diff --git a/src/language/ast/ASTNodeExpressionBuilder.cpp b/src/language/ast/ASTNodeExpressionBuilder.cpp
index f8d26d197e94ae33fef09eed675bfdd688c6cc44..d9b3b4e439183e1f3a205a7c94e47252ac7a42f2 100644
--- a/src/language/ast/ASTNodeExpressionBuilder.cpp
+++ b/src/language/ast/ASTNodeExpressionBuilder.cpp
@@ -25,6 +25,19 @@
 #include <language/node_processor/WhileProcessor.hpp>
 #include <language/utils/ParseError.hpp>
 
+void
+ASTNodeExpressionBuilder::_checkIsPureFunction(const ASTNode& node) const
+{
+  if (node.is_type<language::unary_plusplus>() or node.is_type<language::unary_minusminus>() or
+      node.is_type<language::post_plusplus>() or node.is_type<language::post_minusminus>()) {
+    throw ParseError("invalid function definition. Function data must be constant!", node.begin());
+  }
+
+  for (auto&& child : node.children) {
+    this->_checkIsPureFunction(*child);
+  }
+}
+
 void
 ASTNodeExpressionBuilder::_buildExpression(ASTNode& n)
 {
@@ -42,6 +55,8 @@ ASTNodeExpressionBuilder::_buildExpression(ASTNode& n)
   } else if (n.is_type<language::tuple_expression>()) {
     n.m_node_processor = std::make_unique<TupleToVectorProcessor<ASTNodeExpressionListProcessor>>(n);
   } else if (n.is_type<language::function_definition>()) {
+    this->_checkIsPureFunction(n);
+
     n.m_node_processor = std::make_unique<FakeProcessor>();
 
   } else if (n.is_type<language::function_evaluation>()) {
diff --git a/src/language/ast/ASTNodeExpressionBuilder.hpp b/src/language/ast/ASTNodeExpressionBuilder.hpp
index 74a2f720970df49a6ca5315dfd0b20fb2905f7d7..0e36b65289593d8f59074b7dcc9e8fbd1faeaef2 100644
--- a/src/language/ast/ASTNodeExpressionBuilder.hpp
+++ b/src/language/ast/ASTNodeExpressionBuilder.hpp
@@ -5,7 +5,9 @@
 
 class ASTNodeExpressionBuilder
 {
+ private:
   void _buildExpression(ASTNode& n);
+  void _checkIsPureFunction(const ASTNode& n) const;
 
  public:
   ASTNodeExpressionBuilder(ASTNode& n);