diff --git a/core/src/main/java/org/nwapw/abacus/exception/AbacusException.java b/core/src/main/java/org/nwapw/abacus/exception/AbacusException.java
new file mode 100644
index 0000000..f71cfe1
--- /dev/null
+++ b/core/src/main/java/org/nwapw/abacus/exception/AbacusException.java
@@ -0,0 +1,9 @@
+package org.nwapw.abacus.exception;
+
+public class AbacusException extends RuntimeException {
+
+    public AbacusException(String baseMessage, String description){
+        super(baseMessage + ((description.equals("")) ? "." : (": " + description)));
+    }
+
+}
diff --git a/core/src/main/java/org/nwapw/abacus/number/ComputationInterruptedException.java b/core/src/main/java/org/nwapw/abacus/exception/ComputationInterruptedException.java
similarity index 56%
rename from core/src/main/java/org/nwapw/abacus/number/ComputationInterruptedException.java
rename to core/src/main/java/org/nwapw/abacus/exception/ComputationInterruptedException.java
index 2f7b3fd..5674a27 100644
--- a/core/src/main/java/org/nwapw/abacus/number/ComputationInterruptedException.java
+++ b/core/src/main/java/org/nwapw/abacus/exception/ComputationInterruptedException.java
@@ -1,16 +1,16 @@
-package org.nwapw.abacus.number;
+package org.nwapw.abacus.exception;
 
 /**
  * Exception thrown when the computation is interrupted by
  * the user.
  */
-public class ComputationInterruptedException extends RuntimeException {
+public class ComputationInterruptedException extends AbacusException {
 
     /**
      * Creates a new exception of this type.
      */
     public ComputationInterruptedException() {
-        super("Computation interrupted by user.");
+        super("Computation interrupted", "");
     }
 
 }
diff --git a/core/src/main/java/org/nwapw/abacus/function/DomainException.java b/core/src/main/java/org/nwapw/abacus/exception/DomainException.java
similarity index 71%
rename from core/src/main/java/org/nwapw/abacus/function/DomainException.java
rename to core/src/main/java/org/nwapw/abacus/exception/DomainException.java
index a7a449c..e1cf139 100644
--- a/core/src/main/java/org/nwapw/abacus/function/DomainException.java
+++ b/core/src/main/java/org/nwapw/abacus/exception/DomainException.java
@@ -1,24 +1,24 @@
-package org.nwapw.abacus.function;
+package org.nwapw.abacus.exception;
 
 /**
  * Exception thrown if the function parameters do not match
  * requirements.
  */
-public class DomainException extends RuntimeException {
+public class DomainException extends AbacusException {
 
     /**
      * Creates a new DomainException.
      * @param reason the reason for which the exception is thrown.
      */
     public DomainException(String reason) {
-        super(reason);
+        super("Domain error", reason);
     }
 
     /**
      * Creates a new DomainException with a default message.
      */
     public DomainException(){
-        this("Domain Error");
+        this("");
     }
 
 }
diff --git a/core/src/main/java/org/nwapw/abacus/function/EvaluationException.java b/core/src/main/java/org/nwapw/abacus/exception/EvaluationException.java
similarity index 74%
rename from core/src/main/java/org/nwapw/abacus/function/EvaluationException.java
rename to core/src/main/java/org/nwapw/abacus/exception/EvaluationException.java
index bad85b9..5eed066 100644
--- a/core/src/main/java/org/nwapw/abacus/function/EvaluationException.java
+++ b/core/src/main/java/org/nwapw/abacus/exception/EvaluationException.java
@@ -1,17 +1,17 @@
-package org.nwapw.abacus.function;
+package org.nwapw.abacus.exception;
 
 /**
  * An exception thrown primarily from Tree Value operators and functions,
  * which have to deal with the result of a Reducer as well as the results
  * of Applicable.
  */
-public class EvaluationException extends RuntimeException {
+public class EvaluationException extends AbacusException {
 
