diff --git a/src/main/kotlin/gay/pizza/pork/eval/CallableFunction.kt b/src/main/kotlin/gay/pizza/pork/eval/CallableFunction.kt
new file mode 100644
index 0000000..7b310fd
--- /dev/null
+++ b/src/main/kotlin/gay/pizza/pork/eval/CallableFunction.kt
@@ -0,0 +1,5 @@
+package gay.pizza.pork.eval
+
+fun interface CallableFunction {
+  fun call(argument: Any): Any
+}
diff --git a/src/main/kotlin/gay/pizza/pork/eval/Evaluator.kt b/src/main/kotlin/gay/pizza/pork/eval/Evaluator.kt
index 6ef30d0..266bd34 100644
--- a/src/main/kotlin/gay/pizza/pork/eval/Evaluator.kt
+++ b/src/main/kotlin/gay/pizza/pork/eval/Evaluator.kt
@@ -1,7 +1,6 @@
 package gay.pizza.pork.eval
 
 import gay.pizza.pork.ast.*
-import java.util.function.Function
 
 class Evaluator(root: Scope) : Visitor<Any> {
   private var currentScope: Scope = root
@@ -21,8 +20,8 @@ class Evaluator(root: Scope) : Visitor<Any> {
     return Unit
   }
 
-  override fun visitLambda(node: Lambda): Function<Any, Any> {
-    return Function { _ ->
+  override fun visitLambda(node: Lambda): CallableFunction {
+    return CallableFunction { _ ->
       currentScope = currentScope.fork()
       try {
         var value: Any? = null
diff --git a/src/main/kotlin/gay/pizza/pork/eval/Scope.kt b/src/main/kotlin/gay/pizza/pork/eval/Scope.kt
index 2c2aee8..dcaf64b 100644
--- a/src/main/kotlin/gay/pizza/pork/eval/Scope.kt
+++ b/src/main/kotlin/gay/pizza/pork/eval/Scope.kt
@@ -1,7 +1,5 @@
 package gay.pizza.pork.eval
 
-import java.util.function.Function
-
 class Scope(val parent: Scope? = null) {
   private val variables = mutableMapOf<String, Any>()
 
@@ -25,12 +23,11 @@ class Scope(val parent: Scope? = null) {
 
   fun call(name: String, argument: Any = Unit): Any {
     val value = value(name)
-    if (value !is Function<*, *>) {
+    if (value !is CallableFunction) {
       throw RuntimeException("$value is not callable.")
     }
-    @Suppress("UNCHECKED_CAST")
-    val casted = value as Function<Any, Any>
-    return casted.apply(argument)
+    val function = value as CallableFunction
+    return function.call(argument)
   }
 
   fun fork(): Scope {