From 5b09c1b52597051ad319f0c3333c4e4200ce669b Mon Sep 17 00:00:00 2001
From: Paul Campbell <pcampbell@kemitix.net>
Date: Thu, 19 Aug 2021 13:46:22 +0100
Subject: [PATCH] Add Reader.map and Reader.andThen methods (#227)

---
 .../java/net/kemitix/mon/reader/Reader.java   | 27 +++++++++++
 src/test/java/net/kemitix/mon/ReaderTest.java | 45 +++++++++++++++++++
 2 files changed, 72 insertions(+)

diff --git a/src/main/java/net/kemitix/mon/reader/Reader.java b/src/main/java/net/kemitix/mon/reader/Reader.java
index 5a9c7a4..4ae091b 100644
--- a/src/main/java/net/kemitix/mon/reader/Reader.java
+++ b/src/main/java/net/kemitix/mon/reader/Reader.java
@@ -2,6 +2,9 @@ package net.kemitix.mon.reader;
 
 import org.apiguardian.api.API;
 
+import java.util.function.BiFunction;
+import java.util.function.Function;
+
 /**
  * Returns a program ready to run upon the supply of a suitable environment.
  *
@@ -20,4 +23,28 @@ public interface Reader<E, R> {
      */
     R run(E env);
 
+    /**
+     * Applies the function provided to the reader when it is run.
+     *
+     * @param f the function, which takes an {@link E} as its only parameter
+     * @param <V> the type of the functions output
+     * @return a new Reader to provide the result of the supplied function
+     */
+    @API(status = API.Status.EXPERIMENTAL)
+    default <V> Reader<E, V> map(Function<R, V> f) {
+        return e -> f.apply(run(e));
+    }
+
+    /**
+     * Applies the function provided to the reader when it is run.
+     *
+     * @param f the function, which takes an {@link E} and the previously
+     *          generated value as its two parameters
+     * @param <V> the type of the functions output
+     * @return a new Reader to provided the result of the supplied function
+     */
+    @API(status = API.Status.EXPERIMENTAL)
+    default <V> Reader<E, V> andThen(BiFunction<E, R, V> f) {
+        return env -> f.apply(env, run(env));
+    }
 }
diff --git a/src/test/java/net/kemitix/mon/ReaderTest.java b/src/test/java/net/kemitix/mon/ReaderTest.java
index 0cf801f..5313dd0 100644
--- a/src/test/java/net/kemitix/mon/ReaderTest.java
+++ b/src/test/java/net/kemitix/mon/ReaderTest.java
@@ -5,6 +5,8 @@ import org.assertj.core.api.WithAssertions;
 import org.junit.jupiter.api.DisplayName;
 import org.junit.jupiter.api.Test;
 
+import java.util.concurrent.atomic.AtomicInteger;
+
 public class ReaderTest
         implements WithAssertions {
 
@@ -30,6 +32,49 @@ public class ReaderTest
         assertThat(result).isEqualTo(value);
     }
 
+    @Test
+    @DisplayName("map")
+    void map() {
+        //given
+        AtomicInteger value = new AtomicInteger(123);
+        Environment env = new Environment() {
+            @Override
+            public Integer intValue() {
+                return value.incrementAndGet();
+            }
+        };
+        Reader<Environment, String> program =
+                ((Reader<Environment, Integer>) e -> e.intValue())
+                        .map(i -> i * 2)
+                        .map(i -> Integer.toString(i));
+        //when
+        String result = program.run(env);
+        //then
+        assertThat(result).isEqualTo("248");// (123 + 1) * 2
+    }
+
+    @Test
+    @DisplayName("andThen")
+    void andThen() {
+        //given
+        AtomicInteger value = new AtomicInteger(123);
+        Environment env = new Environment() {
+            @Override
+            public Integer intValue() {
+                return value.incrementAndGet();
+            }
+        };
+        Reader<Environment, String> program =
+                ((Reader<Environment, Integer>) e -> e.intValue())
+                        .andThen((e, i) -> e.intValue() + (i * 2))
+                        .andThen((e, i) -> String.format("%.2f",
+                                i.floatValue() / e.intValue()));
+        //when
+        String result = program.run(env);
+        //then
+        assertThat(result).isEqualTo("2.96");// ((123 * 2) + (123 + 1)) / (123 + 2)
+    }
+
     private interface Environment {
 
         Integer intValue();