diff --git a/core/src/main/kotlin/org/nwapw/abacus/function/NumberFunction.kt b/core/src/main/kotlin/org/nwapw/abacus/function/NumberFunction.kt
index d0946e6..f76f147 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/function/NumberFunction.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/function/NumberFunction.kt
@@ -1,5 +1,6 @@
 package org.nwapw.abacus.function
 
+import org.nwapw.abacus.function.applicable.Applicable
 import org.nwapw.abacus.number.NumberInterface
 
 /**
diff --git a/core/src/main/kotlin/org/nwapw/abacus/function/NumberOperator.kt b/core/src/main/kotlin/org/nwapw/abacus/function/NumberOperator.kt
index 9b1f2f1..81349c3 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/function/NumberOperator.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/function/NumberOperator.kt
@@ -1,5 +1,6 @@
 package org.nwapw.abacus.function
 
+import org.nwapw.abacus.function.applicable.Applicable
 import org.nwapw.abacus.number.NumberInterface
 
 /**
diff --git a/core/src/main/kotlin/org/nwapw/abacus/function/TreeValueFunction.kt b/core/src/main/kotlin/org/nwapw/abacus/function/TreeValueFunction.kt
index 42568ea..7b31d92 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/function/TreeValueFunction.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/function/TreeValueFunction.kt
@@ -1,5 +1,6 @@
 package org.nwapw.abacus.function
 
+import org.nwapw.abacus.function.applicable.ReducerApplicable
 import org.nwapw.abacus.number.NumberInterface
 import org.nwapw.abacus.tree.TreeNode
 
diff --git a/core/src/main/kotlin/org/nwapw/abacus/function/TreeValueOperator.kt b/core/src/main/kotlin/org/nwapw/abacus/function/TreeValueOperator.kt
index beed6b2..990ef1c 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/function/TreeValueOperator.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/function/TreeValueOperator.kt
@@ -1,5 +1,6 @@
 package org.nwapw.abacus.function
 
+import org.nwapw.abacus.function.applicable.ReducerApplicable
 import org.nwapw.abacus.number.NumberInterface
 import javax.swing.tree.TreeNode
 
diff --git a/core/src/main/java/org/nwapw/abacus/function/Applicable.java b/core/src/main/kotlin/org/nwapw/abacus/function/applicable/Applicable.kt
similarity index 68%
rename from core/src/main/java/org/nwapw/abacus/function/Applicable.java
rename to core/src/main/kotlin/org/nwapw/abacus/function/applicable/Applicable.kt
index 2133848..4988c2a 100644
--- a/core/src/main/java/org/nwapw/abacus/function/Applicable.java
+++ b/core/src/main/kotlin/org/nwapw/abacus/function/applicable/Applicable.kt
@@ -1,27 +1,28 @@
-package org.nwapw.abacus.function;
+package org.nwapw.abacus.function.applicable
 
 /**
- * A class that represents something that can be applied to one or more
+ * A class that can be applied to arguments.
+ *
+ * Applicable is a class that represents something that can be applied to one or more
  * arguments of the same type, and returns a single value from that application.
  * @param <T> the type of the parameters passed to this applicable.
  * @param <O> the return type of the applicable.
  */
-public interface Applicable<T, O> {
+interface Applicable<in T: Any, out O: Any> {
 
     /**
      * Checks if the given applicable can be used with the given parameters.
      * @param params the parameter array to verify for compatibility.
      * @return whether the array can be used with applyInternal.
      */
-    boolean matchesParams(T[] params);
-
+    fun matchesParams(params: Array<out T>): Boolean
     /**
      * Applies the applicable object to the given parameters,
      * without checking for compatibility.
      * @param params the parameters to apply to.
      * @return the result of the application.
      */
-    O applyInternal(T[] params);
+    fun applyInternal(params: Array<out T>): O?
 
     /**
      * If the parameters can be used with this applicable, returns
@@ -30,9 +31,9 @@ public interface Applicable<T, O> {
      * @param params the parameters to apply to.
      * @return the result of the operation, or null if parameters do not match.
      */
-     default O apply(T... params){
-        if(!matchesParams(params)) return null;
-        return applyInternal(params);
+    fun apply(vararg params: T): O? {
+        if(!matchesParams(params)) return null
+        return applyInternal(params)
     }
 
-}
+}
\ No newline at end of file
diff --git a/core/src/main/java/org/nwapw/abacus/function/ReducerApplicable.java b/core/src/main/kotlin/org/nwapw/abacus/function/applicable/ReducerApplicable.kt
similarity index 54%
rename from core/src/main/java/org/nwapw/abacus/function/ReducerApplicable.java
rename to core/src/main/kotlin/org/nwapw/abacus/function/applicable/ReducerApplicable.kt
index bff7934..6de6a27 100644
--- a/core/src/main/java/org/nwapw/abacus/function/ReducerApplicable.java
+++ b/core/src/main/kotlin/org/nwapw/abacus/function/applicable/ReducerApplicable.kt
@@ -1,24 +1,24 @@
-package org.nwapw.abacus.function;
+package org.nwapw.abacus.function.applicable
 
-import org.nwapw.abacus.tree.Reducer;
+import org.nwapw.abacus.tree.Reducer
 
 /**
- * A slightly more specific Applicable that requires a reducer
+ * Applicable that requires a reducer.
+ *
+ * ReducerApplicable slightly more specific Applicable that requires a reducer
  * to be passed to it along with the parameters.
  * @param <T> the type of the input arguments.
  * @param <O> the return type of the application.
  * @param <R> the required type of the reducer.
  */
-public interface ReducerApplicable<T, O, R> extends Applicable<T, O> {
+interface ReducerApplicable<in T: Any, out O: Any, in R: Any> : Applicable<T, O> {
 
-    @Override
-    default O applyInternal(T[] params) {
-        return null;
+    override fun applyInternal(params: Array<out T>): O? {
+        return null
     }
 
-    @Override
-    default O apply(T... params) {
-        return null;
+    override fun apply(vararg params: T): O? {
+        return null
     }
 
     /**
@@ -27,8 +27,7 @@ public interface ReducerApplicable<T, O, R> extends Applicable<T, O> {
      * @param args the arguments to apply to.
      * @return the result of the application.
      */
-    O applyWithReducerInternal(Reducer<R> reducer, T[] args);
-
+    fun applyWithReducerInternal(reducer: Reducer<R>, params: Array<out T>): O?
     /**
      * Applies this applicable to the given arguments, and reducer,
      * if the arguments and reducer are compatible with this applicable.
@@ -36,9 +35,9 @@ public interface ReducerApplicable<T, O, R> extends Applicable<T, O> {
      * @param args the arguments to apply to.
      * @return the result of the application, or null if the arguments are incompatible.
      */
-    default O applyWithReducer(Reducer<R> reducer, T...args) {
-        if(!matchesParams(args) || reducer == null) return null;
-        return applyWithReducerInternal(reducer, args);
+    fun applyWithReducer(reducer: Reducer<R>, vararg params: T): O? {
+        if(!matchesParams(params)) return null
+        return applyWithReducerInternal(reducer, params)
     }
 
-}
+}
\ No newline at end of file