3 files changed, 249 insertions, 0 deletions

diff --git a/src/main/java/org/pacien/lemonad/validation/ValidationResult.java b/src/main/java/org/pacien/lemonad/validation/ValidationResult.java
new file mode 100644
index 0000000..65bb389
--- /dev/null
+++ b/src/main/java/org/pacien/lemonad/validation/ValidationResult.java
@@ -0,0 +1,129 @@
+/*
+ * lemonad - Some functional sweetness for Java
+ * Copyright (C) 2019  Pacien TRAN-GIRARD
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package org.pacien.lemonad.validation;
+
+import org.pacien.lemonad.attempt.Attempt;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.Objects;
+import java.util.function.BiConsumer;
+import java.util.function.Consumer;
+import java.util.function.Function;
+import java.util.stream.Stream;
+
+import lombok.NonNull;
+
+import static java.util.stream.Collectors.toUnmodifiableList;
+import static org.pacien.lemonad.attempt.Attempt.failure;
+import static org.pacien.lemonad.attempt.Attempt.success;
+
+/**
+ * Wraps the result of the validation of a subject.
+ *
+ * @param <S> the subject type,
+ * @param <E> the error type.
+ * @author pacien
+ */
+public interface ValidationResult<S, E> {
+  /**
+   * @return whether no error have been reported during the validation.
+   */
+  boolean isValid();
+
+  /**
+   * @return whether some error have been reported during the validation.
+   */
+  boolean isInvalid();
+
+  /**
+   * @return the subject of the validation.
+   */
+  S getSubject();
+
+  /**
+   * @return the potentially empty list of reported validation errors.
+   */
+  List<E> getErrors();
+
+  /**
+   * @param consumer a subject consumer called if the validation is successful.
+   * @return the current object.
+   */
+  default ValidationResult<S, E> ifValid(@NonNull Consumer<? super S> consumer) {
+    if (isValid()) consumer.accept(getSubject());
+    return this;
+  }
+
+  /**
+   * @param consumer the consumer called with the validation subject and reported errors if the validation is failed.
+   * @return the current object.
+   */
+  default ValidationResult<S, E> ifInvalid(@NonNull BiConsumer<? super S, ? super List<? super E>> consumer) {
+    if (!isValid()) consumer.accept(getSubject(), getErrors());
+    return this;
+  }
+
+  /**
+   * @return an {@link Attempt} with a state corresponding to the one of the validation.
+   */
+  default Attempt<S, List<E>> toAttempt() {
+    return isValid() ? success(getSubject()) : failure(getErrors());
+  }
+
+  /**
+   * @param mapper a function transforming a {@link ValidationResult}.
+   * @return the transformed {@link ValidationResult}.
+   */
+  default <SS, EE> ValidationResult<SS, EE> flatMap(@NonNull Function<? super ValidationResult<? super S, ? super E>, ? extends ValidationResult<? extends SS, ? extends EE>> mapper) {
+    //noinspection unchecked
+    return (ValidationResult<SS, EE>) mapper.apply(this);
+  }
+
+  /**
+   * @param subject           an overriding subject.
+   * @param validationResults a {@link Stream} of {@link ValidationResult}s to merge.
+   * @return the merged {@link ValidationResult} containing all errors from the supplied ones.
+   */
+  static <S, E> ValidationResult<S, E> merge(S subject, @NonNull Stream<? extends ValidationResult<?, ? extends E>> validationResults) {
+    return new ValidationResultContainer<>(
+      subject,
+      validationResults.flatMap(res -> res.getErrors().stream()).collect(toUnmodifiableList()));
+  }
+
+  /**
+   * @param subject the suject of the validation.
+   * @return a successful {@link ValidationResult}.
+   */
+  static <S, E> ValidationResult<S, E> valid(S subject) {
+    return new ValidationResultContainer<>(subject, List.of());
+  }
+
+  /**
+   * @param subject the suject of the validation.
+   * @param error   a validation error.
+   * @param errors  additional validation errors.
+   * @return a failed {@link ValidationResult} for the supplied subject.
+   */
+  @SafeVarargs static <S, E> ValidationResult<S, E> invalid(S subject, E error, E... errors) {
+    return new ValidationResultContainer<>(
+      subject,
+      Stream.concat(Stream.of(error), Arrays.stream(errors)).map(Objects::requireNonNull).collect(toUnmodifiableList()));
+  }
+}
diff --git a/src/main/java/org/pacien/lemonad/validation/ValidationResultContainer.java b/src/main/java/org/pacien/lemonad/validation/ValidationResultContainer.java
new file mode 100644
index 0000000..2c752f6
--- /dev/null
+++ b/src/main/java/org/pacien/lemonad/validation/ValidationResultContainer.java
@@ -0,0 +1,40 @@
+/*
+ * lemonad - Some functional sweetness for Java
+ * Copyright (C) 2019  Pacien TRAN-GIRARD
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package org.pacien.lemonad.validation;
+
+import java.util.List;
+
+import lombok.NonNull;
+import lombok.Value;
+
+/**
+ * @author pacien
+ */
+@Value class ValidationResultContainer<S, E> implements ValidationResult<S, E> {
+  S subject;
+  @NonNull List<E> errors;
+
+  @Override public boolean isValid() {
+    return errors.isEmpty();
+  }
+
+  @Override public boolean isInvalid() {
+    return !isValid();
+  }
+}
diff --git a/src/main/java/org/pacien/lemonad/validation/Validator.java b/src/main/java/org/pacien/lemonad/validation/Validator.java
new file mode 100644
index 0000000..a8a9e3f
--- /dev/null
+++ b/src/main/java/org/pacien/lemonad/validation/Validator.java
@@ -0,0 +1,80 @@
+/*
+ * lemonad - Some functional sweetness for Java
+ * Copyright (C) 2019  Pacien TRAN-GIRARD
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package org.pacien.lemonad.validation;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.Objects;
+import java.util.function.Function;
+import java.util.function.Predicate;
+
+import lombok.NonNull;