Implement the ability to reduce a tree to a single variable of a type.

src/org/nwapw/abacus/tree/NumberNode.java

@@ -44,4 +44,9 @@ public class NumberNode extends TreeNode {
     public NumberInterface getNumber() {
         return number;
     }
+
+    @Override
+    public <T> T reduce(Reducer<T> reducer) {
+        return reducer.reduceNode(this);
+    }
 }

src/org/nwapw/abacus/tree/OpNode.java

@@ -18,6 +18,8 @@ public class OpNode extends TreeNode {
      */
     private TreeNode right;
 
+    private OpNode() {}
+
     /**
      * Creates a new operation node with the given operation
      * and no child nodes.
@@ -79,4 +81,11 @@ public class OpNode extends TreeNode {
     public void setRight(TreeNode right) {
         this.right = right;
     }
+
+    @Override
+    public <T> T reduce(Reducer<T> reducer) {
+        T leftReduce = left.reduce(reducer);
+        T rightReduce = right.reduce(reducer);
+        return reducer.reduceNode(this, leftReduce, rightReduce);
+    }
 }

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

@@ -0,0 +1,17 @@
+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/org/nwapw/abacus/tree/TreeNode.java

@@ -137,4 +137,6 @@ public abstract class TreeNode {
         return fromStringRecursive(string, matches);
     }
 
+    public abstract <T> T reduce(Reducer<T> reducer);
+
 }