diff --git a/pom.xml b/pom.xml
index bd817f6..685e2d2 100644
--- a/pom.xml
+++ b/pom.xml
@@ -10,7 +10,7 @@
 
   <properties>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
-    <kotlin.version>1.2.60</kotlin.version>
+    <kotlin.version>1.2.70</kotlin.version>
     <junit5.version>5.2.0</junit5.version>
     <skipTests>false</skipTests>
   </properties>
diff --git a/src/main/kotlin/function/ArgumentValidator.java b/src/main/kotlin/function/ArgumentValidator.java
deleted file mode 100644
index dc0db05..0000000
--- a/src/main/kotlin/function/ArgumentValidator.java
+++ /dev/null
@@ -1,234 +0,0 @@
-package function;
-
-import error.LispException;
-import sexpression.Cons;
-import sexpression.DisplayName;
-import sexpression.SExpression;
-
-import java.math.BigInteger;
-
-import static function.builtin.cons.LENGTH.getLength;
-import static java.text.MessageFormat.format;
-
-public class ArgumentValidator {
-
-    private Class<? extends SExpression> firstArgumentType;
-    private Class<? extends SExpression> trailingArgumentType;
-    private Class<? extends SExpression> excludedFirstArgumentType;
-    private Class<? extends SExpression> excludedTrailingArgumentType;
-    private String functionName;
-    private BigInteger maximumNumberOfArguments;
-    private BigInteger minimumNumberOfArguments;
-
-    public ArgumentValidator(String functionName) {
-        this.firstArgumentType = SExpression.class;
-        this.trailingArgumentType = SExpression.class;
-        this.excludedFirstArgumentType = null;
-        this.excludedTrailingArgumentType = null;
-        this.functionName = functionName;
-        this.minimumNumberOfArguments = null;
-        this.maximumNumberOfArguments = null;
-    }
-
-    public void setFirstArgumentExpectedType(Class<? extends SExpression> argumentType) {
-        this.firstArgumentType = argumentType;
-    }
-
-    public void setTrailingArgumentExpectedType(Class<? extends SExpression> argumentType) {
-        this.trailingArgumentType = argumentType;
-    }
-
-    public void setEveryArgumentExpectedType(Class<? extends SExpression> argumentType) {
-        this.firstArgumentType = argumentType;
-        this.trailingArgumentType = argumentType;
-    }
-
-    public void setFirstArgumentExcludedType(Class<? extends SExpression> argumentType) {
-        this.excludedFirstArgumentType = argumentType;
-    }
-
-    public void setTrailingArgumentExcludedType(Class<? extends SExpression> argumentType) {
-        this.excludedTrailingArgumentType = argumentType;
-    }
-
-    public void setEveryArgumentExcludedType(Class<? extends SExpression> argumentType) {
-        this.excludedFirstArgumentType = argumentType;
-        this.excludedTrailingArgumentType = argumentType;
-    }
-
-    public void setMaximumNumberOfArguments(int maximumNumberOfArguments) {
-        this.maximumNumberOfArguments = BigInteger.valueOf(maximumNumberOfArguments);
-    }
-
-    public void setMinimumNumberOfArguments(int minimumNumberOfArguments) {
-        this.minimumNumberOfArguments = BigInteger.valueOf(minimumNumberOfArguments);
-    }
-
-    public void setExactNumberOfArguments(int exactNumberOfArguments) {
-        this.minimumNumberOfArguments = BigInteger.valueOf(exactNumberOfArguments);
-        this.maximumNumberOfArguments = BigInteger.valueOf(exactNumberOfArguments);
-    }
-
-    public void validate(Cons argumentList) {
-        validateListNotDotted(argumentList);
-        validateListLength(argumentList);
-        validateArgumentTypes(argumentList);
-    }
-
-    private void validateListNotDotted(Cons argumentList) {
-        SExpression next = argumentList.getRest();
-
-        for (Cons current = argumentList; next.isCons(); next = current.getRest())
-            current = (Cons) next;
-
-        if (!next.isNull())
-            throw new DottedArgumentListException(functionName, argumentList);
-    }
-
-    private void validateListLength(Cons argumentList) {
-        if (containsTooFewArguments(argumentList))
-            throw new TooFewArgumentsException(functionName, argumentList);
-        else if (containsTooManyArguments(argumentList))
-            throw new TooManyArgumentsException(functionName, argumentList);
-    }
-
-    private boolean containsTooFewArguments(Cons argumentList) {
-        return isMinimum() && isLengthLessThanMinimum(argumentList);
-    }
-
-    private boolean isMinimum() {
