From 762b1d0bd1613af07d8320fc4f01f508561b3ecb Mon Sep 17 00:00:00 2001
From: Alex Shelkovskiy <alexshelkov@gmail.com>
Date: Mon, 24 Jan 2022 02:29:48 +0600
Subject: [PATCH] Better isErr function

---
 package.json  |  2 +-
 src/checks.ts |  6 +++++-
 src/index.ts  |  2 +-
 src/utils.ts  | 11 +++++++++--
 4 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/package.json b/package.json
index 2b6434c..987768c 100644
--- a/package.json
+++ b/package.json
@@ -8,7 +8,7 @@
     "error",
     "error-handling"
   ],
-  "version": "1.0.20",
+  "version": "1.0.21",
   "author": "Alex Shelkovskiy <alexshelkov@gmail.com>",
   "repository": "https://github.com/alexshelkov/result",
   "license": "MIT",
diff --git a/src/checks.ts b/src/checks.ts
index 3a2d14f..9b2a3db 100644
--- a/src/checks.ts
+++ b/src/checks.ts
@@ -1,10 +1,14 @@
 import { Err } from './err';
 import { Failure, Success } from './result';
 
-export const isErr = (input: unknown): input is Err => {
+export const isUnknownErr = (input: unknown): input is Err => {
   return typeof input === 'object' && input !== null && 'type' in input;
 };
 
+export const isErr = <Error = Err>(input: Error): input is Error extends Err ? Error : never => {
+  return isUnknownErr(input);
+};
+
 export const isErrType = <
   Type extends string,
   Fail,
diff --git a/src/index.ts b/src/index.ts
index 3786ecc..861c6e6 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -2,7 +2,7 @@ export { ErrUtil as Err, Err as ErrInfo, ErrLevel, Errs } from './err';
 
 export { Success, Failure, Result, Response } from './result';
 
-export { isErr, isErrType, isSuccessLike, isFailureLike } from './checks';
+export { isUnknownErr, isErr, isErrType, isSuccessLike, isFailureLike } from './checks';
 
 export {
   ok,
diff --git a/src/utils.ts b/src/utils.ts
index 7361d29..3e59223 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -9,7 +9,14 @@ import {
   Response,
 } from './result';
 
-import { isPromise, isHaveStatus, isSuccessLike, isFailureLike, isErr } from './checks';
+import {
+  isPromise,
+  isHaveStatus,
+  isSuccessLike,
+  isFailureLike,
+  isErr,
+  isUnknownErr,
+} from './checks';
 
 import { Err, ErrUtil } from './err';
 
@@ -56,7 +63,7 @@ const maybeFailToResult = <Data, Fail>(
         prevResult.message,
         prevResult.code
       );
-    } else if (isErr(result)) {
+    } else if (isUnknownErr(result)) {
       exception = new FailureException(
         result.type,
         result as unknown as Fail,