From c1025cd036938254cf00d0ce0da83de761525eb4 Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Tue, 22 May 2018 20:03:13 -0700
Subject: txr: support variable in postive match.

In the @{var mod} syntax in the pattern language, allow mod to
be a variable which contains a regex or integer, not just an
integer or regex literal.

* match.c (h_var): Check for modifier being a variable, and
resolve it.

* parser.y (modifiers): Allow a SYMTOK phrase.

* txr.1: Documented.
---
 match.c  |  5 +++++
 parser.y |  1 +
 txr.1    | 21 +++++++++++++++++++++
 3 files changed, 27 insertions(+)

diff --git a/match.c b/match.c
index 3a1024e9..a7a65098 100644
--- a/match.c
+++ b/match.c
@@ -596,6 +596,11 @@ static val h_var(match_line_ctx *c)
               sym, nao);
   }
 
+  if (bindable(modifier)) {
+    val mpair = tx_lookup_var_ubc(modifier, c->bindings, elem);
+    modifier = cdr(mpair);
+  }
+
   if (pair) {
     /* If the variable already has a binding, we replace
        it with its value, and treat it as a string match.
diff --git a/parser.y b/parser.y
index abc12d1c..a64d489b 100644
--- a/parser.y
+++ b/parser.y
@@ -788,6 +788,7 @@ modifiers : NUMBER              { $$ = cons($1, nil); }
                                   rlc($$, $1); }
           | compound            { $$ = rlc(cons(expand_meta($1, nil),
                                                  nil), $1); }
+          | SYMTOK              { $$ = cons(symhlpr($1, nil), nil); }
           ;
 
 o_var : SYMTOK                  { val expr = symhlpr($1, nil);
diff --git a/txr.1 b/txr.1
index 7d6e00bb..206cdd16 100644
--- a/txr.1
+++ b/txr.1
@@ -2179,6 +2179,7 @@ enclosed with the variable in braces:
 .mets >> @{ bident <> / regex /}
 .mets >> @{ bident >> ( fun >> [args ...])}
 .mets >> @{ bident << number }
+.mets >> @{ bident << bident }
 .cble
 
 These specify a variable binding that is driven by a positive match derived
@@ -2227,6 +2228,26 @@ This syntax is processed without consideration of what other
 syntax follows.  A positive match may be directly followed by an unbound
 variable.
 
+The
+.cblk
+.mets >> @{ bident << bident }
+.cble
+syntax allows the
+.meta number
+or
+.meta regex
+modifier to come from a variable. The variable must be bound and contain
+a non-negative integer or regular expression.
+For example,
+.code "@{x y}"
+behaves like
+.code "@{x 3}"
+if
+.code y
+is bound to the integer 3. It is an error if
+.code y
+is unbound.
+
 .coSS Special Symbols @ nil and @ t
 
 Just like in the Common Lisp language, the names
-- 
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