     /**
      * Creates a new EvaluationException with the default string.
      */
     public EvaluationException() {
-        super("Error evaluating expression.");
+        this("");
     }
 
     /**
@@ -19,7 +19,7 @@ public class EvaluationException extends RuntimeException {
      * @param message the message string.
      */
     public EvaluationException(String message) {
-        super(message);
+        super("Evaluation error", message);
     }
 
 }
diff --git a/core/src/main/java/org/nwapw/abacus/number/NumberInterface.java b/core/src/main/java/org/nwapw/abacus/number/NumberInterface.java
index 4a3fe5c..8727425 100755
--- a/core/src/main/java/org/nwapw/abacus/number/NumberInterface.java
+++ b/core/src/main/java/org/nwapw/abacus/number/NumberInterface.java
@@ -1,5 +1,7 @@
 package org.nwapw.abacus.number;
 
+import org.nwapw.abacus.exception.ComputationInterruptedException;
+
 /**
  * An interface used to represent a number.
  */
diff --git a/core/src/main/java/org/nwapw/abacus/parsing/ShuntingYardParser.java b/core/src/main/java/org/nwapw/abacus/parsing/ShuntingYardParser.java
index 9d326a8..611f5c9 100644
--- a/core/src/main/java/org/nwapw/abacus/parsing/ShuntingYardParser.java
+++ b/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;
-            if (matchType == TokenType.FUNCTION) {
-                node = new FunctionNode(functionName);
-            } else {
-                node = new TreeValueFunctionNode(functionName);
-            }
+            List<TreeNode> children = new ArrayList<>();
             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;
diff --git a/core/src/main/java/org/nwapw/abacus/plugin/StandardPlugin.java b/core/src/main/java/org/nwapw/abacus/plugin/StandardPlugin.java
index b3b16f0..3d20019 100755
--- a/core/src/main/java/org/nwapw/abacus/plugin/StandardPlugin.java
+++ b/core/src/main/java/org/nwapw/abacus/plugin/StandardPlugin.java
@@ -29,8 +29,7 @@ public class StandardPlugin extends Plugin {
         @Override
         public NumberInterface applyInternal(MutableEvaluationContext context, TreeNode[] params) {
             String assignTo = ((VariableNode) params[0]).getVariable();
-            NumberInterface value = params[1].reduce(context.getReducer());
-            if(value == null) throw new EvaluationException();
+            NumberInterface value = params[1].reduce(context.getInheritedReducer());
             context.setVariable(assignTo, value);
             return value;
         }
@@ -48,9 +47,7 @@ public class StandardPlugin extends Plugin {
         public NumberInterface applyInternal(MutableEvaluationContext context, TreeNode[] params) {
             String assignTo = ((VariableNode) params[0]).getVariable();
             context.setDefinition(assignTo, params[1]);
-            NumberInterface value = params[1].reduce(context.getReducer());
-            if(value == null) throw new EvaluationException();
-            return value;
+            return params[1].reduce(context.getInheritedReducer());
         }
     };
     /**
diff --git a/core/src/main/kotlin/org/nwapw/abacus/function/applicable/Applicable.kt b/core/src/main/kotlin/org/nwapw/abacus/function/applicable/Applicable.kt
index 8d824a8..0d64ba6 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/function/applicable/Applicable.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/function/applicable/Applicable.kt
@@ -1,7 +1,7 @@
 package org.nwapw.abacus.function.applicable
 
 import org.nwapw.abacus.context.MutableEvaluationContext
-import org.nwapw.abacus.function.DomainException
+import org.nwapw.abacus.exception.DomainException
 
 /**
  * A class that can be applied to arguments.
@@ -36,7 +36,8 @@ interface Applicable<in T : Any, out O : Any> {
      * @return the result of the operation, or null if parameters do not match.
      */
     fun apply(context: MutableEvaluationContext, vararg params: T): O {
-        if (!matchesParams(context, params)) throw DomainException()
+        if (!matchesParams(context, params))
+            throw DomainException("parameters do not match function requirements.")
         return applyInternal(context, params)
     }
 
