Function isErr

isErr<R extends Result.Base>(result: R): result is NarrowToErr<R>
Checks if a Result is Result.Err. Acts as a type guard, narrowing the type to the error variant.

This function is essential for type-safe Result handling, allowing TypeScript to understand that subsequent operations will work with the error value rather than the success value.
Type Parameters
- R extends Result.Base
  The Result.Base type to check.
Parameters
- result: R
  The Result to check.
Returns result is NarrowToErr<R>
true if the Result is Result.Err, otherwise false.
Example
```
const operation = Result.ok(3);
const failure = Result.err('error');

if (Result.isOk(operation)) {
  const value: number = operation.value;
  assert(value === 3);
}

assert.ok(Result.isErr(failure));
```
- Defined in src/functional/result.mts:252