From 1d9f2661a2f151b9b264dd696821730e2e6e3e46 Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Wed, 2 Aug 2017 20:06:18 -0700
Subject: Bi-directional string tree match for non-vars.

There is an inconsistency in @(bind) in that
given @(bind x y) where x is a variable, both
directions are tried for a string tree match.
x could be tree of strings and y a string atom,
or vice versa. But if x is just an atom, or
a Lisp evaluation, then only one direction is
tried. @(bind @(list "a" "b") "a") succeeds,
but @(bind "a" @(list "a" "b")) fails.

* match.c (dest_bind): Test both directions
in the scalar and Lisp evaluated cases of the
left hand side. Subject to compatibility,
just in case.

* txr.1: Compat note added.
---
 match.c | 14 ++++++++++++--
 txr.1   | 28 ++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+), 2 deletions(-)

diff --git a/match.c b/match.c
index c77a4e7d..6f151c7b 100644
--- a/match.c
+++ b/match.c
@@ -366,8 +366,14 @@ static val dest_bind(val spec, val bindings, val pattern,
       lisp_evaled = t;
     }
 
-    if (lisp_evaled)
-      return if3(tree_find(value, ret, swap_12_21(testfun)), bindings, t);
+    if (lisp_evaled) {
+      if (!opt_compat || opt_compat >= 184)
+        if (tree_find(ret, value, testfun))
+          return bindings;
+      if (tree_find(value, ret, swap_12_21(testfun)))
+        return bindings;
+      return t;
+    }
 
     while (consp(piter) && consp(viter))
     {
@@ -386,6 +392,10 @@ static val dest_bind(val spec, val bindings, val pattern,
       return funcall2(testfun, piter, viter) ? bindings : t;
     }
     return bindings;
+  } else if ((!opt_compat || opt_compat >= 184) &&
+             tree_find(pattern, value, testfun))
+  {
+    return bindings;
   } else if (tree_find(value, pattern, swap_12_21(testfun))) {
     return bindings;
   }
diff --git a/txr.1 b/txr.1
index 22e24752..8d88ff38 100644
--- a/txr.1
+++ b/txr.1
@@ -60685,6 +60685,34 @@ of these version values, the described behaviors are provided if
 is given an argument which is equal or lower. For instance
 .code "-C 103"
 selects the behaviors described below for version 105, but not those for 102.
+.IP 183
+A value of 183 or lower restores an inconsistent behavior in the
+.code "@(bind)"
+directive and other places in the \*(TX pattern language where binding
+takes place. Prior to version 184, a string tree match was only tried in
+both directions when the left hand side of a binding (the "pattern") was a
+variable. For non-variable pattern terms, such as Lisp expressions or atoms,
+the string tree match was tried in one direction only: a string tree arising
+out of the pattern could match a string atom value on the right side.
+A string tree is a nested list structure whose leaves are strings: a list
+of strings, a list of lists of strings, and so on, in any mixture.
+Concretely, before \*(TX 184,
+.cblk
+@(bind "a" ("a" "b" "c"))
+.cble
+didn't match, but
+.cblk
+@(bind ("a" "b" "c") "a")
+.cble
+did. However, if the variable
+.code a
+contained
+.strn a
+then
+.cblk
+@(bind a ("a" "b" "c"))
+.cble
+did match: an inconsistency.
 .IP 177
 A value of 177 or lower causes the emulation of a bug which was present in the
 .code rng
-- 
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