-        return minimumNumberOfArguments != null;
-    }
-
-    private boolean isLengthLessThanMinimum(Cons argumentList) {
-        return getLength(argumentList).compareTo(minimumNumberOfArguments) < 0;
-    }
-
-    private boolean containsTooManyArguments(Cons argumentList) {
-        return isMaximum() && isLengthGreaterThanMaximum(argumentList);
-    }
-
-    private boolean isMaximum() {
-        return maximumNumberOfArguments != null;
-    }
-
-    private boolean isLengthGreaterThanMaximum(Cons argumentList) {
-        return getLength(argumentList).compareTo(maximumNumberOfArguments) > 0;
-    }
-
-    private void validateArgumentTypes(Cons argumentList) {
-        validateFirstArgument(argumentList);
-        validateTrailingArguments(argumentList);
-    }
-
-    private void validateFirstArgument(Cons argumentList) {
-        if (!isFirstArgumentValid(argumentList))
-            throw new BadArgumentTypeException(functionName, argumentList.getFirst(), firstArgumentType);
-    }
-
-    private boolean isFirstArgumentValid(Cons argumentList) {
-        return argumentList.isNull() || isExpectedFirstArgumentType(argumentList.getFirst());
-    }
-
-    private boolean isExpectedFirstArgumentType(SExpression firstArgument) {
-        return firstArgumentType.isInstance(firstArgument) && !isExcludedFirstArgumentType(firstArgument);
-    }
-
-    private boolean isExcludedFirstArgumentType(SExpression firstArgument) {
-        return excludedFirstArgumentType != null && excludedFirstArgumentType.isInstance(firstArgument);
-    }
-
-    private void validateTrailingArguments(Cons argumentList) {
-        for (Cons cons = (Cons) argumentList.getRest(); !cons.isNull(); cons = (Cons) cons.getRest())
-            if (!isExpectedTrailingArgumentType(cons.getFirst()))
-                throw new BadArgumentTypeException(functionName, cons.getFirst(), trailingArgumentType);
-    }
-
-    private boolean isExpectedTrailingArgumentType(SExpression trailingArgument) {
-        return trailingArgumentType.isInstance(trailingArgument) && !isExcludedTrailingArgumentType(trailingArgument);
-    }
-
-    private boolean isExcludedTrailingArgumentType(SExpression trailingArgument) {
-        return excludedTrailingArgumentType != null && excludedTrailingArgumentType.isInstance(trailingArgument);
-    }
-
-    public static class TooFewArgumentsException extends LispException {
-
-        private static final long serialVersionUID = 1L;
-        private String functionName;
-        private Cons argumentList;
-
-        public TooFewArgumentsException(String functionName, Cons argumentList) {
-            this.functionName = functionName;
-            this.argumentList = argumentList;
-        }
-
-        @Override
-        public String getMessage() {
-            return format("too few arguments given to {0}: {1}", functionName, argumentList);
-        }
-    }
-
-    public static class TooManyArgumentsException extends LispException {
-
-        private static final long serialVersionUID = 1L;
-        private String functionName;
-        private Cons argumentList;
-
-        public TooManyArgumentsException(String functionName, Cons argumentList) {
-            this.functionName = functionName;
-            this.argumentList = argumentList;
-        }
-
-        @Override
-        public String getMessage() {
-            return format("too many arguments given to {0}: {1}", functionName, argumentList);
-        }
-    }
-
-    public static class DottedArgumentListException extends LispException {
-
-        private static final long serialVersionUID = 1L;
-        private String functionName;
-        private Cons argumentList;
-
-        public DottedArgumentListException(String functionName, Cons argumentList) {
-            this.functionName = functionName;
-            this.argumentList = argumentList;
-        }
-
-        @Override
-        public String getMessage() {
-            return format("dotted argument list given to {0}: {1}", functionName, argumentList);
-        }
-    }
-
-    public static class BadArgumentTypeException extends LispException {
-
-        private static final long serialVersionUID = 1L;
-        private String functionName;
-        private String argument;
-        private Class<? extends SExpression> expectedType;
-
-        public BadArgumentTypeException(String functionName, SExpression argument,
