From 279eced7ef32c8989f228f0f94ce7591cca45b55 Mon Sep 17 00:00:00 2001
From: Martin Wittlinger <wittlinger.martin@gmail.com>
Date: Tue, 19 Dec 2023 15:16:12 +0100
Subject: [PATCH] feat: Add result/error return type (#1369)

---
 .../laughing_train/commons/result/Error.java  | 99 +++++++++++++++++++
 .../laughing_train/commons/result/Ok.java     | 89 +++++++++++++++++
 .../laughing_train/commons/result/Result.java | 77 +++++++++++++++
 3 files changed, 265 insertions(+)
 create mode 100644 commons/src/main/java/io/github/martinwitt/laughing_train/commons/result/Error.java
 create mode 100644 commons/src/main/java/io/github/martinwitt/laughing_train/commons/result/Ok.java
 create mode 100644 commons/src/main/java/io/github/martinwitt/laughing_train/commons/result/Result.java

diff --git a/commons/src/main/java/io/github/martinwitt/laughing_train/commons/result/Error.java b/commons/src/main/java/io/github/martinwitt/laughing_train/commons/result/Error.java
new file mode 100644
index 000000000..eae2416d1
--- /dev/null
+++ b/commons/src/main/java/io/github/martinwitt/laughing_train/commons/result/Error.java
@@ -0,0 +1,99 @@
+package io.github.martinwitt.laughing_train.commons.result;
+
+import static java.util.Objects.requireNonNull;
+
+import java.util.Objects;
+import java.util.function.Consumer;
+import java.util.function.Function;
+import java.util.function.Supplier;
+
+class Error<T> implements Result<T> {
+
+  private final Throwable throwable;
+
+  Error(Throwable throwable) {
+    this.throwable = throwable;
+  }
+
+  @SuppressWarnings("unchecked")
+  private <E extends Throwable> T propagate(Throwable throwable) throws E {
+    throw (E) throwable;
+  }
+
+  @Override
+  public boolean isOk() {
+    return false;
+  }
+
+  @Override
+  public void ifOk(Consumer<? super T> consumer) {
+    // Do nothing when trying to consume the value of an Error result.
+  }
+
+  @Override
+  public boolean isError() {
+    return true;
+  }
+
+  @Override
+  public void ifError(Consumer<? super Throwable> consumer) {
+    requireNonNull(consumer, "The error consumer cannot be null");
+    consumer.accept(throwable);
+  }
+
+  @Override
+  public Result<T> switchIfError(Function<? super Throwable, ? extends Result<T>> fallbackMethod) {
+    requireNonNull(fallbackMethod, "The fallback method cannot be null");
+    return fallbackMethod.apply(throwable);
+  }
+
+  @Override
+  public <U> Result<U> map(Function<? super T, ? extends U> mapper) {
+    return new Error<>(throwable);
+  }
+
+  @Override
+  public <U> Result<U> flatMap(Function<? super T, ? extends Result<U>> mapper) {
+    return new Error<>(throwable);
+  }
+
+  @Override
+  public Result<T> mapError(Function<? super Throwable, ? extends Throwable> mapper) {
+    requireNonNull(mapper, "The error mapper cannot be null");
+    return new Error<>(mapper.apply(throwable));
+  }
+
+  @Override
+  public T get() {
+    return propagate(throwable);
+  }
+
+  @Override
+  public T getOrElse(Supplier<? extends T> supplier) {
+    requireNonNull(supplier);
+    return supplier.get();
+  }
+
+  @Override
+  public Throwable getError() {
+    return throwable;
+  }
+
+  @Override
+  public boolean equals(Object o) {
+    if (this == o) return true;
+    if (null == o || getClass() != o.getClass()) return false;
+    Error<?> error = (Error<?>) o;
+    return Objects.equals(throwable, error.throwable);
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(throwable);
+  }
+
+  @Override
+  public String toString() {
+    return "Error{" + "throwable=" + throwable + '}';
+  }
+}
diff --git a/commons/src/main/java/io/github/martinwitt/laughing_train/commons/result/Ok.java b/commons/src/main/java/io/github/martinwitt/laughing_train/commons/result/Ok.java
new file mode 100644
index 000000000..add3f81ec
--- /dev/null
+++ b/commons/src/main/java/io/github/martinwitt/laughing_train/commons/result/Ok.java
@@ -0,0 +1,89 @@
+package io.github.martinwitt.laughing_train.commons.result;
+
+import static java.util.Objects.requireNonNull;
+
+import java.util.NoSuchElementException;
+import java.util.Objects;
+import java.util.function.Consumer;
+import java.util.function.Function;
+import java.util.function.Supplier;
+
+class Ok<T> implements Result<T> {
+
+  private final T value;
+
+  Ok(T value) {
+    this.value = value;
+  }
+
+  @Override
+  public boolean isOk() {
+    return true;
+  }
+
+  @Override
+  public void ifOk(Consumer<? super T> consumer) {
+    requireNonNull(consumer, "The value consumer cannot be null");
+    consumer.accept(value);
+  }
+
+  @Override
+  public boolean isError() {
+    return false;
+  }
+
+  @Override
+  public void ifError(Consumer<? super Throwable> consumer) {
+    // Do nothing when trying to consume the error of an Ok result.
