Added MonadError instance to EitherT

7h3kk1d · jnape · commit de1ae8a48078 · 2020-07-30T14:32:21.000-05:00
diff --git a/src/main/java/com/jnape/palatable/lambda/monad/transformer/builtin/EitherT.java b/src/main/java/com/jnape/palatable/lambda/monad/transformer/builtin/EitherT.java
@@ -10,6 +10,7 @@
 import com.jnape.palatable.lambda.functor.builtin.Compose;
 import com.jnape.palatable.lambda.functor.builtin.Lazy;
 import com.jnape.palatable.lambda.monad.Monad;
+import com.jnape.palatable.lambda.monad.MonadError;
 import com.jnape.palatable.lambda.monad.MonadRec;
 import com.jnape.palatable.lambda.monad.transformer.MonadT;
 
@@ -28,7 +29,8 @@
  */
 public final class EitherT<M extends MonadRec<?, M>, L, R> implements
         Bifunctor<L, R, EitherT<M, ?, ?>>,
-        MonadT<M, R, EitherT<M, L, ?>, EitherT<?, L, ?>> {
+        MonadT<M, R, EitherT<M, L, ?>, EitherT<?, L, ?>>,
+        MonadError<L, R, EitherT<M, L, ?>> {
 
     private final MonadRec<Either<L, R>, M> melr;
 
@@ -120,6 +122,24 @@ public <B> EitherT<M, L, R> discardR(Applicative<B, EitherT<M, L, ?>> appB) {
         return MonadT.super.discardR(appB).coerce();
     }
 
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    public EitherT<M, L, R> throwError(L l) {
+        return eitherT(melr.pure(left(l)));
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    public EitherT<M, L, R> catchError(Fn1<? super L, ? extends Monad<R, EitherT<M, L, ?>>> recoveryFn) {
+        return eitherT(runEitherT().flatMap(e -> e.match(
+                l -> recoveryFn.apply(l).<EitherT<M, L, R>>coerce().runEitherT(),
+                r -> melr.pure(r).fmap(Either::right))));
+    }
+
     /**
      * {@inheritDoc}
      */
diff --git a/src/test/java/com/jnape/palatable/lambda/monad/transformer/builtin/EitherTTest.java b/src/test/java/com/jnape/palatable/lambda/monad/transformer/builtin/EitherTTest.java
@@ -2,6 +2,7 @@
 
 import com.jnape.palatable.lambda.adt.Either;
 import com.jnape.palatable.lambda.adt.Unit;
+import com.jnape.palatable.lambda.functions.specialized.Pure;
 import com.jnape.palatable.lambda.functor.builtin.Identity;
 import com.jnape.palatable.lambda.io.IO;
 import com.jnape.palatable.traitor.annotations.TestTraits;
@@ -27,6 +28,7 @@
 import static com.jnape.palatable.lambda.io.IO.io;
 import static com.jnape.palatable.lambda.monad.transformer.builtin.EitherT.eitherT;
 import static com.jnape.palatable.lambda.monad.transformer.builtin.EitherT.liftEitherT;
+import static com.jnape.palatable.lambda.monad.transformer.builtin.EitherT.pureEitherT;
 import static com.jnape.palatable.traitor.framework.Subjects.subjects;
 import static org.junit.Assert.assertEquals;
 
@@ -73,4 +75,15 @@ public void staticPure() {
                 .apply(1);
         assertEquals(eitherT(new Identity<>(right(1))), eitherT);
     }
+
+    @Test
+    public void monadError() {
+        Pure<EitherT<Identity<?>, String, ?>> pure = pureEitherT(pureIdentity());
+
+        assertEquals(eitherT(new Identity<>(left("Hello"))),
+                     pure.<Integer, EitherT<Identity<?>, String, Integer>>apply(1).throwError("Hello"));
+        assertEquals(pure.apply(5),
+                     EitherT.<Identity<?>, String, Integer>eitherT(new Identity<>(left("Hello")))
+                             .catchError(s -> pure.apply(s.length())));
+    }
 }
