Add !expr function to allow inserting the result of evaluating an expression

app/Engine/Cfnpp/Functions.php

@@ -141,6 +141,11 @@ class Functions
 		$expression = $parser->parse($condition->getValue());
 		$result = $expression->evaluate($this->options->getVariables());
 
+		if (is_array($result))
+		{
+			throw new \Exception('!if expression must evaluate down to a single value: '.$condition->getValue());
+		}
+
 		if ($result)
 		{
 			$if_true->setName($node->hasName() ? $node->getName() : null);
@@ -153,4 +158,28 @@ class Functions
 		}
 		return $if_false;
 	}
+
+	/**
+	 * Calculate an expression.
+	 *
+	 * @param  Node         $node
+	 * @param  NodeFunction $function
+	 * @return ?Node
+	 */
+	public function f_expr(Node $node, NodeFunction $function): ?Node
+	{
+		if (!($function instanceof NodeFunctionValue))
+		{
+			throw new \Exception('!if requires scalar argument');
+		}
+
+		$parser = new \App\Engine\Cfnpp\Expression\Parser();
+		$expression = $parser->parse($function->getValue());
+		$result = $expression->evaluate($this->options->getVariables());
+
+		$result = Node::fromPhp($result);
+		$result->setName($node->hasName() ? $node->getName() : null);
+
+		return $result;
+	}
 }