uref: the a.b.c syntax extended to .a.b.c

Now it is possible to use a leading dot on the referencing
dot syntax. This is the is the "unbound reference dot". It
expands to the uref macro, which denotes an unbound-reference:
it produces a function which takes an object as the argument,
and curries the reference implied by the remaining arguments.

* eval.c (uref_s): New global symbol variable.
(eval_init): Intern uref symbol and init uref_s.

* eval.h (uref_s): Declared.

* lib.c (simple_qref_args_p): A qref expression is now
also not simple if it contains an embedded uref, meaning
that it cannot be rendered into the dot notation without
ambiguity.
(obj_print_impl): Support printing (uref a b c) as .a.b.c.

* lisplib.c (struct_set_entries): Add uref to the list of
autoload triggers for struct.tl.

* parser.l (DOTDOT): Consume any leading whitespace as part
of recognizing the DOTDOT token. Otherwise the new rule
for UREFDOT, which matches (mandatory) leading space
will take precedence, causing " .." to be scanned wrong.
(UREFDOT): Rule for new kind of dot token, which is
preceded by mandatory whitespace, and isn't consing
dot (which has mandatory trailing whitespace too,
matched by an earlier rule).

* parser.y (UREFDOT): New token type.
(i_dot_expr, n_dot_expr): New grammar rules.
(list): Handle a leading dot on the first element of a list as
a special case. Things are done this way because trying to
work a UREFDOT into the grammar otherwise causes intractable
conflicts.
(i_expr): The ^, ' and , punctuators are now followed by
an i_dot_expr, so that the expression can be an unbound
dot.
(n_expr): Same change as in i_expr, but using n_dot_expr.
Plus new UREFDOT n_expr production.

* share/txr/stdlib/struct.tl (uref): New macro.

* txr.1: Documented.

1 files changed, 10 insertions, 1 deletions

diff --git a/lib.c b/lib.c
index f7029785..99e7da76 100644
--- a/lib.c
+++ b/lib.c
@@ -9572,7 +9572,10 @@ static val simple_qref_args_p(val args, val pos)
     return nil;
   } else {
     val arg = car(args);
-    if (symbolp(arg) || (consp(arg) && car(arg) != qref_s)) {
+    if (symbolp(arg) || (consp(arg) &&
+                         car(arg) != qref_s &&
+                         car(arg) != uref_s))
+    {
       return simple_qref_args_p(cdr(args), succ(pos));
     }
     return nil;
@@ -9800,6 +9803,12 @@ val obj_print_impl(val obj, val out, val pretty, struct strm_ctx *ctx)
             put_string(lit("."), out);
           iter = next;
         }
+      } else if (sym == uref_s && simple_qref_args_p(cdr(obj), one)) {
+        val iter;
+        for (iter = cdr(obj); iter; iter = cdr(iter)) {
+          put_string(lit("."), out);
+          obj_print_impl(car(iter), out, pretty, ctx);
+        }
       } else if (sym == quasi_s && consp(cdr(obj))) {
         put_char(chr('`'), out);
         out_quasi_str(obj, out, ctx);