Made err() infer strings narrowly for easier error tagging

.changeset/three-mice-act.md

@@ -0,0 +1,5 @@
+---
+"neverthrow": patch
+---
+
+Made err() infer strings narrowly for easier error tagging.

src/result.ts

@@ -62,7 +62,11 @@ export type Result<T, E> = Ok<T, E> | Err<T, E>
 
 export const ok = <T, E = never>(value: T): Ok<T, E> => new Ok(value)
 
-export const err = <T = never, E = unknown>(err: E): Err<T, E> => new Err(err)
+export function err<T = never, E extends string = string>(err: E): Err<T, E>
+export function err<T = never, E = unknown>(err: E): Err<T, E>
+export function err<T = never, E = unknown>(err: E): Err<T, E> {
+  return new Err(err)
+}
 
 /**
  * Evaluates the given generator to a Result returned or an Err yielded from it,

tests/typecheck-tests.ts

@@ -338,11 +338,11 @@ type CreateTuple<L, V = string> =
     });
 
     (function it(_ = 'combines only err results into one') {
-      type Expectation = Result<[ never, never ], number | string>;
+      type Expectation = Result<[ never, never ], number | 'abc'>;
 
       const result = Result.combine([
         err(1),
-        err('string'),
+        err('abc'),
       ]);
 
       const assignableToCheck: Expectation = result;
@@ -928,7 +928,7 @@ type CreateTuple<L, V = string> =
     });
 
     (function it(_ = 'combines only err results into one') {
-      type Expectation = Result<[ never, never ], [number, string]>;
+      type Expectation = Result<[ never, never ], [number, 'string']>;
 
       const result = Result.combineWithAllErrors([
         err(1),
@@ -999,6 +999,24 @@ type CreateTuple<L, V = string> =
       });
     });
   });
+
+  (function describe(_ = 'err') {
+    (function it(_ = 'infers the error type narrowly when it is a string') {
+      type Expectation = Result<never, 'error'>
+
+      const result = err('error')
+
+      const assignableToCheck: Expectation = result;
+    });
+
+    (function it(_ = 'infers the error type widely when it is not a string') {
+      type Expectation = Result<never, { abc: number }>
+
+      const result = err({ abc: 123 })
+
+      const assignableToCheck: Expectation = result;
+    });
+  })
 });