Adding special function from-list.

* lib.c (from_list_s): New symbol variable.
(make_like): Handle a COBJ. If it's a structure with a
from-list method, then use it, otherwise the
default handling applies of returning the list.
(obj_init): Initialize from_list_s.

* txr.1: Documented.

2 files changed, 51 insertions, 1 deletions

diff --git a/lib.c b/lib.c
index cfa2d2bb..8326d34a 100644
--- a/lib.c
+++ b/lib.c
@@ -101,7 +101,7 @@ val error_s, type_error_s, internal_error_s, panic_s;
 val numeric_error_s, range_error_s;
 val query_error_s, file_error_s, process_error_s, syntax_error_s;
 val timeout_error_s, system_error_s;
-val gensym_counter_s, nullify_s;
+val gensym_counter_s, nullify_s, from_list_s;
 
 val nothrow_k, args_k, colon_k, auto_k, fun_k;
 val wrap_k, reflect_k;
@@ -662,6 +662,13 @@ val make_like(val list, val thatobj)
       if (is_chr(car(list)))
         return cat_str(list, nil);
       break;
+    case COBJ:
+      if (structp(thatobj)) {
+        val from_list_meth = maybe_slot(thatobj, from_list_s);
+        if (from_list_meth)
+          return funcall1(from_list_meth, list);
+      }
+      break;
     case NIL:
     case CONS:
     case LCONS:
@@ -8652,6 +8659,7 @@ static void obj_init(void)
   assert_s = intern(lit("assert"), user_package);
   name_s = intern(lit("name"), user_package);
   nullify_s = intern(lit("nullify"), user_package);
+  from_list_s = intern(lit("from-list"), user_package);
 
   args_k = intern(lit("args"), keyword_package);
   nothrow_k = intern(lit("nothrow"), keyword_package);
diff --git a/txr.1 b/txr.1
index 00e15ca5..5a2e79ae 100644
--- a/txr.1
+++ b/txr.1
@@ -21113,6 +21113,48 @@ method should return
 if the object is considered to denote an empty sequence. Otherwise it
 should return that object itself.
 
+.SS Function @ from-list
+.synb
+.mets << object .[from-list << list ]
+.syne
+.desc
+If a
+.code from-list
+structure function is defined for a structure type, it is called in certain
+situations with an argument which is a list object. The function's purpose
+is to construct a new instance of the structure type, derived from that
+list.
+
+Note: the
+.code from-list
+function isn't a method; it doesn't receive
+.meta object
+as an argument. In the style of call depicted by the syntax description
+above,
+.meta object
+is used to identify the structure type whose
+.meta from-list
+static slot provides the function definition.
+
+The purpose of this function is to allow sequence processing operations
+such as
+.code mapcar
+and
+.code remove
+to operate on a structure object as if it were a sequence, and return a
+transformed sequence of the same type.  This is analogous to the way such
+functions can operate on a vector or string, and return a vector or string.
+
+If a structure object behaves as a sequence thanks to providing
+.codn car ,
+.code cdr
+and
+.code nullify
+methods, but does not have a
+.code from-list
+function, then those sequence-processing operations which return a sequence
+will always return a plain list of items.
+
 .SS* Sequence Manipulation
 .coNP Function @ seqp
 .synb