diff --git a/gcc/elna-generic.cc b/gcc/elna-generic.cc
index 0dccd6b..633a277 100644
--- a/gcc/elna-generic.cc
+++ b/gcc/elna-generic.cc
@@ -13,15 +13,40 @@ namespace gcc
 {
     void generic_visitor::visit(source::call_statement *statement)
     {
-        empty_visitor::visit(statement);
+        if (statement->name() != "writei")
+        {
+            error_at(elna_gcc_location(&statement->position()),
+                    "procedure '%s' not declared",
+                    statement->name().c_str());
+            return;
+        }
+        if (statement->arguments().size() != 1)
+        {
+            error_at(elna_gcc_location(&statement->position()),
+                    "procedure '%s' expects 1 argument, %i given",
+                    statement->name().c_str(), statement->arguments().size());
+            return;
+        }
+        auto& argument = statement->arguments().at(0);
+        argument->accept(this);
+        auto argument_type = TREE_TYPE(this->current_expression);
+        auto argument_tree = this->current_expression;
+        this->current_expression = NULL_TREE;
 
-        const char *format_integer = "%d\n";
+        if (argument_type != integer_type_node)
+        {
+            error_at(elna_gcc_location(&argument->position()),
+                    "invalid argument of type %s for procedure %s",
+                    elna_gcc_print_type(argument_type), statement->name().c_str());
+            return;
+        }
+        constexpr const char *format_integer = "%d\n";
         tree args[] = {
             build_string_literal(strlen(format_integer) + 1, format_integer),
-            this->current_expression
+            argument_tree
         };
         tree fndecl_type_param[] = {
-            build_pointer_type (build_qualified_type(char_type_node, TYPE_QUAL_CONST)) /* const char* */
+            build_pointer_type(build_qualified_type(char_type_node, TYPE_QUAL_CONST)) /* const char* */
         };
         tree fndecl_type = build_varargs_function_type_array(integer_type_node, 1, fndecl_type_param);
 
@@ -84,11 +109,14 @@ namespace gcc
     {
         expression->lhs().accept(this);
         auto left = this->current_expression;
+        auto left_type = TREE_TYPE(left);
 
         expression->rhs().accept(this);
         auto right = this->current_expression;
+        auto right_type = TREE_TYPE(right);
 
-        tree_code operator_code{};
+        auto expression_location = elna_gcc_location(&expression->position());
+        tree_code operator_code = ERROR_MARK;
 
         switch (expression->operation())
         {
@@ -104,11 +132,51 @@ namespace gcc
             case source::binary_operator::multiplication:
                 operator_code = MULT_EXPR;
                 break;
-            default:
-                gcc_unreachable();
         }
-
-        this->current_expression = build2(operator_code, integer_type_node, left, right);
+        if (operator_code != ERROR_MARK) // An arithmetic operation.
+        {
+            if (left_type != integer_type_node || right_type != integer_type_node)
+            {
+                error_at(expression_location,
+                        "invalid operands of type %s and %s for operator %s",
+                        elna_gcc_print_type(left_type), elna_gcc_print_type(right_type),
+                        elna::source::print_binary_operator(expression->operation()));
+                this->current_expression = error_mark_node;
+                return;
+            }
+        }
+        switch (expression->operation())
+        {
+            case source::binary_operator::equals:
+                operator_code = PLUS_EXPR;
+                break;
+            case source::binary_operator::not_equals:
+                operator_code = MINUS_EXPR;
+                break;
+            case source::binary_operator::less:
+                operator_code = TRUNC_DIV_EXPR;
+                break;
+            case source::binary_operator::greater:
+                operator_code = MULT_EXPR;
+                break;
+            case source::binary_operator::less_equal:
+                operator_code = TRUNC_DIV_EXPR;
+                break;
+            case source::binary_operator::greater_equal:
+                operator_code = MULT_EXPR;
+                break;
+        }
+        if (left_type != right_type)
+        {
+            error_at(expression_location,
+                    "invalid operands of type %s and %s for operator %s",
+                    elna_gcc_print_type(left_type), elna_gcc_print_type(right_type),
+                    elna::source::print_binary_operator(expression->operation()));
+            this->current_expression = error_mark_node;
+            return;
+        }
+        this->current_expression = build2_loc(expression_location,
+                operator_code, integer_type_node, left, right);
     }
 
     void generic_visitor::visit(source::declaration *declaration)
diff --git a/include/elna/source/ast.h b/include/elna/source/ast.h
index 1e6e797..cc3188a 100644
--- a/include/elna/source/ast.h
+++ b/include/elna/source/ast.h
@@ -486,5 +486,7 @@ namespace source
         expression& operand();
         unary_operator operation() const noexcept;
     };
+
+    const char *print_binary_operator(const binary_operator operation);
 }
 }
diff --git a/source/ast.cc b/source/ast.cc
index 696569d..cbca9fe 100644
--- a/source/ast.cc
+++ b/source/ast.cc
@@ -525,5 +525,32 @@ namespace source
     {
         return *m_body;
     }
+
+    const char *print_binary_operator(const binary_operator operation)
+    {
+        switch (operation)
+        {
+            case binary_operator::sum:
+                return "+";
+            case binary_operator::subtraction: 
+                return "-";
+            case binary_operator::multiplication:
+                return "*";
+            case binary_operator::division:
+                return "/";
+            case binary_operator::equals:
+                return "=";
+            case binary_operator::not_equals:
+                return "/=";
+            case binary_operator::less:
+                return "<";
+            case binary_operator::less_equal:
+                return "<=";
+            case binary_operator::greater:
+                return ">";
+            case binary_operator::greater_equal:
+                return ">=";
+        }
+    };
 }
 }