diff --git a/core/src/main/java/org/nwapw/abacus/lexing/pattern/Transformation.java b/core/src/main/java/org/nwapw/abacus/lexing/pattern/Transformation.java
index 5bab0e8..4758f54 100644
--- a/core/src/main/java/org/nwapw/abacus/lexing/pattern/Transformation.java
+++ b/core/src/main/java/org/nwapw/abacus/lexing/pattern/Transformation.java
@@ -13,4 +13,4 @@ public interface Transformation<T> {
      */
     PatternChain<T> transform(PatternChain<T> from);
 
-}
+}
\ No newline at end of file
diff --git a/core/src/main/java/org/nwapw/abacus/plugin/NumberImplementation.java b/core/src/main/java/org/nwapw/abacus/plugin/NumberImplementation.java
index ad5d1d1..623a2c9 100644
--- a/core/src/main/java/org/nwapw/abacus/plugin/NumberImplementation.java
+++ b/core/src/main/java/org/nwapw/abacus/plugin/NumberImplementation.java
@@ -1,10 +1,10 @@
 package org.nwapw.abacus.plugin;
 
 import org.nwapw.abacus.number.NumberInterface;
+import org.nwapw.abacus.number.promotion.PromotionFunction;
 
 import java.util.HashMap;
 import java.util.Map;
-import java.util.function.Function;
 
 /**
  * A class that holds data about a number implementation.
@@ -14,7 +14,7 @@ public abstract class NumberImplementation {
     /**
      * The list of paths through which this implementation can be promoted.
      */
-    private Map<String, Function<NumberInterface, NumberInterface>> promotionPaths;
+    private Map<String, PromotionFunction> promotionPaths;
     /**
      * The implementation class for this implementation.
      */
@@ -41,7 +41,7 @@ public abstract class NumberImplementation {
      *
      * @return the map of documentation paths.
      */
-    public final Map<String, Function<NumberInterface, NumberInterface>> getPromotionPaths() {
+    public final Map<String, PromotionFunction> getPromotionPaths() {
         return promotionPaths;
     }
 
diff --git a/core/src/main/kotlin/org/nwapw/abacus/number/promotion/NumberUtils.kt b/core/src/main/kotlin/org/nwapw/abacus/number/promotion/NumberUtils.kt
index 4d765c5..7054994 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/number/promotion/NumberUtils.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/number/promotion/NumberUtils.kt
@@ -3,7 +3,6 @@ package org.nwapw.abacus.number.promotion
 
 import org.nwapw.abacus.number.NumberInterface
 
-typealias PromotionFunction = java.util.function.Function<NumberInterface, NumberInterface>
 typealias PromotionPath = List<PromotionFunction>
 typealias NumberClass = Class<NumberInterface>
 
@@ -14,5 +13,5 @@ typealias NumberClass = Class<NumberInterface>
  * @param from the number to start from.
  */
 fun PromotionPath.promote(from: NumberInterface): NumberInterface {
-    return fold(from, { current, function -> function.apply(current) })
+    return fold(from, { current, function -> function.promote(current) })
 }
\ No newline at end of file
diff --git a/core/src/main/kotlin/org/nwapw/abacus/number/promotion/PromotionFunction.kt b/core/src/main/kotlin/org/nwapw/abacus/number/promotion/PromotionFunction.kt
new file mode 100644
index 0000000..4242fe2
--- /dev/null
+++ b/core/src/main/kotlin/org/nwapw/abacus/number/promotion/PromotionFunction.kt
@@ -0,0 +1,20 @@
+package org.nwapw.abacus.number.promotion
+
+import org.nwapw.abacus.number.NumberInterface
+
+/**
+ * Function that is used to promote a number from one type to another.
+ *
+ * A promotion function is used in the promotion system as a mean to
+ * actually "travel" down the promotion path.
+ */
+interface PromotionFunction {
+
+    /**
+     * Promotes the given [number] into another type.
+     * @param number the number to promote from.
+     * @return the new number with the same value.
+     */
+    fun promote(number: NumberInterface): NumberInterface
+
+}
diff --git a/core/src/main/kotlin/org/nwapw/abacus/number/promotion/PromotionManager.kt b/core/src/main/kotlin/org/nwapw/abacus/number/promotion/PromotionManager.kt
index fd7004b..c6318d8 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/number/promotion/PromotionManager.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/number/promotion/PromotionManager.kt
@@ -6,7 +6,6 @@ import org.nwapw.abacus.number.NumberInterface
 import org.nwapw.abacus.plugin.NumberImplementation
 import org.nwapw.abacus.plugin.PluginListener
 import org.nwapw.abacus.plugin.PluginManager
-import java.util.function.Function
 
 /**
  * A class that handles promotions based on priority and the
@@ -32,7 +31,11 @@ class PromotionManager(val abacus: Abacus) : PluginListener {
         val fromName = abacus.pluginManager.interfaceImplementationNameFor(from.implementation)
         val toName = abacus.pluginManager.interfaceImplementationNameFor(to.implementation)
 
-        if(fromName == toName) return listOf(Function { it })
+        if(fromName == toName) return listOf(object : PromotionFunction {
+            override fun promote(number: NumberInterface): NumberInterface {
+                return number
+            }
+        })
 
         if(from.promotionPaths.containsKey(toName))
             return listOf(from.promotionPaths[toName] ?: return null)
diff --git a/core/src/test/java/org/nwapw/abacus/tests/RangeTests.java b/core/src/test/java/org/nwapw/abacus/tests/RangeTests.java
index 9b87ef3..7f07b67 100644
--- a/core/src/test/java/org/nwapw/abacus/tests/RangeTests.java
+++ b/core/src/test/java/org/nwapw/abacus/tests/RangeTests.java
@@ -5,19 +5,17 @@ import org.junit.BeforeClass;
 import org.junit.Test;
 import org.nwapw.abacus.Abacus;
 import org.nwapw.abacus.config.Configuration;
+import org.nwapw.abacus.number.promotion.PromotionFunction;
 import org.nwapw.abacus.number.standard.NaiveNumber;
-import org.nwapw.abacus.number.NumberInterface;
 import org.nwapw.abacus.number.range.NumberRange;
 import org.nwapw.abacus.number.standard.PreciseNumber;
 import org.nwapw.abacus.plugin.standard.StandardPlugin;
 
-import java.util.function.Function;
-
 public class RangeTests {
 
     private static Abacus abacus = new Abacus(new Configuration( "precise", new String[]{}));
-    private static Function<NumberInterface, NumberInterface> naivePromotion = i -> new NaiveNumber((i.toString()));
-    private static Function<NumberInterface, NumberInterface> precisePromotion = i -> new PreciseNumber((i.toString()));
+    private static PromotionFunction naivePromotion = i -> new NaiveNumber((i.toString()));
+    private static PromotionFunction precisePromotion = i -> new PreciseNumber((i.toString()));
 
     @BeforeClass
     public static void prepareTests() {