Methods for turning objects into sequences.

Struct objects can now define methods car, cdr and nullify.
With these, they can participate in operations on sequences.

* eval.h (car_s, cdr_s): Declared.

* lib.c (nullify_s): New symbol variable.
(car, cdr): Implement for struct objects via, respectively,
their car and cdr methods.
(tolist): Handle objects by mapping through identity.
(nullify): Implement for objects optionally: if an object
is a struct with a nullify method, use it, otherwise go
through default case of just returning the object.
(empty): Implement for objects that have nullify method.
(obj_init): Initialize nullify_s.

* struct.c (maybe_slot): New function.

* struct.h (maybe_slot): Declared.

* txr.1: Documented car, cdr and nullify method
convention.

1 files changed, 1 insertions, 0 deletions

diff --git a/eval.h b/eval.h
index 7af95058..fb9c34f4 100644
--- a/eval.h
+++ b/eval.h
@@ -28,6 +28,7 @@ extern val dwim_s, lambda_s, vector_lit_s, vec_list_s, list_s;
 extern val hash_lit_s, hash_construct_s, struct_lit_s, qref_s;
 extern val eval_error_s, if_s, call_s;
 extern val eq_s, eql_s, equal_s;
+extern val car_s, cdr_s;
 extern val last_form_evaled, last_form_expanded;
 extern val self_load_path_s;