-                                        Class<? extends SExpression> expectedType) {
-            this.functionName = functionName;
-            this.argument = argument.toString();
-            this.expectedType = expectedType;
-        }
-
-        @Override
-        public String getMessage() {
-            return format("{0}: {1} is not the expected type of ''{2}''",
-                          functionName,
-                          argument,
-                          getExpectedTypeName());
-        }
-
-        private String getExpectedTypeName() {
-            DisplayName displayName = expectedType.getAnnotation(DisplayName.class);
-
-            return (displayName == null) ? "unknown" : displayName.value();
-        }
-    }
-}
diff --git a/src/main/kotlin/function/ArgumentValidator.kt b/src/main/kotlin/function/ArgumentValidator.kt
new file mode 100644
index 0000000..c22dcce
--- /dev/null
+++ b/src/main/kotlin/function/ArgumentValidator.kt
@@ -0,0 +1,164 @@
+package function
+
+import error.LispException
+import function.builtin.cons.LENGTH.getLength
+import sexpression.Cons
+import sexpression.DisplayName
+import sexpression.SExpression
+import java.math.BigInteger
+import java.text.MessageFormat.format
+
+class ArgumentValidator(private val functionName: String) {
+
+    private var firstArgumentType: Class<out SExpression> = SExpression::class.java
+    private var trailingArgumentType: Class<out SExpression> = SExpression::class.java
+    private var excludedFirstArgumentType: Class<out SExpression>? = null
+    private var excludedTrailingArgumentType: Class<out SExpression>? = null
+    private var maximumNumberOfArguments: BigInteger? = null
+    private var minimumNumberOfArguments: BigInteger? = null
+
+    fun setFirstArgumentExpectedType(argumentType: Class<out SExpression>) {
+        this.firstArgumentType = argumentType
+    }
+
+    fun setTrailingArgumentExpectedType(argumentType: Class<out SExpression>) {
+        this.trailingArgumentType = argumentType
+    }
+
+    fun setEveryArgumentExpectedType(argumentType: Class<out SExpression>) {
+        this.firstArgumentType = argumentType
+        this.trailingArgumentType = argumentType
+    }
+
+    fun setFirstArgumentExcludedType(argumentType: Class<out SExpression>) {
+        this.excludedFirstArgumentType = argumentType
+    }
+
+    fun setTrailingArgumentExcludedType(argumentType: Class<out SExpression>) {
+        this.excludedTrailingArgumentType = argumentType
+    }
+
+    fun setEveryArgumentExcludedType(argumentType: Class<out SExpression>) {
+        this.excludedFirstArgumentType = argumentType
+        this.excludedTrailingArgumentType = argumentType
+    }
+
+    fun setMaximumNumberOfArguments(maximumNumberOfArguments: Int) {
+        this.maximumNumberOfArguments = BigInteger.valueOf(maximumNumberOfArguments.toLong())
+    }
+
+    fun setMinimumNumberOfArguments(minimumNumberOfArguments: Int) {
+        this.minimumNumberOfArguments = BigInteger.valueOf(minimumNumberOfArguments.toLong())
+    }
+
+    fun setExactNumberOfArguments(exactNumberOfArguments: Int) {
+        this.minimumNumberOfArguments = BigInteger.valueOf(exactNumberOfArguments.toLong())
+        this.maximumNumberOfArguments = BigInteger.valueOf(exactNumberOfArguments.toLong())
+    }
+
+    fun validate(argumentList: Cons) {
+        validateListNotDotted(argumentList)
+        validateListLength(argumentList)
+        validateArgumentTypes(argumentList)
+    }
+
+    private fun validateListNotDotted(argumentList: Cons) {
+        var current = argumentList
+        var next = current.rest
+
+        while (next.isCons) {
+            current = next as Cons
+            next = current.rest
+        }
+
+        if (!next.isNull)
+            throw DottedArgumentListException(functionName, argumentList)
+    }
+
+    private fun validateListLength(argumentList: Cons) {
+        if (containsTooFewArguments(argumentList))
+            throw TooFewArgumentsException(functionName, argumentList)
+        else if (containsTooManyArguments(argumentList))
+            throw TooManyArgumentsException(functionName, argumentList)
+    }
+
+    private fun containsTooFewArguments(argumentList: Cons) = isMinimum() && isLengthLessThanMinimum(argumentList)
+    private fun isMinimum() = minimumNumberOfArguments != null
+    private fun isLengthLessThanMinimum(argumentList: Cons) = getLength(argumentList) < minimumNumberOfArguments!!
