WIP

assert.ts

@@ -1,4 +1,4 @@
-import type { Predicate } from "./type.ts";
+import type { Notifier, Predicate } from "./type.ts";
 
 /**
  * A factory function that generates assertion error messages.
@@ -7,6 +7,7 @@ export type AssertMessageFactory = (
   x: unknown,
   pred: Predicate<unknown>,
   name?: string,
+  keys?: PropertyKey[],
 ) => string;
 
 /**
@@ -20,9 +21,8 @@ export const defaultAssertMessageFactory: AssertMessageFactory = (
   const p = pred.name || "anonymous predicate";
   const t = typeof x;
   const v = JSON.stringify(x, null, 2);
-  return `Expected ${
-    name ?? "a value"
-  } that satisfies the predicate ${p}, got ${t}: ${v}`;
+  const n = name ?? "a value";
+  return `Expected ${n} that satisfies the predicate ${p}, got ${t}: ${v}`;
 };
 
 let assertMessageFactory = defaultAssertMessageFactory;
@@ -94,9 +94,26 @@ export function assert<T>(
   pred: Predicate<T>,
   options: { message?: string; name?: string } = {},
 ): asserts x is T {
-  if (!pred(x)) {
+  const ns: [
+    key: PropertyKey,
+    value: unknown,
+    pred: Predicate<unknown>,
+  ][] = [];
+  const notifier: Notifier = (key, value, pred) => {
+    ns.unshift([key, value, pred]);
+  };
+  if (!pred(x, notifier)) {
+    const cause = ns.length === 0 ? undefined : {
+      path: ns.map(([k]) => k),
+      value: ns[0][1],
+      pred: ns[0][2],
+    };
     throw new AssertError(
-      options.message ?? assertMessageFactory(x, pred, options.name),
+      options.message ?? assertMessageFactory(
+        x,
+        pred,
+        options.name,
+      ),
     );
   }
 }

assert_test.ts

@@ -5,6 +5,7 @@ import {
   defaultAssertMessageFactory,
   setAssertMessageFactory,
 } from "./assert.ts";
+import { is } from "./is/mod.ts";
 
 const x: unknown = Symbol("x");
 
@@ -61,6 +62,21 @@ Deno.test("assert", async (t) => {
       );
     },
   );
+
+  await t.step("throws an `AssertError` on isObjectOf", () => {
+    const pred = is.ObjectOf({
+      a: is.ObjectOf({
+        b: is.ObjectOf({
+          c: is.String,
+        }),
+      }),
+    });
+    assertThrows(
+      () => assert({ a: { b: { c: 0 } } }, pred),
+      AssertError,
+      `Expected a value that satisfies the predicate falsePredicate, got symbol: undefined`,
+    );
+  });
 });
 
 Deno.test("setAssertMessageFactory", async (t) => {

is/__snapshots__/object_of_test.ts.snap

@@ -33,3 +33,66 @@ snapshot[`isObjectOf<T> > with symbol properties > returns properly named predic
   })
 })"
 `;
+
+snapshot[`isObjectOf<T> > with 'notifier' argument > it is not invoked when x satisfies the predicate 1`] = `[]`;
+
+snapshot[`isObjectOf<T> > with 'notifier' argument > it is invoked when x does not satisfies the predicate (c) 1`] = `
+[
+  [
+    "c",
+    0,
+    [Function: isString],
+  ],
+  [
+    "b",
+    {
+      c: 0,
+    },
+    [Function: isObjectOf({c: isString})],
+  ],
+  [
+    "a",
+    {
+      b: {
+        c: 0,
+      },
+    },
+    [Function: isObjectOf({
+  b: isObjectOf({c: isString})
+})],
+  ],
+]
+`;
+
+snapshot[`isObjectOf<T> > with 'notifier' argument > it is invoked when x does not satisfies the predicate (b) 1`] = `
+[
+  [
+    "b",
+    0,
+    [Function: isObjectOf({c: isString})],
+  ],
+  [
+    "a",
+    {
+      b: 0,
+    },
+    [Function: isObjectOf({
+  b: isObjectOf({c: isString})
+})],
+  ],
+]
+`;
+
+snapshot[`isObjectOf<T> > with 'notifier' argument > it is invoked when x does not satisfies the predicate (a) 1`] = `
+[
+  [
+    "a",
+    0,
+    [Function: isObjectOf({
+  b: isObjectOf({c: isString})
+})],
+  ],
+]
+`;
+
+snapshot[`isObjectOf<T> > with 'notifier' argument > it is invoked when x does not satisfies the predicate (<root>) 1`] = `[]`;

