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diff --git a/build2/module b/build2/module
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+// file      : build2/module -*- C++ -*-
+// copyright : Copyright (c) 2014-2015 Code Synthesis Ltd
+// license   : MIT; see accompanying LICENSE file
+
+#ifndef BUILD2_MODULE
+#define BUILD2_MODULE
+
+#include <map>
+
+#include <build2/types>
+#include <build2/utility>
+
+#include <build2/diagnostics>
+
+namespace build2
+{
+  class scope;
+  class location;
+
+  class module
+  {
+  public:
+    virtual
+    ~module () = default;
+  };
+
+  extern "C"
+  using module_boot_function =
+    void (scope& root, const location&, unique_ptr<module>&);
+
+  // Return false if the module configuration (normally based on the default
+  // values) was unsuccessful but this is not (yet) an error. One example
+  // would be the optional use of a module. Or a module might remain
+  // unconfigured for as long as it is actually not used (e.g., install,
+  // dist). The return value is used to set the <module>.configured variable.
+  //
+  extern "C"
+  using module_init_function =
+    bool (scope& root,
+          scope& base,
+          const location&,
+          unique_ptr<module>&,
+          bool first,     // First time for this project.
+          bool optional); // Loaded with 'using?' (optional module).
+
+
+  struct module_state
+  {
+    bool boot; // True if the module boot'ed but not yet init'ed.
+    module_init_function* init;
+    unique_ptr<build2::module> module;
+    const location loc; // Boot location.
+  };
+
+  using loaded_module_map = std::map<string, module_state>;
+
+  // Load and boot the specified module.
+  //
+  void
+  boot_module (const string& name, scope& root, const location&);
+
+  // Load (if not already loaded) and initialize the specified module. Used
+  // by the parser but also by some modules to load prerequisite modules.
+  // Return true if the module was both successfully loaded and configured
+  // (false can only be returned if optional).
+  //
+  bool
+  load_module (bool optional,
+               const std::string& name,
+               scope& root,
+               scope& base,
+               const location&);
+
+  // Builtin modules.
+  //
+  struct module_functions
+  {
+    module_boot_function* boot;
+    module_init_function* init;
+  };
+
+  using available_module_map = std::map<string, module_functions>;
+  extern available_module_map builtin_modules;
+}
+
+#endif // BUILD2_MODULE