Convert Reducer interface to Kotlin.

src/main/java/org/nwapw/abacus/tree/Reducer.java

@@ -1,19 +0,0 @@
-package org.nwapw.abacus.tree;
-
-/**
- * Interface used to reduce a tree into a single value.
- *
- * @param <T> the value to reduce into.
- */
-public interface Reducer<T> {
-
-    /**
-     * Reduces the given tree into a single value of type T.
-     *
-     * @param node     the node being passed in to be reduced.
-     * @param children the already-reduced children of this node.
-     * @return the resulting value from the reduce.
-     */
-    public T reduceNode(TreeNode node, Object... children);
-
-}

src/main/kotlin/org/nwapw/abacus/tree/FunctionNode.kt

@@ -16,7 +16,7 @@ data class FunctionNode(val function: String) : TreeNode() {
     val children: MutableList<TreeNode> = mutableListOf()
 
     override fun <T : Any> reduce(reducer: Reducer<T>): T? {
-        val children = Array<Any?>(children.size, { children[it].reduce(reducer) ?: return null; })
+        val children = Array<Any>(children.size, { children[it].reduce(reducer) ?: return null; })
         return reducer.reduceNode(this, *children)
     }
 

src/main/kotlin/org/nwapw/abacus/tree/Reducer.kt

@@ -0,0 +1,19 @@
+package org.nwapw.abacus.tree
+
+/**
+ * Reducer interface that takes a tree and returns a single value.
+ *
+ * The reducer walks the tree, visiting the children first, converting them into
+ * a value, and then attempts to reduce the parent. Eventually, the single final value is returned.
+ */
+interface Reducer <out T> {
+
+    /**
+     * Reduces the given tree node, given its already reduced children.
+     *
+     * @param treeNode the tree node to reduce.
+     * @param children the list of children, of type T.
+     */
+    fun reduceNode(treeNode: TreeNode, vararg children: Any) : T?
+
+}