+
+    private fun containsTooManyArguments(argumentList: Cons) = isMaximum() && isLengthGreaterThanMaximum(argumentList)
+    private fun isMaximum() = maximumNumberOfArguments != null
+    private fun isLengthGreaterThanMaximum(argumentList: Cons) = getLength(argumentList) > maximumNumberOfArguments!!
+
+    private fun validateArgumentTypes(argumentList: Cons) {
+        validateFirstArgument(argumentList)
+        validateTrailingArguments(argumentList)
+    }
+
+    private fun validateFirstArgument(argumentList: Cons) {
+        if (!isFirstArgumentValid(argumentList))
+            throw BadArgumentTypeException(functionName, argumentList.first, firstArgumentType)
+    }
+
+    private fun isFirstArgumentValid(argumentList: Cons) =
+        argumentList.isNull || isExpectedFirstArgumentType(argumentList.first)
+
+    private fun isExpectedFirstArgumentType(firstArgument: SExpression) =
+        firstArgumentType.isInstance(firstArgument) && !isExcludedFirstArgumentType(firstArgument)
+
+    private fun isExcludedFirstArgumentType(firstArgument: SExpression) =
+        excludedFirstArgumentType != null && excludedFirstArgumentType!!.isInstance(firstArgument)
+
+    private fun validateTrailingArguments(argumentList: Cons) {
+        var cons = argumentList.rest as Cons
+
+        while (!cons.isNull) {
+            if (!isExpectedTrailingArgumentType(cons.first))
+                throw BadArgumentTypeException(functionName, cons.first, trailingArgumentType)
+
+            cons = cons.rest as Cons
+        }
+    }
+
+    private fun isExpectedTrailingArgumentType(trailingArgument: SExpression) =
+        trailingArgumentType.isInstance(trailingArgument) && !isExcludedTrailingArgumentType(trailingArgument)
+
+    private fun isExcludedTrailingArgumentType(trailingArgument: SExpression) =
+        excludedTrailingArgumentType != null && excludedTrailingArgumentType!!.isInstance(trailingArgument)
+
+    class TooFewArgumentsException(functionName: String, argumentList: Cons) : LispException() {
+
+        override val message: String by lazy {
+            format("too few arguments given to {0}: {1}", functionName, argumentList)
+        }
+    }
+
+    class TooManyArgumentsException(functionName: String, argumentList: Cons) : LispException() {
+
+        override val message: String by lazy {
+            format("too many arguments given to {0}: {1}", functionName, argumentList)
+        }
+    }
+
+    class DottedArgumentListException(functionName: String, argumentList: Cons) : LispException() {
+
+        override val message: String by lazy {
+            format("dotted argument list given to {0}: {1}", functionName, argumentList)
+        }
+    }
+
+    class BadArgumentTypeException(functionName: String,
+                                   argument: SExpression,
+                                   expectedType: Class<out SExpression>?) : LispException() {
+
+        private val expectedTypeName: String by lazy {
+            val displayName = expectedType?.getAnnotation(DisplayName::class.java)
+
+            displayName?.value ?: "unknown"
+        }
+
+        override val message: String by lazy {
+            format("{0}: {1} is not the expected type of ''{2}''", functionName, argument.toString(), expectedTypeName)
+        }
+    }
+}