diff --git a/include/impl.h b/include/impl.h
new file mode 100644
index 0000000..d4f31c8
--- /dev/null
+++ b/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
diff --git a/include/interpreter.h b/include/interpreter.h
index 62bd817..f0eee73 100644
--- a/include/interpreter.h
+++ b/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);
diff --git a/include/libabacus.h b/include/libabacus.h
index 998a79b..4c425a0 100644
--- a/include/libabacus.h
+++ b/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;
diff --git a/src/interpreter.c b/src/interpreter.c
index 2fa5a82..cc496ff 100644
--- a/src/interpreter.c
+++ b/src/interpreter.c
@@ -2,18 +2,22 @@
 #include "util.h"
 
 void libab_interpreter_init(libab_interpreter* intr,
-                            libab_ref* table) {
+                            libab_ref* table,
+                            libab_impl* impl) {
     libab_ref_copy(table, &intr->base_table);
+    intr->impl = impl;
 }
 
 struct interpreter_state {
     libab_ref num_ref;
+    libab_impl* impl;
 };
 
 libab_result _interpreter_init(struct interpreter_state* state, libab_interpreter* intr) {
     libab_result result = LIBAB_SUCCESS;
     libab_basetype* num_type;
     libab_ref_null(&state->num_ref);
+    state->impl = intr->impl;
 
     num_type = libab_table_search_basetype(libab_ref_get(&intr->base_table), "num");
     if(num_type != NULL) {
diff --git a/src/libabacus.c b/src/libabacus.c
index 9956686..6283853 100644
--- a/src/libabacus.c
+++ b/src/libabacus.c
@@ -12,7 +12,7 @@ libab_result libab_init(libab* ab) {
 
     if(result == LIBAB_SUCCESS) {
         libab_parser_init(&ab->parser, &ab->table);
-        libab_interpreter_init(&ab->intr, &ab->table);
+        libab_interpreter_init(&ab->intr, &ab->table, &ab->impl);
         result = libab_lexer_init(&ab->lexer);
     }