Add Result.recover(Function) and Result.onError(Consumer)

src/main/java/net/kemitix/mon/result/Err.java

@@ -80,6 +80,16 @@ class Err<T> implements Result<T> {
         return this;
     }
 
+    @Override
+    public Result<T> recover(final Function<Throwable, Result<T>> f) {
+        return f.apply(error);
+    }
+
+    @Override
+    public void onError(final Consumer<Throwable> errorConsumer) {
+        errorConsumer.accept(error);
+    }
+
     @Override
     public boolean equals(final Object other) {
         return other instanceof Err && Objects.equals(error, ((Err) other).error);

src/main/java/net/kemitix/mon/result/Result.java

@@ -108,6 +108,7 @@ public interface Result<T> extends Functor<T, Result<?>> {
 
     /**
      * Swaps the inner Result of a Maybe, so that a Result is on the outside.
+     *
      * @param maybeResult the Maybe the contains a Result
      * @param <T>         the type of the value that may be in the Result
      * @return a Result containing a Maybe, the value in the Maybe was the value in a successful Result within the
@@ -177,4 +178,22 @@ public interface Result<T> extends Functor<T, Result<?>> {
      * @return this Result
      */
     Result<T> peek(Consumer<T> consumer);
+
+    /**
+     * Provide a way to attempt to recover from an error state.
+     *
+     * @param f the function to recover from the error
+     * @return a new Result, either a Success, or if recovery is not possible an other Err.
+     */
+    Result<T> recover(Function<Throwable, Result<T>> f);
+
+    /**
+     * A handler for error states.
+     *
+     * <p>When this is an error then tne Consumer will be supplier with the error. When this is a success, then nothing
+     * happens.</p>
+     *
+     * @param errorConsumer the consumer to handle the error
+     */
+    void onError(Consumer<Throwable> errorConsumer);
 }

src/main/java/net/kemitix/mon/result/Success.java

@@ -84,6 +84,16 @@ class Success<T> implements Result<T> {
         return this;
     }
 
+    @Override
+    public Result<T> recover(final Function<Throwable, Result<T>> f) {
+        return this;
+    }
+
+    @Override
+    public void onError(final Consumer<Throwable> errorConsumer) {
+        // do nothing - this is not an error
+    }
+
     @Override
     public boolean equals(final Object other) {
         return other instanceof Success && Objects.equals(value, ((Success) other).value);

src/test/java/net/kemitix/mon/ResultTest.java

@@ -452,6 +452,54 @@ public class ResultTest implements WithAssertions {
         assertThat(peeked).isSameAs(result);
     }
 
+    @Test public void success_whenOnError_thenIgnore() {
+        //given
+        final Result<Integer> ok = Result.ok(1);
+        //when
+        ok.onError(e -> fail("not an error"));
+    }
+
+    @Test public void error_whenOnError_thenConsume() {
+        //given
+        final RuntimeException exception = new RuntimeException();
+        final Result<Integer> error = Result.error(exception);
+        final AtomicReference<Throwable> capture = new AtomicReference<>();
+        //when
+        error.onError(capture::set);
+        //then
+        assertThat(capture).hasValue(exception);
+    }
+
+    @Test
+    public void success_whenRecover_thenNoChange() {
+        //given
+        final Result<Integer> ok = Result.ok(1);
+        //when
+        final Result<Integer> recovered = ok.recover(e -> Result.ok(2));
+        //then
+        recovered.peek(v -> assertThat(v).isEqualTo(1));
+    }
+
+    @Test
+    public void error_whenRecover_thenSuccess() {
+        //given
+        final Result<Integer> error = Result.error(new RuntimeException());
+        //when
+        final Result<Integer> recovered = error.recover(e -> Result.ok(2));
+        //then
+        recovered.peek(v -> assertThat(v).isEqualTo(2));
+    }
+
+    @Test
+    public void error_whenRecoverFails_thenUpdatedError() {
+        //given
+        final Result<Integer> error = Result.error(new RuntimeException("original"));
+        //when
+        final Result<Integer> recovered = error.recover(e -> Result.error(new RuntimeException("updated")));
+        //then
+        recovered.onError(e -> assertThat(e).hasMessage("updated"));
+    }
+
     @RequiredArgsConstructor
     private static class UseCase {