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, 14 insertions, 0 deletions

diff --git a/struct.c b/struct.c
index 21a3a18a..2075b42f 100644
--- a/struct.c
+++ b/struct.c
@@ -759,6 +759,20 @@ val slot(val strct, val sym)
   no_such_slot(self, si->type->self, sym);
 }
 
+val maybe_slot(val strct, val sym)
+{
+  const val self = lit("slot");
+  struct struct_inst *si = struct_handle(strct, self);
+
+  if (symbolp(sym)) {
+    loc ptr = lookup_slot_load(strct, si, sym);
+    if (!nullocp(ptr))
+      return deref(ptr);
+  }
+
+  return nil;
+}
+
 val slotset(val strct, val sym, val newval)
 {
   const val self = lit("slotset");