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/BinaryNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/BinaryNode.kt
index 4d0d9dd..aa04571 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/BinaryNode.kt
+++ b/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() + ")"
     }
 
 }
\ No newline at end of file
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/CallNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/CallNode.kt
index 5f99494..38595dd 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/CallNode.kt
+++ b/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() {
-
-    /**
-     * The list of children this node has.
-     */
-    val children: MutableList<TreeNode> = mutableListOf()
+abstract class CallNode(val callTo: String, val children: List<TreeNode>) : TreeNode() {
 
     override fun toString(): String {
         val buffer = StringBuffer()
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/EvaluationResult.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/EvaluationResult.kt
index e94c9d6..a499877 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/EvaluationResult.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/EvaluationResult.kt
@@ -3,4 +3,4 @@ package org.nwapw.abacus.tree
 import org.nwapw.abacus.context.MutableEvaluationContext
 import org.nwapw.abacus.number.NumberInterface
 
-data class EvaluationResult(val value: NumberInterface?, val resultingContext: MutableEvaluationContext)
\ No newline at end of file
+data class EvaluationResult(val value: NumberInterface, val resultingContext: MutableEvaluationContext)
\ No newline at end of file
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/FunctionNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/FunctionNode.kt
index 813ad52..e4d2acb 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/FunctionNode.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/FunctionNode.kt
@@ -8,10 +8,10 @@ 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; })
+    override fun <T : Any> reduce(reducer: Reducer<T>): T {
+        val children = Array<Any>(children.size, { children[it].reduce(reducer) })
         return reducer.reduceNode(this, *children)
     }
 
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/NumberBinaryNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/NumberBinaryNode.kt
index 74c2206..3742477 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/NumberBinaryNode.kt
+++ b/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 right = right?.reduce(reducer) ?: return null
+    override fun <T : Any> reduce(reducer: Reducer<T>): T {
+        val left = left.reduce(reducer)
+        val right = right.reduce(reducer)
         return reducer.reduceNode(this, left, right)
     }
 
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/NumberNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/NumberNode.kt
index 3507568..9278301 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/NumberNode.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/NumberNode.kt
@@ -10,7 +10,7 @@ package org.nwapw.abacus.tree
  */
 class NumberNode(val number: String) : TreeNode() {
 
-    override fun <T : Any> reduce(reducer: Reducer<T>): T? {
+    override fun <T : Any> reduce(reducer: Reducer<T>): T {
         return reducer.reduceNode(this)
     }
 
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/NumberReducer.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/NumberReducer.kt
index c1a1da8..7a80ca7 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/NumberReducer.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/NumberReducer.kt
@@ -2,6 +2,7 @@ package org.nwapw.abacus.tree
 
 import org.nwapw.abacus.Abacus
 import org.nwapw.abacus.context.EvaluationContext
+import org.nwapw.abacus.exception.EvaluationException
 import org.nwapw.abacus.number.NumberInterface
 
 class NumberReducer(val abacus: Abacus, context: EvaluationContext) : Reducer<NumberInterface> {
@@ -12,18 +13,19 @@ class NumberReducer(val abacus: Abacus, context: EvaluationContext) : Reducer<Nu
         this.context.reducer = this
     }
 
