* match.c (dest_bind): Remove the restriction of not allowing

@(expr ...) and @var on the left side of a bind. This
is useful, and necessary for @(line @(lisp expr)) to work:
matching computed line numbers and character positions.

* txr.1: Document use of Lisp on left hand side of bind,
that there is a restriction on the left hand side of a set,
and that Lisp can be used in a line or chr directive for
computed matches.

1 files changed, 55 insertions, 0 deletions

diff --git a/txr.1 b/txr.1
index 4541c485..006d3b0a 100644
--- a/txr.1
+++ b/txr.1
@@ -3678,6 +3678,19 @@ takes the entire line of input:
   @(chr 0)@(chr 0)@x
 .cble
 
+The argument of
+.code line
+or
+.code chr
+may be a
+.codn @ -delimited
+Lisp expression. This is useful for matching computed lines or
+character positions:
+
+.cblk
+  @(line @(+ a (* b c)))
+.cble
+
 .dirs some all none maybe cases choose
 
 These directives, called the parallel directives, combine multiple subqueries,
@@ -5351,6 +5364,33 @@ matches
 .str c
 if these are upcased.
 
+.coNP Lisp forms in the @ bind directive
+
+\*(TL forms, introduced by
+.code @
+may be used on either side of
+.codn bind .
+
+
+Example:
+
+.cblk
+  @(bind a @(+ 2 2))
+  @(bind @(+ 2 2) @(* 2 2))
+.cble
+
+Here,
+.code a
+is bound to the integer
+.codn 4 .
+The second
+.code bind
+then succeeds because the forms
+.code (+ 2 2)
+and
+.code (* 2 2)
+produce equal values.
+
 .dir set
 
 The
@@ -5420,6 +5460,21 @@ binds to
   @(set (A B . C) D)
 .cble
 
+Note that
+.code set
+does not support a \*(TL expression on the left side, so the following
+are invalid syntax:
+
+.cblk
+  @(set @(+ 1 1) @(* 2 2))
+  @(set @b @(list "a"))
+.cble
+
+The second one is erroneous even though there is a variable on the left.
+Because it is preceded by the
+.code @
+escape, it is a Lisp variable, and not a pattern variable.
+
 .dir rebind
 
 The