Lens#both for dually focusing with two lenses at once

jnape · jnape · commit 5d451d9aa15f · 2018-02-25T15:02:11.000-06:00
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -24,6 +24,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/).
 - `MagnetizeBy` and `Magnetize`, for grouping elements by pairwise predicate tests
 - `Both`, for dually applying two functions and producing a `Tuple2` of their results
 - `Lens#both`, for dually focusing with two lenses at once
+- `IfThenElse`, an expression form for `if` statements
 
 ### Deprecated
 - `Either#trying` in favor of `Try#trying` 
diff --git a/src/main/java/com/jnape/palatable/lambda/functions/builtin/fn4/IfThenElse.java b/src/main/java/com/jnape/palatable/lambda/functions/builtin/fn4/IfThenElse.java
@@ -0,0 +1,50 @@
+package com.jnape.palatable.lambda.functions.builtin.fn4;
+
+import com.jnape.palatable.lambda.functions.Fn1;
+import com.jnape.palatable.lambda.functions.Fn2;
+import com.jnape.palatable.lambda.functions.Fn3;
+import com.jnape.palatable.lambda.functions.Fn4;
+
+import java.util.function.Function;
+
+public final class IfThenElse<A, B> implements Fn4<Function<? super A, Boolean>, Function<? super A, ? extends B>, Function<? super A, ? extends B>, A, B> {
+
+    private static final IfThenElse INSTANCE = new IfThenElse();
+
+    private IfThenElse() {
+    }
+
+    @Override
+    public B apply(Function<? super A, Boolean> predicate, Function<? super A, ? extends B> thenCase,
+                   Function<? super A, ? extends B> elseCase, A a) {
+        return predicate.apply(a) ? thenCase.apply(a) : elseCase.apply(a);
+    }
+
+    @SuppressWarnings("unchecked")
+    public static <A, B> IfThenElse<A, B> ifThenElse() {
+        return INSTANCE;
+    }
+
+    public static <A, B> Fn3<Function<? super A, ? extends B>, Function<? super A, ? extends B>, A, B> ifThenElse(
+            Function<? super A, Boolean> predicate) {
+        return IfThenElse.<A, B>ifThenElse().apply(predicate);
+    }
+
+    public static <A, B> Fn2<Function<? super A, ? extends B>, A, B> ifThenElse(
+            Function<? super A, Boolean> predicate, Function<? super A, ? extends B> thenCase) {
+        return IfThenElse.<A, B>ifThenElse(predicate).apply(thenCase);
+    }
+
+    public static <A, B> Fn1<A, B> ifThenElse(
+            Function<? super A, Boolean> predicate, Function<? super A, ? extends B> thenCase,
+            Function<? super A, ? extends B> elseCase) {
+        return IfThenElse.<A, B>ifThenElse(predicate, thenCase).apply(elseCase);
+    }
+
+    public static <A, B> B ifThenElse(
+            Function<? super A, Boolean> predicate, Function<? super A, ? extends B> thenCase,
+            Function<? super A, ? extends B> elseCase,
+            A a) {
+        return ifThenElse(predicate, thenCase, elseCase).apply(a);
+    }
+}
diff --git a/src/test/java/com/jnape/palatable/lambda/functions/builtin/fn4/IfThenElseTest.java b/src/test/java/com/jnape/palatable/lambda/functions/builtin/fn4/IfThenElseTest.java
@@ -0,0 +1,21 @@
+package com.jnape.palatable.lambda.functions.builtin.fn4;
+
+import com.jnape.palatable.lambda.functions.Fn1;
+import com.jnape.palatable.lambda.functions.specialized.Predicate;
+import org.junit.Test;
+
+import static com.jnape.palatable.lambda.functions.builtin.fn4.IfThenElse.ifThenElse;
+import static org.junit.Assert.assertEquals;
+
+public class IfThenElseTest {
+
+    @Test
+    public void standardLogic() {
+        Predicate<Integer> even = x -> x % 2 == 0;
+        Fn1<Integer, Integer> inc = x -> x + 1;
+        Fn1<Integer, Integer> dec = x -> x - 1;
+
+        assertEquals((Integer) 3, ifThenElse(even, inc, dec, 2));
+        assertEquals((Integer) 0, ifThenElse(even, inc, dec, 1));
+    }
+}