-    override fun reduceNode(treeNode: TreeNode, vararg children: Any): NumberInterface? {
+    override fun reduceNode(treeNode: TreeNode, vararg children: Any): NumberInterface {
         val promotionManager = abacus.promotionManager
         return when(treeNode){
             is NumberNode -> {
                 context.inheritedNumberImplementation?.instanceForString(treeNode.number)
+                        ?: throw EvaluationException("no number implementation selected.")
             }
             is VariableNode -> {
                 val variable = context.getVariable(treeNode.variable)
                 if(variable != null) return variable
                 val definition = context.getDefinition(treeNode.variable)
                 if(definition != null) return definition.reduce(this)
-                null
+                throw EvaluationException("variable is not defined.")
             }
             is NumberUnaryNode -> {
                 val child = children[0] as NumberInterface
@@ -34,29 +36,31 @@ class NumberReducer(val abacus: Abacus, context: EvaluationContext) : Reducer<Nu
             is NumberBinaryNode -> {
                 val left = children[0] as NumberInterface
                 val right = children[1] as NumberInterface
-                val promotionResult = promotionManager.promote(left, right) ?: return null
+                val promotionResult = promotionManager.promote(left, right) ?:
+                        throw EvaluationException("promotion failed.")
                 context.numberImplementation = promotionResult.promotedTo
                 abacus.pluginManager.operatorFor(treeNode.operation).apply(context, *promotionResult.items)
             }
             is FunctionNode -> {
                 val promotionResult = promotionManager
-                        .promote(*children.map { it as NumberInterface }.toTypedArray()) ?: return null
+                        .promote(*children.map { it as NumberInterface }.toTypedArray()) ?:
+                        throw EvaluationException("promotion failed.")
                 context.numberImplementation = promotionResult.promotedTo
                 abacus.pluginManager.functionFor(treeNode.callTo).apply(context, *promotionResult.items)
             }
             is TreeValueUnaryNode -> {
                 abacus.pluginManager.treeValueOperatorFor(treeNode.operation)
-                        .apply(context, treeNode.applyTo ?: return null)
+                        .apply(context, treeNode.applyTo)
             }
             is TreeValueBinaryNode -> {
                 abacus.pluginManager.treeValueOperatorFor(treeNode.operation)
-                        .apply(context, treeNode.left ?: return null, treeNode.right ?: return null)
+                        .apply(context, treeNode.left, treeNode.right)
             }
             is TreeValueFunctionNode -> {
                 abacus.pluginManager.treeValueFunctionFor(treeNode.callTo)
                         .apply(context, *treeNode.children.toTypedArray())
             }
-            else -> null
+            else -> throw EvaluationException("unrecognized tree node.")
         }
     }
 
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/NumberUnaryNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/NumberUnaryNode.kt
index e4fee3f..01a494f 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/NumberUnaryNode.kt
+++ b/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
+    override fun <T : Any> reduce(reducer: Reducer<T>): T {
+        val child = applyTo.reduce(reducer)
         return reducer.reduceNode(this, child)
     }
 
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/Reducer.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/Reducer.kt
index 412af29..45cb4dc 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/Reducer.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/Reducer.kt
@@ -14,6 +14,6 @@ interface Reducer<out T> {
      * @param treeNode the tree node to reduce.
      * @param children the list of children, of type T.
      */
-    fun reduceNode(treeNode: TreeNode, vararg children: Any): T?
+    fun reduceNode(treeNode: TreeNode, vararg children: Any): T
 
 }
\ No newline at end of file
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/TreeNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/TreeNode.kt
index 2017739..a17b191 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/TreeNode.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/TreeNode.kt
@@ -5,6 +5,6 @@ package org.nwapw.abacus.tree
  */
 abstract class TreeNode {
 
-    abstract fun <T : Any> reduce(reducer: Reducer<T>): T?
+    abstract fun <T : Any> reduce(reducer: Reducer<T>): T
 
 }
\ No newline at end of file
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueBinaryNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueBinaryNode.kt
index 4edfef3..97cd0cf 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueBinaryNode.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueBinaryNode.kt
@@ -11,10 +11,10 @@ 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? {
+    override fun <T : Any> reduce(reducer: Reducer<T>): T {
         return reducer.reduceNode(this)
     }
 
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueFunctionNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueFunctionNode.kt
index 68c358e..18301d8 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueFunctionNode.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueFunctionNode.kt
@@ -7,9 +7,9 @@ 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? {
+    override fun <T : Any> reduce(reducer: Reducer<T>): T {
         return reducer.reduceNode(this)
     }
 
