Add Result.errors utility

Counterpart to Result.successes

Author: Elias Mulhall

src/result.ts

@@ -99,6 +99,12 @@ export const withException = <V>(r: Result<V, any>): V => {
 export const successes = <A>(results: Result<A, any>[]): A[] =>
   results.reduce((acc: A[], r: Result<A, any>) => (r.ok === true ? acc.concat(r.result) : acc), []);
 
+/**
+ * Given an array of `Result`s, return the error values.
+ */
+export const errors = <E>(results: Result<any, E>[]): E[] =>
+  results.reduce((acc: E[], r: Result<any, E>) => (r.ok === false ? acc.concat(r.error) : acc), []);
+
 /**
  * Apply `f` to the result of an `Ok`, or pass the error through.
  */

test/json-decode.test.ts

@@ -1059,11 +1059,16 @@ describe('Result', () => {
     });
   });
 
-  it('can return successes from an array of decoded values', () => {
+  describe('can filter and unwrap arrays of Results', () => {
     const json: unknown = [1, true, 2, 3, 'five', 4, []];
-    const jsonArray: unknown[] = Result.withDefault([], array().run(json));
-    const numbers: number[] = Result.successes(jsonArray.map(number().run));
+    const numberResults = Result.withDefault([], array(result(number())).run(json));
 
-    expect(numbers).toEqual([1, 2, 3, 4]);
+    it('can return successes from an array of decoded values', () => {
+      expect(Result.successes(numberResults)).toEqual([1, 2, 3, 4]);
+    });
+
+    it('can return failures from an array of decoded values', () => {
+      expect(Result.errors(numberResults).map(({input}) => input)).toEqual([true, 'five', []]);
+    });
   });
 });