Add a new implementation struct with only one function.

include/impl.h

@@ -0,0 +1,16 @@
+#ifndef LIBABACUS_IMPL_H
+#define LIBABACUS_IMPL_H
+
+/**
+ * Implementation functions for things like numbers.
+ */
+struct libab_impl_s {
+    /**
+     * Function to parse a number from a string.
+     */
+    void* (*parse_num)(const char*);
+};
+
+typedef struct libab_impl_s libab_impl;
+
+#endif

include/interpreter.h

@@ -3,15 +3,18 @@
 
 #include "table.h"
 #include "tree.h"
+#include "impl.h"
 
 struct libab_interpreter_s {
     libab_ref base_table;
+    libab_impl* impl;
 };
 
 typedef struct libab_interpreter_s libab_interpreter;
 
 void libab_interpreter_init(libab_interpreter* intr,
-                            libab_ref* table);
+                            libab_ref* table,
+                            libab_impl* impl);
 libab_result libab_interpreter_run(libab_interpreter* intr,
                                    libab_tree* tree, libab_ref* into);
 void libab_interpreter_free(libab_interpreter* intr);

include/libabacus.h

@@ -8,6 +8,7 @@
 #include "interpreter.h"
 #include "result.h"
 #include "table.h"
+#include "impl.h"
 
 /**
  * The main struct of libabacus,
@@ -35,6 +36,11 @@ struct libab_s {
      * things like functions and operators.
      */
     libab_ref table;
+
+    /**
+     * The allocator used to construct number instances.
+     */
+    libab_impl impl;
 };
 
 typedef struct libab_s libab;