Got most of the new memory architecture roughed out.

src/c/payloads/function.h

@@ -0,0 +1,47 @@
+/**
+ *  payloads/function.h
+ *
+ *  an ordinary Lisp function - one whose arguments are pre-evaluated.
+ *
+ *  (c) 2026 Simon Brooke <simon@journeyman.cc>
+ *  Licensed under GPL version 2.0, or, at your option, any later version.
+ */
+
+#ifndef __psse_payloads_function_h
+#define __psse_payloads_function_h
+
+#include "memory/pointer.h"
+
+/**
+ * @brief Tag for an ordinary Lisp function - one whose arguments are pre-evaluated.
+ * \see LAMBDATAG for interpretable functions.
+ * \see SPECIALTAG for functions whose arguments are not pre-evaluated.
+ */
+#define FUNCTIONTAG "FUN"
+
+/**
+ * @brief Payload of a function cell.
+ * `source` points to the source from which the function was compiled, or NIL
+ * if it is a primitive.
+ * `executable` points to a function which takes a pointer to a stack frame
+ * (representing its stack frame) and a cons pointer (representing its
+ * environment) as arguments and returns a cons pointer (representing its
+ * result).
+ */
+struct function_payload {
+    /**
+     * pointer to metadata (e.g. the source from which the function was compiled).
+     */
+    struct cons_pointer meta;
+    /**  pointer to a function which takes a cons pointer (representing
+     * its argument list) and a cons pointer (representing its environment) and a
+     * stack frame (representing the previous stack frame) as arguments and returns
+     * a cons pointer (representing its result).
+     * \todo check this documentation is current!
+     */
+    struct cons_pointer ( *executable ) ( struct stack_frame *,
+                                          struct cons_pointer,
+                                          struct cons_pointer );
+};
+
+#endif