a minor patch to awk invocation

[Greg Minshall <minshall@acm.org>]

hi.  i just wandered down a rathole others could avoid.  the following
program fails in (what was to me) a mysterious way:
----
#+BEGIN_SRC awk :var a=2
BEGIN{print $a;}
#+END_SRC
----

it turns out values for variables to awk need to be strings (rather than
a number, as above).  below is a patch to give what might be a less
mysterious error message.

also: for an awk invocation with ":var a=b", all occurrences of "$a" in
the body of the awk code are changed to "b".  i'm curious why this is
done rather than invoking awk with "-v a=b"?  or, moving the
initialization into a "BEGIN{}" block?  maybe because of the variety of
awk variants loose in the world?

cheers, Greg
----
diff --git a/lisp/ob-awk.el b/lisp/ob-awk.el
index ed98afd..162ddfb 100644
--- a/lisp/ob-awk.el
+++ b/lisp/ob-awk.el
@@ -47,6 +47,8 @@
 (defun org-babel-expand-body:awk (body params)
   "Expand BODY according to PARAMS, return the expanded body."
   (dolist (pair (mapcar #'cdr (org-babel-get-header params :var)))
+    (if (not (stringp (cdr pair)))
+	(error "awk variable values must be strings"))
     (setf body (replace-regexp-in-string
                 (regexp-quote (format "$%s" (car pair))) (cdr pair) body)))
   body)