1. Lecture 1-2CS251: Intro to AI/Lisp II “And now for something completely different…”

2. Lecture 1-2CS251: Intro to AI/Lisp II Announcements Web site is coming up Office hours Allegro for Windows Next few weeks –Today and Tuesday: Lisp (Micros/Macros/CLOS) –Thursday: Review of 250/350 final, plan on planning –Tuesday (4/13): Read Chapters 11 & 12

3. Lecture 1-2CS251: Intro to AI/Lisp II Today’s Topics More Lisp –Macros –Micros

4. Lecture 1-2CS251: Intro to AI/Lisp II Micros “Micros are Lisp runtime objects that have dynamic type resolution and support multiple dispatch, but otherwise do very little.” (Guy Steele) When should you use micros? –Good when you don’t have much to do –Lots of examples from the reading

5. Lecture 1-2CS251: Intro to AI/Lisp II A Quick Quiz Give two examples where micros are useful When should you use micros, and when are macros appropriate instead? Do micros support inheritance?

6. Lecture 1-2CS251: Intro to AI/Lisp II Just Kidding

7. Lecture 1-2CS251: Intro to AI/Lisp II Lisp, Lisp & More Lisp Common Lisp can do a lot for you, but… … what if you need another language? Functions just don’t cut it –when is this true?

8. Lecture 1-2CS251: Intro to AI/Lisp II Extending Lisp Lisp is a “programmable programming language” (John Foderaro) Build your own language with macros Macros let you treat data as code

9. Lecture 1-2CS251: Intro to AI/Lisp II Functions vs. Macros Evaluate all its arguments Produce expressions

10. Lecture 1-2CS251: Intro to AI/Lisp II How Macros Work At toplevel, Lisp recognizes macro Builds the macro expression inline Evaluates the expression inline

11. Lecture 1-2CS251: Intro to AI/Lisp II The defmacro Jam Use defmacro to create new macros (just like defun ) > (defmacro nil! (var) (list `setq var nil)) NIL!

12. Lecture 1-2CS251: Intro to AI/Lisp II “Big, Fat, Hairy Deal!” (Garfield) Everybody and their dog has a macro language How Lisp works –Parser reads Lisp source, sends its output to the… –Compiler, which compiles it for –Execution Parser sends a list of Lisp objects

13. Lecture 1-2CS251: Intro to AI/Lisp II Macros in the Middle Macros work between the parser and compiler –Output a list of Lisp objects, so compiler doesn’t notice An expanding macro has access to all of Lisp Cool, isn’t it?

14. Lecture 1-2CS251: Intro to AI/Lisp II Macros, Evaluation and You Argument evaluation Remember the quote? > ‘ (a b c) (A B C) > (quote (a b c) Now we’ve got backquote

15. Lecture 1-2CS251: Intro to AI/Lisp II Backquote Backquote lets you control evaluation –Backquote (`) stops evaluation –Comma (,) starts it up again –Comma-at (,@) starts it with a list > (setq a 1 b 2 c 3) 3 > `(a,b c) ??? > `(a (b,c)) ???

16. Lecture 1-2CS251: Intro to AI/Lisp II Backquotes & Commas One comma balances one backquote Don’t worry unless you have nested backquotes > ‘ (a,(b,c))

17. Lecture 1-2CS251: Intro to AI/Lisp II Where’s Waldo? Use defmacro to create new macros (just like defun ) Backquote is used for making lists, but there are other ways > (defmacro nil! (var) (list `setq var nil)) NIL!

18. Lecture 1-2CS251: Intro to AI/Lisp II nif With and Without > (defmacro nif (expr pos zero neg) (list ‘case (list ‘truncate (list ‘signum expr)) (list 1 pos) (list 0 zero) (list -1 neg))) > (defmacro nif (expr pos zero neg) `(case (truncate (signum,expr)) (1,pos) (0,zero) (-1,neg)))

19. Lecture 1-2CS251: Intro to AI/Lisp II And the macro expansion is... (case (truncate (signum x)) (1 ‘p) (0 ‘z) (-1 ‘n)) (nif x ‘p ‘z ‘n) expands into...

20. Lecture 1-2CS251: Intro to AI/Lisp II Back to the Comma Turn evaluation back on > (setq b ‘(1 2 3)) (1 2 3) > `(a,b c) ??? > `(a,@b c) Comma-at splices an expression into a list –Insert and remove the ()’s

21. Lecture 1-2CS251: Intro to AI/Lisp II When is,@ Handy? Macros that take an indeterminate number of arguments –Pass them on to functions or macros that also take an indeterminate number of arguments Example: Code blocks like progn Remember when?

22. Lecture 1-2CS251: Intro to AI/Lisp II Splicing Away Doing when our way (defmacro our-when (test &body body) `(if,test (progn,@body)))

23. Lecture 1-2CS251: Intro to AI/Lisp II Macro Expansion Why is macro expansion special? > (pprint (macroexpand-1 ‘(our-when (able) (laugh) (laughAgain)))) (if (able) (progn (laugh) (laughAgain)))

24. Lecture 1-2CS251: Intro to AI/Lisp II Macro Headaches Variable capture –Macro variable names collide with the variable names of the expansion context (defmacro ntimes (n &rest body) `(do ((x 0) (+ x 1))) ((>= x,n)),@body))

25. Lecture 1-2CS251: Intro to AI/Lisp II What happens here? > (let ((x 10)) (ntimes 5 (setf x (+ x 1))) x) ???

26. Lecture 1-2CS251: Intro to AI/Lisp II Finding the Problem Expand the macro (defmacro ntimes (n &rest body) `(do ((x 0) (+ x 1))) ((>= x,n)),@body)) (let ((x 10)) (do ((x 0) (+ x 1))) ((>= x 5)) (setf x (+ x 1))) x)

27. Lecture 1-2CS251: Intro to AI/Lisp II A Name to Call Our Own Generate a new symbol –Use gensym

28. Lecture 1-2CS251: Intro to AI/Lisp II A Macro Recipe Figure out what you’re starting with Break it down into small chunks Reassemble the chunks into what you need