Title: Common LISP awk macro for easy text file operations

	Title: Common LISP awk macro for easy text file operations
	Author: Solène
	Date: 04 February 2020
	Tags: awk lisp
	Description:

	I like Common LISP and I also like awk. Dealing with text files in
	Common LISP
	is often painful. So I wrote a small awk like common lisp macro, which
	helps a
	lot dealing with text files.

	Here is the implementation, I used the uiop package for split-string
	function,
	it comes with sbcl. But it's possible to write your own split-string or
	reused
	the infamous split-str function shared on the Internet.


	(defmacro awk(file separator &body code)
	"allow running code for each line of a text file,
	giving access to NF and NR variables, and also to
	fields list containing fields, and line containing $0"
	`(progn
	(let ((stream (open ,file :if-does-not-exist nil)))
	(when stream
	(loop for line = (read-line stream nil)
	counting t into NR
	while line do
	(let* ((fields (uiop:split-string line :separator
	,separator))
	(NF (length fields)))
	,@code))))))


	It's interesting that the "do" in the loop could be replaced with a
	"collect",
	allowing to reuse awk output as a list into another function, a quick
	example I
	have in mind is this:

	;; equivalent of awk '{ print NF }' file \| sort \| uniq
	;; for counting how many differents fields long line we have
	(uniq (sort (awk "file" " " NF)))


	Now, here are a few examples of usage of this macro, I've written the
	original
	awk command in the comments in comparison:

	;; numbering lines of a text file with NR
	;; awk '{ print NR": "$0 }' file.txt
	;;
	(awk "file.txt" " "
	(format t "~a: ~a~%" NR line))

	last field in the list)
	;; awk -F ';' '{ print NF-1 }' file.csv
	;;
	(awk "file.csv" ";"
	(print (nth (- NF 2) fields)))

	;; awk '/unbound/ { print }' /var/log/messages
	;;
	(awk "/var/log/messages" " "
	(when (search "unbound" line)
	(print line)))

	;; awk -F ';' '{ print $4 }' data.csv
	;;
	(awk "data.csv" ";"
	(print (nth 4 fields)))