Macros.

Macros - grasped. Kind of.
When you're dealing with lisp, sooner or later, you're going to hear about macros, and how utterly, astoundingly fantastic they are.
Macros are for:

• Creating new operators and saving you a gang of time by "extending the language"

Um, that's all I can think of. There is probably a lot more to it than that, but if you're in desperate need of a starting point, that would probably be quite a goody.

Of course, when you're a total monkey like me, you very quickly pick up on the emotional content of any essays,blogs or books, but the logic escapes you for a bit.

Even when you think you've wrapped your brain around the issue, you still have this nagging feeling that you're not getting something vital.

A few things fell nicely into place when I was reading about anaphoric macros. I think they should be used to introduce the idea of lisp macros, rather than being interesting items you only get to see after a while...

So let's look at AIF. AIF is the anaphoric version of if. Anaphoric meaning something like "referring to itself". You can define and test it like this:

(defmacro aif (test-form then-form &optional else-form)
 `(let ((it ,test-form))
    (if it ,then-form ,else-form)))

(defun big-long-calculation ()
 t)

(aif (big-long-calculation)
    (format t "hello"))

;running the above will give you "hello" in the repl. 
;changing big-long-calculation to return nil will result
;in the aif test doing nothing.

Which allows you to avoid this:

(let ((result (big-long-calculation)))
 (if result
     (foo result)))

... and use this instead:

(aif (big-long-calculation)
    (foo it))

(all this stuff stolen from Paul Graham's "On Lisp" - fantastic book that I've not finished reading, because it's too advanced, or I'm too much of a monkey)


... so you've "extended" the language by adding a new operator, as elemental as, say, "+" or "format" (I think).

Which is pretty powerful.

Lisp conditions - atomic example

Lisp allows you to use restarts - magical stuff that allows your app to recover from errors, instead of vomiting everything at you.

Lisp gives you options. Hopefully. One day, I might even be able to use them somewhat.

Here's my grossly simplified, probably wrong internal model of conditions:

• Something goes wrong
• A continuation is created, or maybe several are. Conditions are mystical "pause" buttons in the app, or something.
  
• The debugger appears, offering you the options
• When you pick one, lisp magicaly re-injects you where the condition was created.
• Things hopefully work a bit better.
  

Pretty sure this will be totally embarrasing in future.

Any way. From what I can gather, the CL condition system is implemented in terms of objects, and a 'condition' is a parent of an 'error'. 'Conditions' allow for signalling of diferent states within your app, but that's quite a fancy-pants subject, for when I know what the heck I'm doing. Typically, you'd define your error condition like this:

(define-condition slave-not-happy (error) ... other code ...)

So you're basically creating your little error as a sub-class of error, which is a subclass of condition.

In your code, you can then enable all sorts of mystical getting-out-of-jail stuff like this:

... other code ... (restart-case (error 'slave-not-happy :useable-key-name useable-key-value
(this-option ... some code)
(that-option ... some other code))

...So when you have a problem, and lisp kicks into the debugger, you'll have something like
..blah blah blah - THERE'S A PROBLEM:
0 - this-option
1 - that-option
2 - abort
3 - maybe some more options

Your stuff appears RIGHT THERE with the system's restarts, and you feel all powerful and stuff. So, basically, you're giving yourself more options for getting out of trouble.

Deciding what to do with the signalled conditions is up to you, but you can start getting really fancy-cool stuff done:

...code
(handler-bind ((slave-not-happy #'(lambda (c) (invoke-restart 'this-option)))
... other code

Of course, you can conditionally decide which restart to invoke, based on some other internal logic. Really spiffy.

here's a complete, working example you can put in a file and play around with:
Also available at

http://ryan-white-lisp.googlecode.com/files/conditions-atomic.lisp

Update:

Nikodemus Siivola (major contributor to SBCL) posted a style guide on conditions and errors:

http://random-state.net/log/3453016738.html

"Learn the difference between HANDLER-BIND and HANDLER-CASE. Short version: HANDLER-CASE always unwinds, with HANDLER-BIND you can eg. log the condition without handling it, or decide whether to unwind or not after inspecting the condition in more detail."

Fantastic resources:
http://www.nhplace.com/kent/Papers/Condition-Handling-2001.html (Kent Pitman)
http://www.gigamonkeys.com/book/beyond-exception-handling-conditions-and-restarts.html (Peter Siebel)
enjoy.

#|
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CONDITIONS - quick and dirty intro/reminder
Originally set up on

Fire up lisp and enter
(load "path-to-this-file")
You should then be able to call
(slave 10)
which will return 10
(slave "foo")
which will give you a set of restarts.
Calling
(master '(1 2 3 "foo"))
will return just (1 2 3)
(master '(1 2 3 "foo") :on-error "change" :chage-value 111)
will return (1 2 3 111)
(emperor '(1 2 3 "foo"))
will return (1 2 3)
(careless-master '(1 2 3 "foo"))
will give a set of restarts
(micromanaging-emperor '(1 2 3 "foo"))
will return (1 2 3 1000000)

Extend.Document.Publish.
|#


#|
Define a condition - an indicator that something isn't
totally and fantastically 100% right
|#

(define-condition slave-not-happy (error)
((given-value :initarg :given-value :reader given-value-read))
(:report (lambda (condition stream)
   (format stream "~A is not a number, dammit!" (given-value-read condition)))))

#|
This is the function at the bottom of the chain of events.
When its unhappy, it uses the condition to let everyone know
|#
(defun slave (item)
(if (numberp item)  ;the test
      item   ;if everything's good, just return the number
      (restart-case (error 'slave-not-happy :given-value item) ;or start this whole error handling trip
 (use-nil () :report "Use nil value." nil)
(replace-item (new-value) :report "Replace the value" new-value)
(replace-item-interactively (new-value)
:report "Provide a value yourself."
:interactive (lambda ()
 (format t "~&Value to use: ")
 (list (eval (read))))
(slave new-value))))) ;finally - a potentially recursive call...

#|
This is a function up the chain of events. It handles the error spit out
|#
(defun master (mylist &key (on-error "ignore") (change-value 0))
(handler-bind ((slave-not-happy ;figuring out what to do with the slave-not-happy error
    (if (equal  on-error "change")
     #'(lambda (c) (invoke-restart 'replace-item change-value)) ;we either do a substitution
        #'(lambda (c) (invoke-restart 'use-nil)) ; or we use nil
        )))
 (let ((output '()) (tst ""))
   (loop for item in mylist do
 (setf tst (slave item))
 (if (eq nil tst)
     ()
     (push tst output)))
   (reverse output))))
#|
This function just orders stuff done, and doesn't want to know
about the details
|#
(defun emperor (list)
(master list))


#|
This version of master doesn't worry about errors at all.
Basically, it just doesn't cater for the error
|#
(defun careless-master (mylist &key)
(let ((output '()) (tst nil))
 (loop for item in mylist do
(setf tst (slave item))
(if (eq nil tst)
   ()
   (push tst output)))
 (reverse output)))
#|
If the emperor wishes, he can do the managing of
the unhappy slave. In a typically self-serving manner
|#
(defun micromanaging-emperor (list)
(handler-bind
   ((slave-not-happy #'(lambda (c) (invoke-restart 'replace-item 1000000))))
 (careless-master list)))