Add a custom UnwrapError type

Author: sck451

src/Option/None.ts

@@ -1,6 +1,7 @@
 import { Option, Some, some } from "./Option.ts";
 import { Err, err } from "../Result/Result.ts";
 import { OptionBase } from "./OptionBase.ts";
+import { UnwrapError } from "../UnwrapError/UnwrapError.ts";
 
 export function none<T = never>(): None<T> {
   return new None();
@@ -24,7 +25,7 @@ export class None<T = never> extends OptionBase<T> {
   }
 
   unwrap(message: string = "Called Option#unwrap on a None value"): T {
-    throw new Error(message);
+    throw new UnwrapError(message, null);
   }
 
   unwrapOr(defaultValue: T): T {

src/Result/Err.ts

@@ -1,6 +1,7 @@
 import { Ok, Result } from "./Result.ts";
 import { None, none, Some, some } from "../Option/Option.ts";
 import { ResultBase } from "./ResultBase.ts";
+import { UnwrapError } from "../UnwrapError/UnwrapError.ts";
 
 export function err<E>(error: E): Err<never, E> {
   return new Err(error);
@@ -63,7 +64,7 @@ export class Err<T = never, E = unknown> extends ResultBase<T, E> {
   *[Symbol.iterator](): Iterator<T> {}
 
   unwrap(message: string = `Expected ok() but got ${this}`): never {
-    throw new Error(message);
+    throw new UnwrapError(message, this.error);
   }
 
   unwrapErr(_message: string = `Expected error but got ${this}`): E {

src/Result/Ok.ts

@@ -1,6 +1,7 @@
 import { Err, Result } from "./Result.ts";
 import { None, none, Some, some } from "../Option/Option.ts";
 import { ResultBase } from "./ResultBase.ts";
+import { UnwrapError } from "../UnwrapError/UnwrapError.ts";
 
 export function ok<T>(value: T): Ok<T> {
   return new Ok(value);
@@ -69,7 +70,7 @@ export class Ok<T, E = never> extends ResultBase<T, E> {
   }
 
   unwrapErr(message: string = `Expected error but got ${this}`): never {
-    throw new Error(message);
+    throw new UnwrapError(message, this.value);
   }
 
   and<U>(resultB: Result<U, E>): Result<U, E> {

src/UnwrapError/UnwrapError.ts

@@ -0,0 +1,10 @@
+export class UnwrapError<E = unknown> extends Error {
+  error: E;
+
+  constructor(message: string, error: E) {
+    super();
+
+    this.message = message;
+    this.error = error;
+  }
+}

tests/option_none_test.ts

@@ -1,5 +1,6 @@
 import { expect, fn } from "@std/expect";
 import { none, type Option, some } from "../main.ts";
+import { UnwrapError } from "../src/UnwrapError/UnwrapError.ts";
 
 Deno.test("none() is None and not Some", () => {
   const option = none();
@@ -24,7 +25,7 @@ Deno.test("none().isNoneOr(n) returns true", () => {
 Deno.test("none().unwrap() throws", () => {
   const option: Option<number> = none();
 
-  expect(() => option.unwrap()).toThrow();
+  expect(() => option.unwrap()).toThrow(UnwrapError);
 });
 
 Deno.test("none().unwrapOr(n) returns n", () => {

tests/result_err_test.ts

@@ -1,5 +1,6 @@
 import { expect } from "@std/expect";
 import { err, ok, type Result } from "../main.ts";
+import { UnwrapError } from "../src/UnwrapError/UnwrapError.ts";
 
 Deno.test("err result is Err and not Ok", () => {
   const result = err(42);
@@ -94,7 +95,7 @@ Deno.test({
 });
 
 Deno.test("err().unwrap() throws", () => {
-  expect(() => err("problem").unwrap()).toThrow();
+  expect(() => err("problem").unwrap()).toThrow(UnwrapError);
 });
 
 Deno.test("err(n).unwrapErr() returns n", () => {

tests/result_ok_test.ts

@@ -1,5 +1,6 @@
 import { expect } from "@std/expect";
 import { err, ok, type Result } from "../main.ts";
+import { UnwrapError } from "../src/UnwrapError/UnwrapError.ts";
 
 Deno.test("OK result is Ok and not Err", () => {
   const result = ok(42);
@@ -93,7 +94,7 @@ Deno.test("ok(n).unwrap() returns n", () => {
 });
 
 Deno.test("ok().unwrapErr() throws", () => {
-  expect(() => ok(42).unwrapErr()).toThrow();
+  expect(() => ok(42).unwrapErr()).toThrow(UnwrapError);
 });
 
 Deno.test("ok().and(b) returns b", () => {