Adding $, function application represented as a higher-order Fn2

jnape · jnape · commit 8ac26203b600 · 2020-07-30T14:32:21.000-05:00
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -5,7 +5,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/).
 
 ## [Unreleased]
 
-There are currently no unreleased changes
+### Added
+- `$`, function application represented as a higher-order `Fn2`
 
 ## [5.2.0] - 2020-02-12
 
diff --git a/src/main/java/com/jnape/palatable/lambda/functions/builtin/fn2/$.java b/src/main/java/com/jnape/palatable/lambda/functions/builtin/fn2/$.java
@@ -0,0 +1,43 @@
+package com.jnape.palatable.lambda.functions.builtin.fn2;
+
+import com.jnape.palatable.lambda.functions.Fn1;
+import com.jnape.palatable.lambda.functions.Fn2;
+
+/**
+ * Function application, represented as a higher-order {@link Fn2} that receives an {@link Fn1} and its argument, and
+ * applies it. Useful for treating application as a combinator, e.g.:
+ * <pre>
+ * {@code
+ * List<Fn1<Integer, Integer>> fns     = asList(x -> x + 1, x -> x, x -> x - 1);
+ * List<Integer>               args    = asList(0, 1, 2);
+ * Iterable<Integer>           results = zipWith($(), fns, args); // [1, 1, 1]
+ * }
+ * </pre>
+ *
+ * @param <A> the applied {@link Fn1 Fn1's} input type
+ * @param <B> the applied {@link Fn1 Fn1's} output type
+ */
+public final class $<A, B> implements Fn2<Fn1<? super A, ? extends B>, A, B> {
+    private static final $<?, ?> INSTANCE = new $<>();
+
+    private $() {
+    }
+
+    @Override
+    public B checkedApply(Fn1<? super A, ? extends B> fn, A a) {
+        return fn.apply(a);
+    }
+
+    @SuppressWarnings("unchecked")
+    public static <A, B> $<A, B> $() {
+        return ($<A, B>) INSTANCE;
+    }
+
+    public static <A, B> Fn1<A, B> $(Fn1<? super A, ? extends B> fn) {
+        return $.<A, B>$().apply(fn);
+    }
+
+    public static <A, B> B $(Fn1<? super A, ? extends B> fn, A a) {
+        return $.<A, B>$(fn).apply(a);
+    }
+}
diff --git a/src/test/java/com/jnape/palatable/lambda/functions/builtin/fn2/$Test.java b/src/test/java/com/jnape/palatable/lambda/functions/builtin/fn2/$Test.java
@@ -0,0 +1,21 @@
+package com.jnape.palatable.lambda.functions.builtin.fn2;
+
+import org.junit.Test;
+
+import static com.jnape.palatable.lambda.functions.Fn2.fn2;
+import static com.jnape.palatable.lambda.functions.builtin.fn2.$.$;
+import static org.junit.Assert.assertEquals;
+
+public class $Test {
+
+    @Test
+    public void application() {
+        assertEquals((Integer) 1, $(x -> x + 1, 0));
+        assertEquals((Integer) 1, $.<Integer, Integer>$(x -> x + 1).apply(0));
+    }
+
+    @Test
+    public void curryingInference() {
+        assertEquals((Integer) 1, $($(fn2(Integer::sum), 0), 1));
+    }
+}