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueUnaryNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueUnaryNode.kt
index fdc0db5..dab3f52 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueUnaryNode.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueUnaryNode.kt
@@ -9,11 +9,11 @@ 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? {
-        return reducer.reduceNode(this);
+    override fun <T : Any> reduce(reducer: Reducer<T>): T {
+        return reducer.reduceNode(this)
     }
 
 }
\ No newline at end of file
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/UnaryNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/UnaryNode.kt
index 285231b..ea518e2 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/UnaryNode.kt
+++ b/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
     }
 
 }
\ No newline at end of file
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/VariableNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/VariableNode.kt
index 93d91dc..0fdf887 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/VariableNode.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/VariableNode.kt
@@ -10,7 +10,7 @@ package org.nwapw.abacus.tree
  */
 class VariableNode(val variable: String) : TreeNode() {
 
-    override fun <T : Any> reduce(reducer: Reducer<T>): T? {
+    override fun <T : Any> reduce(reducer: Reducer<T>): T {
         return reducer.reduceNode(this)
     }
 
diff --git a/core/src/test/java/org/nwapw/abacus/tests/CalculationTests.java b/core/src/test/java/org/nwapw/abacus/tests/CalculationTests.java
index af85be6..1aee10e 100755
--- a/core/src/test/java/org/nwapw/abacus/tests/CalculationTests.java
+++ b/core/src/test/java/org/nwapw/abacus/tests/CalculationTests.java
@@ -5,7 +5,7 @@ import org.junit.BeforeClass;
 import org.junit.Test;
 import org.nwapw.abacus.Abacus;
 import org.nwapw.abacus.config.Configuration;
-import org.nwapw.abacus.function.DomainException;
+import org.nwapw.abacus.exception.DomainException;
 import org.nwapw.abacus.number.NumberInterface;
 import org.nwapw.abacus.plugin.StandardPlugin;
 import org.nwapw.abacus.tree.TreeNode;
diff --git a/fx/src/main/java/org/nwapw/abacus/fx/AbacusController.java b/fx/src/main/java/org/nwapw/abacus/fx/AbacusController.java
index 15c75b8..f903618 100644
--- a/fx/src/main/java/org/nwapw/abacus/fx/AbacusController.java
+++ b/fx/src/main/java/org/nwapw/abacus/fx/AbacusController.java
@@ -12,9 +12,12 @@ import javafx.util.Callback;
 import javafx.util.StringConverter;
 import org.nwapw.abacus.Abacus;
 import org.nwapw.abacus.config.Configuration;
+import org.nwapw.abacus.exception.AbacusException;
+import org.nwapw.abacus.exception.ComputationInterruptedException;
 import org.nwapw.abacus.function.Documentation;
 import org.nwapw.abacus.function.DocumentationType;
-import org.nwapw.abacus.function.DomainException;
+import org.nwapw.abacus.exception.DomainException;
+import org.nwapw.abacus.exception.EvaluationException;
 import org.nwapw.abacus.number.*;
 import org.nwapw.abacus.plugin.ClassFinder;
 import org.nwapw.abacus.plugin.PluginListener;
@@ -147,16 +150,11 @@ public class AbacusController implements PluginListener {
                 }
                 EvaluationResult result = abacus.evaluateTree(constructedTree);
                 NumberInterface evaluatedNumber = result.getValue();
-                if (evaluatedNumber == null) {
-                    return ERR_EVAL;
-                }
                 String resultingString = evaluatedNumber.toString();
                 historyData.add(new HistoryModel(inputField.getText(), constructedTree.toString(), resultingString));
                 abacus.applyToContext(result.getResultingContext());
                 return resultingString;
-            } catch (ComputationInterruptedException exception) {
-                return ERR_STOP;
-            } catch (DomainException exception) {
+            } catch (AbacusException exception) {
                 return exception.getMessage();
             } catch (RuntimeException exception) {
                 exception.printStackTrace();