Comments on: Lisp Macro Example

By: amjad

amjad — Mon, 05 Mar 2007 12:11:07 +0000

dear
iam looking for how to write a lisp to raed a data from text file (.prn)for example . please i f you can help me .email me

By: Emre Sevinc

Emre Sevinc — Tue, 26 Jul 2005 20:33:54 +0000

Also there is an error in your dofile macro, you forgot to use a comma which must take place just before filename in `(with-open-file (stream filename) …

If you macroexpand-1 it:

CL-USER> (macroexpand-1 ‘(dofile (line “foo.txt”) (princ line)))

(WITH-OPEN-FILE (STREAM FILENAME)
(DO ((LINE (READ-LINE STREAM NIL NIL) (READ-LINE STREAM NIL NIL)))
((NULL LINE))
(PRINC LINE)))
T

As you may notice in the above expansion, there’s no “foo.txt”. However if you put the comma before the filename: (with-open-file (stream ,filename) …

Then you have:

CL-USER> (macroexpand-1 ‘(dofile (line “foo.txt”) (princ line)))
(WITH-OPEN-FILE (STREAM “foo.txt”)
(DO ((LINE (READ-LINE STREAM NIL NIL) (READ-LINE STREAM NIL NIL)))
((NULL LINE))
(PRINC LINE)))
T

Another point is that, you don’t need the third NIL in function read-line unless you want to be very explicit. The eof-value defaults to NIL according to ANSI Common Lisp standard: http://www.lisp.org/HyperSpec/Body/fun_read-line.html

Also the with-gensyms (and looping) version is included somewhere in http://paste.lisp.org/display/10220#4

BTW, no I’m not a Lisp guru just pointing to the wisdom of #lisp :)

By: RM

RM — Tue, 26 Jul 2005 18:47:35 +0000

It would probably be better not to use LOOP in a macro in this way, for the same reason it’s better to use a gensym instead of the name “stream”. The macro LOOP-FINISH has a meaning lexically within the body of a LOOP, so it’s best to avoid making macros that emit LOOP as part of their expansions:
(loop for file in files
do (dofile (line file)
(when (string= line “STOP PROCESSING HERE”)
(loop-finish))))
would have unexpected results. So my “dofile” macro would be:
(defmacro dofile ((linevar filename &rest open-options) &body body)
(let ((stream (gensym))) ; Or with-gensyms
`(with-open-file (,stream ,filename ,@open-options)
(block nil
(let ((,linevar (read-line ,stream nil)))
(unless ,linevar (return nil))
,@body)))))
This introduces a new NIL-block but lots of looping macros do that in CL, so it’s not unexpected. If you didn’t want it, you could name the block with another gensym.

By: Emre Sevinc

Emre Sevinc — Tue, 26 Jul 2005 17:34:31 +0000

Yes, I prefer loop (sometimes) and after reading PCL I automatically say with-gensyms when I see the gensym in some code.

Did you check the very useful with-gensyms macro from Practical Common Lisp by Peter Seibel?

http://www.gigamonkeys.com/book/macros-defining-your-own.html

The complete source code for the book is available at:

http://www.gigamonkeys.com/book/

Happy hacking,

Emre S.

By: Ralph Richard Cook

Ralph Richard Cook — Thu, 21 Jul 2005 18:06:41 +0000

Congratulations; you have found a common abstraction, and used a macro to extend Lisp by implementing the abstraction. You have implemented dofile. You are on the path to enlightenment.

A couple of things; using the LOOP macro will improve readability. That’s just style, though. A problem is the use of ’stream’ within the body of your macro. If dofile is used, for example, in the body of another with-open-file, say for outputting a file, then a user of your dofile would need to know about how it was implemented in order to avoid using ’stream’ as a variable name. This is solved in Common Lisp with the gensym method. More about this and macros in general can be learned from
http://www.paulgraham.com/onlisptext.html

Here is my dofile:
(defmacro dofile ((line filename) &body body)
(let ((stream (gensym)))
`(with-open-file (,stream ,filename)
(loop for ,line = (read-line ,stream nil nil)
while ,line do ,@body))))