is/object_of.ts

@@ -44,9 +44,14 @@ export function isObjectOf<
     ...Object.getOwnPropertySymbols(predObj),
   ].map((k) => [k, predObj[k]]);
   const pred = rewriteName(
-    (x): x is ObjectOf<T> => {
+    (x, notifier): x is ObjectOf<T> => {
       if (!isRecordT<T>(x)) return false;
-      return preds.every(([k, pred]) => pred(x[k]));
+      return preds.every(([k, pred]) => {
+        const v = x[k];
+        if (pred(v, notifier)) return true;
+        notifier?.(k, v, pred);
+        return false;
+      });
     },
     "isObjectOf",
     predObj,

is/object_of_test.ts

@@ -2,6 +2,7 @@ import { assertEquals } from "@std/assert";
 import { assertSnapshot } from "@std/testing/snapshot";
 import { assertType } from "@std/testing/types";
 import type { Equal } from "../_testutil.ts";
+import type { Notifier, Predicate } from "../type.ts";
 import { is } from "./mod.ts";
 import { as } from "../as/mod.ts";
 import { isObjectOf } from "./object_of.ts";
@@ -268,4 +269,74 @@ Deno.test("isObjectOf<T>", async (t) => {
       }
     });
   });
+
+  await t.step("with 'notifier' argument", async (t) => {
+    const pred = isObjectOf({
+      a: isObjectOf({
+        b: isObjectOf({
+          c: is.String,
+        }),
+      }),
+    });
+
+    await t.step(
+      "it is not invoked when x satisfies the predicate",
+      async (t) => {
+        const notified: [PropertyKey, unknown, Predicate<unknown>][] = [];
+        const notifier: Notifier = (key, value, pred) => {
+          notified.push([key, value, pred]);
+        };
+        assertEquals(pred({ a: { b: { c: "string" } } }, notifier), true);
+        await assertSnapshot(t, notified);
+      },
+    );
+
+    await t.step(
+      "it is not invoked when x does not satisfies the predicate (<root>)",
+      async (t) => {
+        const notified: [PropertyKey, unknown, Predicate<unknown>][] = [];
+        const notifier: Notifier = (key, value, pred) => {
+          notified.push([key, value, pred]);
+        };
+        assertEquals(pred(0, notifier), false);
+        await assertSnapshot(t, notified);
+      },
+    );
+
+    await t.step(
+      "it is invoked when x does not satisfies the predicate (a)",
+      async (t) => {
+        const notified: [PropertyKey, unknown, Predicate<unknown>][] = [];
+        const notifier: Notifier = (key, value, pred) => {
+          notified.push([key, value, pred]);
+        };
+        assertEquals(pred({ a: 0 }, notifier), false);
+        await assertSnapshot(t, notified);
+      },
+    );
+
+    await t.step(
+      "it is invoked when x does not satisfies the predicate (b)",
+      async (t) => {
+        const notified: [PropertyKey, unknown, Predicate<unknown>][] = [];
+        const notifier: Notifier = (key, value, pred) => {
+          notified.push([key, value, pred]);
+        };
+        assertEquals(pred({ a: { b: 0 } }, notifier), false);
+        await assertSnapshot(t, notified);
+      },
+    );
+
+    await t.step(
+      "it is invoked when x does not satisfies the predicate (c)",
+      async (t) => {
+        const notified: [PropertyKey, unknown, Predicate<unknown>][] = [];
+        const notifier: Notifier = (key, value, pred) => {
+          notified.push([key, value, pred]);
+        };
+        assertEquals(pred({ a: { b: { c: 0 } } }, notifier), false);
+        await assertSnapshot(t, notified);
+      },
+    );
+  });
 });