Added expectErr()

src/result.ts

@@ -25,6 +25,12 @@ interface BaseResult<T, E> extends Iterable<T extends Iterable<infer U> ? U : ne
      */
     expect(msg: string): T;
 
+    /**
+     * Returns the contained `Ok` value, if does not exist.  Throws an error if it does.
+     * @param msg the message to throw if Ok value.
+     */
+    expectErr(msg: string): T;
+
     /**
      * Returns the contained `Ok` value.
      * Because this function may throw, its use is generally discouraged.
@@ -136,6 +142,10 @@ export class ErrImpl<E> implements BaseResult<never, E> {
         throw new Error(`${msg} - Error: ${toString(this.val)}\n${this._stack}`);
     }
 
+    expectErr(_msg: string): E {
+        return this.val
+    }
+
     unwrap(): never {
         throw new Error(`Tried to unwrap Error: ${toString(this.val)}\n${this._stack}`);
     }
@@ -220,6 +230,10 @@ export class OkImpl<T> implements BaseResult<T, never> {
         return this.val;
     }
 
+    expectErr(msg: string): never {
+        throw new Error(msg);
+    }
+
     unwrap(): T {
         return this.val;
     }