From f9c0def5b6049bc600051b928910f724e14af4f5 Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Mon, 10 Aug 2015 22:31:01 -0700
Subject: Dot with no whitespace generates qref syntax.

a.b.(expr ...).c -> (qref a b (expr ...) c)
Consing dot requires whitespace.

* eval.c (qref_s): New symbol global variable.
(eval_init): Initialize qref_s.

* eval.h (qref_s): Declared.

* parser.l (REQWS): New pattern definition, required whitespace.
(grammar): New rules to scan CONSDOT (space required on both
sides) and LAMBDOT (space required after).

* parser.y (CONSDOT, LAMBDOT): New token types.
(list): (. n_expr) rule replaced with LAMBDOT and CONSDOT.
(r_exprs): r_exprs . n_expr consing dot rule replaced with CONSDOT.
(n_expr): New n_expr . n_expr rule introduced here for producing
qref expressions.
(yybadtoken): Handle CONSDOT and LAMBDOT.

* txr.1: Documented qref dot.
---
 txr.1 | 41 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 41 insertions(+)

(limited to 'txr.1')

diff --git a/txr.1 b/txr.1
index a2c1ee32..2954fc4b 100644
--- a/txr.1
+++ b/txr.1
@@ -9181,6 +9181,47 @@ or an atom
 as
 .codn "(. sym)" .
 
+.NP* Referencing Dot
+
+A dot token which is flanked by expressions on both sides, without any
+intervening whitespace, is the referencing dot, and not the consing dot.
+The referencing dot is a syntactic sugar which translated to the
+.code qref
+syntax ("quoted ref"), which currently has no assigned meaning and is reserved
+for a future language extension. The referencing dot obeys these
+correspondences:
+
+.cblk
+  ;; a.b may be almost any expressions
+  a.b           <-->  (qref a b)
+  a.b.c         <-->  (qref a b c)
+  a.(qref b c)  <-->  (qref a b c)
+  (qref a b).c  <-->  (qref (qref a b) c)
+.cble
+
+That is to say, this dot operator constructs a
+.code qref
+expression out of its left and right arguments. If the right argument
+of the dot is already a qref expression (whether produced by another instance
+of the dot operator, or expressed directly) it is merged. And the qref dot
+operator is right-to-left associative, so that
+.code a.b.c
+first produces
+.code (qref b c)
+via the right dot, and then
+.code a
+is adjoined into the syntax via the right dot.
+
+Integer tokens cannot be involved in this syntax, because they
+form floating-point constants when juxtaposed with a dot.
+Such ambiguous uses of floating-point tokens are diagnosed as syntax errors:
+
+.cblk
+  (a.4)   ;; error: cramped floating-point literal
+  (a .4)  ;; good: a followed by 0.4
+.cble
+
+
 .NP* Quote and Quasiquote
 
 .meIP >> ' expr
-- 
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