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, 2 insertions, 1 deletions

diff --git a/eval.c b/eval.c
index 62c24170..40847c40 100644
--- a/eval.c
+++ b/eval.c
@@ -94,7 +94,7 @@ val gen_s, gun_s, generate_s, rest_s, plus_s;
 val promise_s, promise_forced_s, promise_inprogress_s, force_s;
 val op_s, ap_s, identity_s, apf_s, ipf_s;
 val ret_s, aret_s;
-val hash_lit_s, hash_construct_s, struct_lit_s, qref_s;
+val hash_lit_s, hash_construct_s, struct_lit_s, qref_s, uref_s;
 val vector_lit_s, vec_list_s;
 val macro_time_s, with_dyn_rebinds_s, macrolet_s;
 val defsymacro_s, symacrolet_s, prof_s, switch_s;
@@ -5417,6 +5417,7 @@ void eval_init(void)
   hash_construct_s = intern(lit("hash-construct"), user_package);
   struct_lit_s = intern(lit("struct-lit"), system_package);
   qref_s = intern(lit("qref"), user_package);
+  uref_s = intern(lit("uref"), user_package);
   vector_lit_s = intern(lit("vector-lit"), system_package);
   vec_list_s = intern(lit("vec-list"), user_package);
   macro_time_s = intern(lit("macro-time"), user_package);