown): _x is string {
+  return true;
+}
+
+function falsePredicate(_x: unknown): _x is string {
+  return false;
+}
+
 Deno.test("assert", async (t) => {
   await t.step("does nothing on true predicate", () => {
-    assert(x, (_x): _x is string => true);
+    assert(x, truePredicate);
   });
 
   await t.step("throws an `AssertError` on false predicate", () => {
-    assertThrows(() => assert(x, (_x): _x is string => false), AssertError);
+    assertThrows(
+      () => assert(x, falsePredicate),
+      AssertError,
+      `Expected a value that satisfies the predicate falsePredicate, got symbol: undefined`,
+    );
+  });
+
+  await t.step("throws an `AssertError` on false predicate", () => {
+    assertThrows(
+      () => assert(x, falsePredicate),
+      AssertError,
+      `Expected a value that satisfies the predicate falsePredicate, got symbol: undefined`,
+    );
   });
 
   await t.step(
     "throws an `AssertError` with a custom message on false predicate",
     () => {
       assertThrows(
-        () => assert(x, (_x): _x is string => false, { message: "Hello" }),
+        () => assert(x, falsePredicate, { message: "Hello" }),
         AssertError,
         "Hello",
       );
@@ -29,18 +56,22 @@ Deno.test("assert", async (t) => {
 
 Deno.test("ensure", async (t) => {
   await t.step("returns `x` as-is on true predicate", () => {
-    assertStrictEquals(ensure(x, (_x): _x is string => true), x);
+    assertStrictEquals(ensure(x, truePredicate), x);
   });
 
   await t.step("throws an `AssertError` on false predicate", () => {
-    assertThrows(() => ensure(x, (_x): _x is string => false), AssertError);
+    assertThrows(
+      () => ensure(x, falsePredicate),
+      AssertError,
+      `Expected a value that satisfies the predicate falsePredicate, got symbol: undefined`,
+    );
   });
 
   await t.step(
     "throws an `AssertError` with a custom message on false predicate",
     () => {
       assertThrows(
-        () => ensure(x, (_x): _x is string => false, { message: "Hello" }),
+        () => ensure(x, falsePredicate, { message: "Hello" }),
         AssertError,
         "Hello",
       );
@@ -50,10 +81,32 @@ Deno.test("ensure", async (t) => {
 
 Deno.test("maybe", async (t) => {
   await t.step("returns `x` as-is on true predicate", () => {
-    assertStrictEquals(maybe(x, (_x): _x is string => true), x);
+    assertStrictEquals(maybe(x, truePredicate), x);
   });
 
   await t.step("returns `undefined` on false predicate", () => {
-    assertStrictEquals(maybe(x, (_x): _x is string => false), undefined);
+    assertStrictEquals(maybe(x, falsePredicate), undefined);
   });
 });
+
+Deno.test("setAssertMessageFactory", async (t) => {
+  setAssertMessageFactory((x, pred) => `Hello ${typeof x} ${pred.name}`);
+
+  await t.step("change `AssertError` message on `assert` failure", () => {
+    assertThrows(
+      () => assert(x, falsePredicate),
+      AssertError,
+      "Hello symbol falsePredicate",
+    );
+  });
+
+  await t.step("change `AssertError` message on `ensure` failure", () => {
+    assertThrows(
+      () => ensure(x, falsePredicate),
+      AssertError,
+      "Hello symbol falsePredicate",
+    );
+  });
+
+  setAssertMessageFactory(defaultAssertMessageFactory);
+});