Adding ReaderT#and, category composition between ReaderTs

jnape · jnape · commit d81d23563f5b · 2020-02-08T16:16:13.000-06:00
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -14,6 +14,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/).
 - `Id#id` overload that accepts an argument and returns it
 - `MaybeT#or`, choose the first `MaybeT` that represents an effect around `just` a value
 - `StateMatcher, StateTMatcher, WriterTMatcher`
+- `ReaderT#and`, category composition between `ReaderT` instances: `(a -> m b) -> (b -> m c) -> (a -> m c)`
 
 ## [5.1.0] - 2019-10-13
 ### Changed
diff --git a/src/main/java/com/jnape/palatable/lambda/monad/transformer/builtin/ReaderT.java b/src/main/java/com/jnape/palatable/lambda/monad/transformer/builtin/ReaderT.java
@@ -61,6 +61,18 @@ public <MA extends MonadRec<A, M>, N extends MonadRec<?, N>, B> ReaderT<R, N, B>
         return readerT(r -> fn.apply(runReaderT(r).coerce()));
     }
 
+    /**
+     * Left-to-right composition between {@link ReaderT} instances running under the same effect and compatible between
+     * their inputs and outputs.
+     *
+     * @param amb the next {@link ReaderT} to run
+     * @param <B> the final output type
+     * @return the composed {@link ReaderT}
+     */
+    public <B> ReaderT<R, M, B> and(ReaderT<A, M, B> amb) {
+        return readerT(r -> runReaderT(r).flatMap(amb::runReaderT));
+    }
+
     /**
      * {@inheritDoc}
      */
diff --git a/src/test/java/com/jnape/palatable/lambda/monad/transformer/builtin/ReaderTTest.java b/src/test/java/com/jnape/palatable/lambda/monad/transformer/builtin/ReaderTTest.java
@@ -8,12 +8,7 @@
 import com.jnape.palatable.traitor.runners.Traits;
 import org.junit.Test;
 import org.junit.runner.RunWith;
-import testsupport.traits.ApplicativeLaws;
-import testsupport.traits.Equivalence;
-import testsupport.traits.FunctorLaws;
-import testsupport.traits.MonadLaws;
-import testsupport.traits.MonadReaderLaws;
-import testsupport.traits.MonadRecLaws;
+import testsupport.traits.*;
 
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.Executors;
@@ -58,6 +53,15 @@ public void mapReaderT() {
                              .runReaderT("foo"));
     }
 
+    @Test
+    public void andComposesLeftToRight() {
+        ReaderT<Integer, Identity<?>, Float> intToFloat    = readerT(x -> new Identity<>(x.floatValue()));
+        ReaderT<Float, Identity<?>, Double>  floatToDouble = readerT(f -> new Identity<>(f.doubleValue()));
+
+        assertEquals(new Identity<>(1.),
+                     intToFloat.and(floatToDouble).runReaderT(1));
+    }
+
     @Test
     public void staticPure() {
         ReaderT<String, Identity<?>, Integer> readerT =
