1 / 65

CSCI 2210: Programming in Lisp

CSCI 2210: Programming in Lisp. ANSI Common Lisp, Chapters 5-10. Progn. Progn Creates a block of code Expressions in body are evaluated Value of last is returned (if (< x 0) (progn (format t "X is less than zero ") (format t "and more than one statement ")

wade-garza
Download Presentation

CSCI 2210: Programming in Lisp

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. CSCI 2210: Programming in Lisp ANSI Common Lisp, Chapters 5-10 CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  2. Progn • Progn • Creates a block of code • Expressions in body are evaluated • Value of last is returned • (if (< x 0) • (progn • (format t "X is less than zero ") • (format t "and more than one statement ") • (format t "needs to be executed in the IF") • (- x) • ) • ) • Prog1 • Same as progn, except value of first expression is returned CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  3. Block • Like a progn with • A name • An "emergency exit" • return-from - Returns from a named block • return - Returns from a block named NIL • Examples • > (block head • (format t "Here we go") • (return-from head 'idea) • (format t "We'll never see this")) • Here we go. • IDEA • > (block nil (return 27)) • 27 CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  4. Implicit use of Blocks • Some Lisp constructs implicitly use blocks • All iteration constructs use a block named NIL (note return) • > (dolist (x '(a b c d e)) • (format t "~A " x) • (if (eql x 'c) (return 'done))) • A B C • DONE • Defun uses a block with same name as the function • > (defun foo () • (return-from foo 27)) CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  5. Iteration • DOTIMES (Review) • (dotimes (<counter> <upper-bound> <final-result>) • <body>) • Example • (dotimes (i 5) (print i)) ;; prints 0 1 2 3 4 • DOLIST • (dolist (<element> <list-of-elements> <final-result>) • <body>) • Example • (dolist (elem '(a b c d)) (print elem)) ;; prints a b c d CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  6. Example of DOLIST • Given a list of ages of people, how many adults? • List of ages • > (setf ages '(3 4 17 21 22 34 2 7)) • Adult defined as: >= 21 years old • > (defun adultp (age) (>= age 21)) • Using Count-if • (defun count-adult (ages) (count-if #'adultp ages)) • Using dolist • (defun count-adult (ages &aux (nadult 0)) • (dolist (age ages nadult) • (if (adultp age) (setf nadult (+ 1 nadult))))) CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  7. Example (cont.) • Get the ages of the first two adults • (defun first-two-adults (ages &aux (nadult 0) (adults nil)) • (dolist (age ages) • (if (adultp age) • (progn (setf nadult (+ nadult 1)) • (push age adults) • (if (= nadult 2) (return adults)))))) • Notes • PROGN (and PROG1) are like C/C++ Blocks { … } • > (prog1 (setf a 'x) (setf b 'y) (setf c 'z)) • X • > (progn (setf a 'x) (setf b 'y) (setf c 'z)) • Z • RETURN exits the DOLIST block • Note: does not necessarily return from the procedure! • Takes an optional return value CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  8. DO • DO is more general than DOLIST or DOTIMES • Example • (defun do-expt (m n) ;; Return M^N • (do ((result 1) ;; Bind variable ‘Result’ to 1 • (exponent n)) ;; Bind variable ‘Exponent’ to N • ((zerop exponent) result) ;; test and return value • (setf result (* m result)) ;; Body • (setf exponent (- exponent 1)) ;; Body • Equivalent C/C++ definition • int do_expt (int m, int n) { • int result, exponent; • for (result=1,exponent=n; (exponent != 0); ) { • result = m * result; • exponent = exponent - 1; • } • return result; • } CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  9. DO Template • Full DO Template (There is also a DO*) • (DO ( (<p1> <i1> <u1>) • (<p2> <i2> <u2>) … • (<pN> <iN> <uN>) ) • ( <term-test> • <a1> <a2> … <aN> • <result> ) • <body> ) • Rough equivalent in C/C++ • for ( <p1>=<i1>, <p2>=<i2>,…,<pN>=<iN>; //Note: Lisp=parallel • !(<term-test>); // C/C++ has a “continuation-test” • <p1>=<u1>, <p2>=<u2>,…,<pN>=<uN>) { • <body> • } • <a1>; <a2>; … ; <aN>; • <result>; // Note: (DO…) in Lisp evaluates to <result> • // Note: <p1>,<p2>,…,<pN> are now restored to original values CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  10. Do-expt, Loop • Here is another (equivalent) definition of do-expt • (defun do-expt (m n) • (do ((result 1 (* m result)) • (exponent n (- exponent 1))) • ((zerop exponent) result) • ;; Note that there is no body! • )) • Loop • An infinite loop, terminated only by a (return) • (loop • (print '(Say uncle)) • (if (equal (read) 'uncle) (return))) CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  11. Multiple Values • We said each lisp expression returns a value • Actually, can return zero or more values (i.e. multiple values) • > (round 7.6) ; Round returns two values • 8 • -0.4 • > (setf a (round 7.6)) • 8 ; Setf expects only one value, so it takes the first (8) • > a • 8 • How can you get all values? • > (multiple-value-list (round 7.6)) • (8 -0.4) • > (multiple-value-setq (intpart dif) (round 7.6)) • 8 • > intpart • 8 • > dif • -0.4 CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  12. Generating Multiple Values • How? Use values • > (values 1 2 3) ; Generates 3 return values • 1 • 2 • 3 • > (defun uncons (aList) (values (first aList) (rest aList))) • UNCONS • > (uncons '(A B C)) • A • (B C) • > (format t "Hello World") • "Hello World" • NIL • > (defun format-without-return (out str &rest args) • (apply #'format out str args) • (values)) ; A function with no return value! • FORMAT-WITHOUT-RETURN • > (format-without-return t "Hello") • Hello • > CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  13. Functions about functions • FBOUNDP - Is a symbol the name of a function? • > (fboundp '+) • T • SYMBOL-FUNCTION - returns function • > (symbol-function '+) • #<Compiled-Function + 17B4E> • > (setf (symbol-function 'sqr) #'(lambda (x) (* x x))) • #<Interpreted-Function SQR> • > (defun sqr (x) (* x x)) ; this is equivalent to above • SQR • Defun and symbol-function define global functions • Local functions can be defined using LABELS • > (defun add3 (x) • (labels ((add1 (x) (+ x 1)) • (add2 (x) (+ x 2))) • (add1 (add2 x)))) CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  14. Closures • Want a function that can save some values for future access/update • Can use global variables for this • > (setf *number-of-hits* 0) ; Global variable • 0 • > (defun increment-hits () (incf *number-of-hits*)) • INCREMENT-HITS • > (increment-hits) • 1 • > (increment-hits) • 2 • But, what if someone clobbers the variable? • > (setf *number-of-hits* "This variable is now a string") • "This variable is now a string" • > (increment-hits) • Error: '*number-of-hits*' is not of the expected type: NUMBER • