diff --git a/app/Engine/Cfnpp/Expression/Expression.php b/app/Engine/Cfnpp/Expression/Expression.php
index 9cca07c..f70d4b3 100644
--- a/app/Engine/Cfnpp/Expression/Expression.php
+++ b/app/Engine/Cfnpp/Expression/Expression.php
@@ -39,6 +39,25 @@ class Expression
 		$this->tokens = $tokens;
 	}
 
+	/**
+	 * Find all referenced variables so we can do proper ordering of variable
+	 * block evaluate.
+	 *
+	 * @return string[]
+	 */
+	public function getReferencedVariables(): array
+	{
+		$variables = [];
+		foreach ($this->tokens as $token)
+		{
+			if ($token instanceof TokenVariable)
+			{
+				$variables[] = $token->getName();
+			}
+		}
+		return array_values(array_unique($variables));
+	}
+
 	/**
 	 * Evaluate the tokens contained in this expression.
 	 *
@@ -56,21 +75,12 @@ class Expression
 			$token = array_shift($tokens);
 			assert(isset($token));
 
-			if ($token instanceof TokenNumericLiteral)
-			{
-				$this->evaluateTokenNumericLiteral($token);
-			}
-			elseif ($token instanceof TokenStringLiteral)
-			{
-				$this->evaluateTokenStringLiteral($token);
-			}
-			elseif ($token instanceof TokenOperator)
+			$token_class = get_class($token);
+			$token_name = substr($token_class, strrpos($token_class, '\\') + 1);
+			$func = 'evaluate'.$token_name;
+			if (method_exists($this, $func))
 			{
-				$this->evaluateTokenOperator($token);
-			}
-			elseif ($token instanceof TokenVariable)
-			{
-				$this->evaluateTokenVariable($token);
+				$this->{$func}($token);
 			}
 			else
 			{
@@ -148,6 +158,32 @@ class Expression
 				$var2 = $this->pop();
 				$this->push($var1 || $var2);
 				break;
+			default:
+				throw new \Exception('Unhandled comparison operator: '.$token->getOperator());
+		}
+	}
+
+	/**
+	 * Evaluate and mutate the stack given a function token.
+	 *
+	 * @param  TokenFunction $token
+	 * @return void
+	 */
+	protected function evaluateTokenFunction(TokenFunction $token): void
+	{
+		switch ($token->getFunction())
+		{
+			case 'concat':
+				$this->push($this->pop(1).$this->pop());
+				break;
+			case 'concat*':
+				while (sizeof($this->stack) > 1)
+				{
+					$this->push($this->pop(1).$this->pop());
+				}
+				break;
+			default:
+				throw new \Exception('Unhandled function: '.$token->getFunction());
 		}
 	}
 
diff --git a/app/Engine/Cfnpp/Expression/Parser.php b/app/Engine/Cfnpp/Expression/Parser.php
index 19cec7c..f7122c4 100644
--- a/app/Engine/Cfnpp/Expression/Parser.php
+++ b/app/Engine/Cfnpp/Expression/Parser.php
@@ -17,6 +17,7 @@ class Parser
 	 * @var string[]
 	 */
 	public const TOKEN_TYPES = [
+		TokenFunction::class,
 		TokenNumericLiteral::class,
 		TokenOperator::class,
 		TokenStringLiteral::class,
diff --git a/app/Engine/Cfnpp/Expression/TokenFunction.php b/app/Engine/Cfnpp/Expression/TokenFunction.php
new file mode 100644
index 0000000..0b99cef
--- /dev/null
+++ b/app/Engine/Cfnpp/Expression/TokenFunction.php
@@ -0,0 +1,90 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Engine\Cfnpp\Expression;
+
+/**
+ * Token representing a function.
+ *
+ * @author Adam Pippin <hello@adampippin.ca>
+ */
+class TokenFunction extends Token
+{
+	/**
+	 * List of valid functions this can parse.
+	 * @var string[]
+	 */
+	public const FUNCTIONS = [
+		'concat*',
+		'concat'
+	];
+
+	/**
+	 * Function this token represents.
+	 * @var string
+	 */
+	protected $function;
+
+	/**
+	 * Create a new function token.
+	 *
+	 * @param string $function
+	 */
+	public function __construct($function)
+	{
+		$this->function = $function;
+	}
+
+	/**
+	 * Get the function this token represents.
+	 *
+	 * @return string
+	 */
+	public function getFunction(): string
+	{
+		return $this->function;
+	}
+
+	/**
+	 * Determine whether a function token can be parsed from a stream.
+	 *
+	 * @param  string $stream
+	 * @return bool
+	 */
+	public static function isToken(string $stream): bool
+	{
+		foreach (static::FUNCTIONS as $function)
+		{
+			if (strlen($stream) > strlen($function) &&
+				substr($stream, 0, strlen($function)) == $function)
+			{
+				return true;
+			}
+		}
+		return false;
+	}
+
+	/**
+	 * Parse a function token from a stream.
+	 *
+	 * Returns token, and modifies stream to remove all consumed characters
+	 *
+	 * @param  string $stream
+	 * @return Token
+	 */
+	public static function getToken(string &$stream): Token
+	{
+		foreach (static::FUNCTIONS as $function)
+		{
+			if (strlen($stream) > strlen($function) &&
+				substr($stream, 0, strlen($function)) == $function)
+			{
+				$function = substr($stream, 0, strlen($function));
+				$stream = substr($stream, strlen($function));
+				return new TokenFunction($function);
+			}
+		}
+		throw new \Exception('Could not parse function token!');
+	}
+}