Add a DomainException that avoids using null in functions.

core/src/main/java/org/nwapw/abacus/number/DomainException.java

@@ -0,0 +1,24 @@
+package org.nwapw.abacus.number;
+
+/**
+ * Exception thrown if the function parameters do not match
+ * requirements.
+ */
+public class DomainException extends RuntimeException {
+
+    /**
+     * Creates a new DomainException.
+     * @param reason the reason for which the exception is thrown.
+     */
+    public DomainException(String reason) {
+        super(reason);
+    }
+
+    /**
+     * Creates a new DomainException with a default message.
+     */
+    public DomainException(){
+        this("Domain Error");
+    }
+
+}

core/src/main/kotlin/org/nwapw/abacus/function/applicable/Applicable.kt

@@ -1,5 +1,6 @@
 package org.nwapw.abacus.function.applicable
 
+import org.nwapw.abacus.number.DomainException
 import org.nwapw.abacus.plugin.NumberImplementation
 
 /**
@@ -25,7 +26,7 @@ interface Applicable<in T : Any, out O : Any> {
      * @param params the parameters to apply to.
      * @return the result of the application.
      */
-    fun applyInternal(implementation: NumberImplementation, params: Array<out T>): O?
+    fun applyInternal(implementation: NumberImplementation, params: Array<out T>): O
 
     /**
      * If the parameters can be used with this applicable, returns
@@ -34,8 +35,8 @@ interface Applicable<in T : Any, out O : Any> {
      * @param params the parameters to apply to.
      * @return the result of the operation, or null if parameters do not match.
      */
-    fun apply(implementation: NumberImplementation, vararg params: T): O? {
+    fun apply(implementation: NumberImplementation, vararg params: T): O {
-        if (!matchesParams(implementation, params)) return null
+        if (!matchesParams(implementation, params)) throw DomainException()
         return applyInternal(implementation, params)
     }
 

core/src/main/kotlin/org/nwapw/abacus/function/applicable/ReducerApplicable.kt

@@ -1,5 +1,6 @@
 package org.nwapw.abacus.function.applicable
 
+import org.nwapw.abacus.number.DomainException
 import org.nwapw.abacus.plugin.NumberImplementation
 import org.nwapw.abacus.tree.Reducer
 
@@ -27,7 +28,7 @@ interface ReducerApplicable<in T : Any, out O : Any, in R : Any> {
      * @param params the arguments to apply to.
      * @return the result of the application.
      */
-    fun applyWithReducerInternal(implementation: NumberImplementation, reducer: Reducer<R>, params: Array<out T>): O?
+    fun applyWithReducerInternal(implementation: NumberImplementation, reducer: Reducer<R>, params: Array<out T>): O
 
     /**
      * Applies this applicable to the given arguments, and reducer,
@@ -36,8 +37,8 @@ interface ReducerApplicable<in T : Any, out O : Any, in R : Any> {
      * @param params the arguments to apply to.
      * @return the result of the application, or null if the arguments are incompatible.
      */
-    fun applyWithReducer(implementation: NumberImplementation, reducer: Reducer<R>, vararg params: T): O? {
+    fun applyWithReducer(implementation: NumberImplementation, reducer: Reducer<R>, vararg params: T): O {
-        if (!matchesParams(implementation, params)) return null
+        if (!matchesParams(implementation, params)) throw DomainException()
         return applyWithReducerInternal(implementation, reducer, params)
     }
 

fx/src/main/java/org/nwapw/abacus/fx/AbacusController.java

@@ -152,6 +152,8 @@ public class AbacusController implements PluginListener {
                 return resultingString;
             } catch (ComputationInterruptedException exception) {
                 return ERR_STOP;
+            } catch (DomainException exception) {
+                return exception.getMessage();
             } catch (RuntimeException exception) {
                 exception.printStackTrace();
                 return ERR_EXCEPTION;