+  }
+
+  @Override
+  public Result<T> switchIfError(Function<? super Throwable, ? extends Result<T>> fallbackMethod) {
+    return new Ok<>(value);
+  }
+
+  @Override
+  public <U> Result<U> map(Function<? super T, ? extends U> mapper) {
+    requireNonNull(mapper, "The value mapper cannot be null");
+    return new Ok<>(mapper.apply(value));
+  }
+
+  @Override
+  public <U> Result<U> flatMap(Function<? super T, ? extends Result<U>> mapper) {
+    requireNonNull(mapper, "The value flat-mapper cannot be null");
+    return mapper.apply(value);
+  }
+
+  @Override
+  public Result<T> mapError(Function<? super Throwable, ? extends Throwable> mapper) {
+    return new Ok<>(value);
+  }
+
+  @Override
+  public T get() {
+    return value;
+  }
+
+  @Override
+  public T getOrElse(Supplier<? extends T> supplier) {
+    return value;
+  }
+
+  @Override
+  public Throwable getError() {
+    throw new NoSuchElementException("Result contains a value: " + value.toString());
+  }
+
+  @Override
+  public boolean equals(Object obj) {
+    if (this == obj) return true;
+    if (null == obj || getClass() != obj.getClass()) return false;
+    Ok<?> ok = (Ok<?>) obj;
+    return Objects.equals(value, ok.value);
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(value);
+  }
+}
diff --git a/commons/src/main/java/io/github/martinwitt/laughing_train/commons/result/Result.java b/commons/src/main/java/io/github/martinwitt/laughing_train/commons/result/Result.java
new file mode 100644
index 000000000..d6759ef1d
--- /dev/null
+++ b/commons/src/main/java/io/github/martinwitt/laughing_train/commons/result/Result.java
@@ -0,0 +1,77 @@
+package io.github.martinwitt.laughing_train.commons.result;
+
+import static java.util.Objects.nonNull;
+import static java.util.Objects.requireNonNull;
+
+import java.util.NoSuchElementException;
+import java.util.Optional;
+import java.util.function.Consumer;
+import java.util.function.Function;
+import java.util.function.Supplier;
+
+public interface Result<T> {
+
+  static <T> Result<T> ok(T value) {
+    requireNonNull(value, "The value of a Result cannot be null");
+    return new Ok<>(value);
+  }
+
+  static <T, E extends Throwable> Result<T> error(E throwable) {
+    requireNonNull(throwable, "The error of a Result cannot be null");
+    return new Error<>(throwable);
+  }
+
+  static <T> Result<T> of(Supplier<T> supplier) {
+    requireNonNull(supplier, "The value supplier cannot be null");
+
+    try {
+      return ok(supplier.get());
+    } catch (Exception error) {
+      return error(error);
+    }
+  }
+
+  @SuppressWarnings("OptionalUsedAsFieldOrParameterType")
+  static <T> Result<T> of(Optional<T> optional) {
+    requireNonNull(optional, "The optional value cannot be null");
+
+    return optional
+        .map(Result::ok)
+        .orElseGet(
+            () ->
+                error(new NoSuchElementException("No value present when unwrapping the optional")));
+  }
+
+  static <T> Result<T> ofNullable(T value) {
+    return ofNullable(
+        value, () -> new NullPointerException("The result was initialized with a null value"));
+  }
+
+  static <T> Result<T> ofNullable(T value, Supplier<? extends Throwable> errorSupplier) {
+    requireNonNull(errorSupplier, "The error supplier cannot be null");
+
+    return nonNull(value) ? ok(value) : error(errorSupplier.get());
+  }
+
+  boolean isOk();
+
+  void ifOk(Consumer<? super T> consumer);
+
+  boolean isError();
+
+  void ifError(Consumer<? super Throwable> consumer);
+
+  Result<T> switchIfError(Function<? super Throwable, ? extends Result<T>> fallbackMethod);
+
+  <U> Result<U> map(Function<? super T, ? extends U> mapper);
+
+  <U> Result<U> flatMap(Function<? super T, ? extends Result<U>> mapper);
+
+  Result<T> mapError(Function<? super Throwable, ? extends Throwable> mapper);
+
+  T get();
+
+  T getOrElse(Supplier<? extends T> supplier);
+
+  Throwable getError();
+}