Implement a Reader monad (#214)

* Add Reader monad

* WrapperTest: clean up

* Reader: add API status annotations

* Version set to 3.1.0

* pom: remove duplicate dependency

pom.xml

@@ -11,7 +11,7 @@
     </parent>
 
     <artifactId>mon</artifactId>
-    <version>3.0.0</version>
+    <version>3.1.0</version>
 
     <name>Mon</name>
     <description>Wrapper, TypeAlias, Maybe, Result, Tree, Lazy, Either and Combinators for Java.
@@ -60,7 +60,6 @@
             <artifactId>apiguardian-api</artifactId>
             <version>${apiguardian-api.version}</version>
         </dependency>
-
         <dependency>
             <groupId>org.junit.jupiter</groupId>
             <artifactId>junit-jupiter-api</artifactId>

src/main/java/net/kemitix/mon/reader/Reader.java

@@ -0,0 +1,23 @@
+package net.kemitix.mon.reader;
+
+import org.apiguardian.api.API;
+
+/**
+ * Returns a program ready to run upon the supply of a suitable environment.
+ *
+ * @param <E> The type of the environment required by the program.
+ * @param <R> The type of the result returned by the program.
+ */
+@API(status = API.Status.EXPERIMENTAL, since = "3.0.1")
+@FunctionalInterface
+public interface Reader<E, R> {
+
+    /**
+     * Executes the program.
+     *
+     * @param env the required environment
+     * @return the result of the program
+     */
+    R run(E env);
+
+}

src/main/java/net/kemitix/mon/reader/package-info.java

@@ -0,0 +1 @@
+package net.kemitix.mon.reader;

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

@@ -0,0 +1,38 @@
+package net.kemitix.mon;
+
+import net.kemitix.mon.reader.Reader;
+import org.assertj.core.api.WithAssertions;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+public class ReaderTest
+        implements WithAssertions {
+
+    @Test
+    @DisplayName("read a value and return it")
+    void readAndReturn() {
+        //given
+        Integer value = 123;
+        Environment env = new Environment() {
+            @Override
+            public Integer intValue() {
+                return value;
+            }
+        };
+        Reader<Environment, Integer> program = (Environment e) -> {
+            return e.intValue();
+        };
+
+        //when
+        Integer result = program.run(env);
+
+        //then
+        assertThat(result).isEqualTo(value);
+    }
+
+    private interface Environment {
+
+        Integer intValue();
+
+    }
+}

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

@@ -1,10 +1,7 @@
 package net.kemitix.mon;
 
-import org.junit.jupiter.api.Disabled;
-import org.junit.jupiter.api.DisplayName;
 import org.junit.jupiter.api.Test;
 
-import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
 
@@ -18,8 +15,6 @@ class WrapperTest {
 
     interface WrappedIterableString extends Wrapper<Iterable<String>> { }
 
-    interface WrappedListInteger extends Wrapper<List<Integer>> {}
-
     interface AWrapper extends Wrapper<String> {}
 
     @Test
@@ -65,6 +60,7 @@ class WrapperTest {
         final WrappedString wrappedString = () -> "1";
         final WrappedInteger wrappedInteger = () -> 1;
         //then
+        //noinspection AssertBetweenInconvertibleTypes
         assertThat(wrappedString).isNotEqualTo(wrappedInteger);
     }