Remove nullability from tree nodes.

core/src/main/java/org/nwapw/abacus/parsing/ShuntingYardParser.java

@@ -154,19 +154,20 @@ public class ShuntingYardParser implements Parser<Match<TokenType>>, PluginListe
             return new VariableNode(match.getContent());
         } else if (matchType == TokenType.FUNCTION || matchType == TokenType.TREE_VALUE_FUNCTION) {
             String functionName = match.getContent();
-            CallNode node;
+            List<TreeNode> children = new ArrayList<>();
-            if (matchType == TokenType.FUNCTION) {
-                node = new FunctionNode(functionName);
-            } else {
-                node = new TreeValueFunctionNode(functionName);
-            }
             while (!matches.isEmpty() && matches.get(0).getType() != TokenType.INTERNAL_FUNCTION_END) {
                 TreeNode argument = constructRecursive(matches);
                 if (argument == null) return null;
-                node.getChildren().add(0, argument);
+                children.add(0, argument);
             }
             if (matches.isEmpty()) return null;
             matches.remove(0);
+            CallNode node;
+            if (matchType == TokenType.FUNCTION) {
+                node = new FunctionNode(functionName, children);
+            } else {
+                node = new TreeValueFunctionNode(functionName, children);
+            }
             return node;
         }
         return null;

core/src/main/kotlin/org/nwapw/abacus/tree/BinaryNode.kt

@@ -11,10 +11,10 @@ package org.nwapw.abacus.tree
  * @param left the left node.
  * @param right the right node.
  */
-abstract class BinaryNode(val operation: String, val left: TreeNode? = null, val right: TreeNode?) : TreeNode() {
+abstract class BinaryNode(val operation: String, val left: TreeNode, val right: TreeNode) : TreeNode() {
 
     override fun toString(): String {
-        return "(" + (left?.toString() ?: "null") + operation + (right?.toString() ?: "null") + ")"
+        return "(" + left.toString() + operation + right.toString() + ")"
     }
 
 }

core/src/main/kotlin/org/nwapw/abacus/tree/CallNode.kt

@@ -7,13 +7,9 @@ package org.nwapw.abacus.tree
  * to extend this functionality.
  *
  * @param callTo the name of the things being called.
+ * @param children the children of this node.
  */
-abstract class CallNode(val callTo: String) : TreeNode() {
+abstract class CallNode(val callTo: String, val children: List<TreeNode>) : TreeNode() {
-
-    /**
-     * The list of children this node has.
-     */
-    val children: MutableList<TreeNode> = mutableListOf()
 
     override fun toString(): String {
         val buffer = StringBuffer()

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

@@ -8,7 +8,7 @@ package org.nwapw.abacus.tree
  *
  * @param function the function string.
  */
-class FunctionNode(function: String) : CallNode(function) {
+class FunctionNode(function: String, children: List<TreeNode>) : CallNode(function, children) {
 
     override fun <T : Any> reduce(reducer: Reducer<T>): T? {
         val children = Array<Any>(children.size, { children[it].reduce(reducer) ?: return null; })

core/src/main/kotlin/org/nwapw/abacus/tree/NumberBinaryNode.kt

@@ -10,12 +10,12 @@ package org.nwapw.abacus.tree
  * @param left the left child of this node.
  * @param right the right child of this node.
  */
-class NumberBinaryNode(operation: String, left: TreeNode?, right: TreeNode?)
+class NumberBinaryNode(operation: String, left: TreeNode, right: TreeNode)
     : BinaryNode(operation, left, right) {
 
     override fun <T : Any> reduce(reducer: Reducer<T>): T? {
-        val left = left?.reduce(reducer) ?: return null
+        val left = left.reduce(reducer) ?: return null
-        val right = right?.reduce(reducer) ?: return null
+        val right = right.reduce(reducer) ?: return null
         return reducer.reduceNode(this, left, right)
     }
 

core/src/main/kotlin/org/nwapw/abacus/tree/NumberUnaryNode.kt

@@ -8,11 +8,11 @@ package org.nwapw.abacus.tree
  * @param operation the operation this node performs.
  * @param child the child this node should be applied to.
  */
-class NumberUnaryNode(operation: String, child: TreeNode?)
+class NumberUnaryNode(operation: String, child: TreeNode)
     : UnaryNode(operation, child) {
 
     override fun <T : Any> reduce(reducer: Reducer<T>): T? {
-        val child = applyTo?.reduce(reducer) ?: return null
+        val child = applyTo.reduce(reducer)
         return reducer.reduceNode(this, child)
     }
 

core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueBinaryNode.kt

@@ -11,7 +11,7 @@ package org.nwapw.abacus.tree
  * @param left the left child of this node.
  * @param right the right child of this node.
  */
-class TreeValueBinaryNode(operation: String, left: TreeNode?, right: TreeNode?)
+class TreeValueBinaryNode(operation: String, left: TreeNode, right: TreeNode)
     : BinaryNode(operation, left, right) {
 
     override fun <T : Any> reduce(reducer: Reducer<T>): T? {

core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueFunctionNode.kt

@@ -7,7 +7,7 @@ package org.nwapw.abacus.tree
  * is mostly to help the reducer. Besides that, this class also does not
  * even attempt to reduce its children.
  */
-class TreeValueFunctionNode(name: String) : CallNode(name) {
+class TreeValueFunctionNode(name: String, children: List<TreeNode>) : CallNode(name, children) {
 
     override fun <T : Any> reduce(reducer: Reducer<T>): T? {
         return reducer.reduceNode(this)

core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueUnaryNode.kt

@@ -9,7 +9,7 @@ package org.nwapw.abacus.tree
  * @param operation the operation this node performs.
  * @param child the node the operation should be applied to.
  */
-class TreeValueUnaryNode(operation: String, child: TreeNode?)
+class TreeValueUnaryNode(operation: String, child: TreeNode)
     : UnaryNode(operation, child) {
 
     override fun <T : Any> reduce(reducer: Reducer<T>): T? {

core/src/main/kotlin/org/nwapw/abacus/tree/UnaryNode.kt

@@ -9,10 +9,10 @@ package org.nwapw.abacus.tree
  * @param operation the operation applied to the given node.
  * @param applyTo the node to which the operation will be applied.
  */
-abstract class UnaryNode(val operation: String, val applyTo: TreeNode? = null) : TreeNode() {
+abstract class UnaryNode(val operation: String, val applyTo: TreeNode) : TreeNode() {
 
     override fun toString(): String {
-        return "(" + (applyTo?.toString() ?: "null") + ")" + operation
+        return "(" + applyTo.toString() + ")" + operation
     }
 
 }