Fix #116: Add set_file_reader callback for custom file loading

Add a customizable file reader callback to ChaiScript_Basic, following
the same pattern as set_print_handler. When set, the callback is invoked
instead of the default filesystem read, enabling use cases like encrypted
files, in-memory virtual filesystems, or platform-specific file access
(e.g., Android assets). The callback is settable from both C++ and
ChaiScript.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

include/chaiscript/language/chaiscript_engine.hpp

@@ -80,6 +80,7 @@ namespace chaiscript {
     std::map<std::string, std::function<Namespace &()>> m_namespace_generators;
 
     std::function<void(const std::string &)> m_print_handler = [](const std::string &) noexcept {};
+    std::function<std::string(const std::string &)> m_file_reader;
 
     /// Evaluates the given string in by parsing it and running the results through the evaluator
     Boxed_Value do_eval(const std::string &t_input, const std::string &t_filename = "__EVAL__", bool /* t_internal*/ = false) {
@@ -139,6 +140,10 @@ namespace chaiscript {
         m_print_handler = t_handler;
       }), "set_print_handler");
 
+      m_engine.add(fun([this](const std::function<std::string(const std::string &)> &t_reader) {
+        m_file_reader = t_reader;
+      }), "set_file_reader");
+
       m_engine.add(fun([this]() { m_engine.dump_system(); }), "dump_system");
       m_engine.add(fun([this](const Boxed_Value &t_bv) { m_engine.dump_object(t_bv); }), "dump_object");
       m_engine.add(fun([this](const Boxed_Value &t_bv, const std::string &t_type) { return m_engine.is_type(t_bv, t_type); }), "is_type");
@@ -236,7 +241,15 @@ namespace chaiscript {
     }
 
     /// Helper function for loading a file
-    static std::string load_file(const std::string &t_filename) {
+    std::string load_file(const std::string &t_filename) const {
+      if (m_file_reader) {
+        return m_file_reader(t_filename);
+      }
+
+      return load_file_default(t_filename);
+    }
+
+    static std::string load_file_default(const std::string &t_filename) {
       std::ifstream infile(t_filename.c_str(), std::ios::in | std::ios::ate | std::ios::binary);
 
       if (!infile.is_open()) {
@@ -278,6 +291,12 @@ namespace chaiscript {
       m_print_handler = std::move(t_handler);
     }
 
+    /// \brief Set a custom handler for reading files, used by eval_file, use, and internal_eval_file
+    /// \param[in] t_reader Function to call with the filename, returning the file contents as a string
+    void set_file_reader(std::function<std::string(const std::string &)> t_reader) {
+      m_file_reader = std::move(t_reader);
+    }
+
     /// \brief Virtual destructor for ChaiScript
     virtual ~ChaiScript_Basic() = default;
      

unittests/compiled_tests.cpp

@@ -1782,3 +1782,42 @@ TEST_CASE("eval_error with AST_Node_Trace call stack compiles in C++20") {
     (void)stack;
   }
 }
+
+TEST_CASE("Test set_file_reader from C++ land") {
+  chaiscript::ChaiScript chai;
+  chai.set_file_reader([](const std::string &) {
+    return std::string("var file_reader_test_val = 42");
+  });
+  chai.eval_file("nonexistent_file.chai");
+  CHECK(chai.eval<int>("file_reader_test_val") == 42);
+}
+
+TEST_CASE("Test set_file_reader from ChaiScript land") {
+  chaiscript::ChaiScript chai;
+  chai.set_file_reader([](const std::string &) {
+    return std::string("var from_custom_reader = true");
+  });
+  chai.eval("set_file_reader(fun(filename) { return \"var from_chai_reader = true\"; })");
+  chai.eval_file("any_file.chai");
+  CHECK(chai.eval<bool>("from_chai_reader") == true);
+}
+
+TEST_CASE("Test set_file_reader receives correct filename") {
+  chaiscript::ChaiScript chai;
+  std::string captured_filename;
+  chai.set_file_reader([&captured_filename](const std::string &t_filename) {
+    captured_filename = t_filename;
+    return std::string("var dummy = 1");
+  });
+  chai.eval_file("my_special_file.chai");
+  CHECK(captured_filename == "my_special_file.chai");
+}
+
+TEST_CASE("Test use with set_file_reader") {
+  chaiscript::ChaiScript chai;
+  chai.set_file_reader([](const std::string &) {
+    return std::string("var use_reader_val = 99");
+  });
+  chai.use("virtual_file.chai");
+  CHECK(chai.eval<int>("use_reader_val") == 99);
+}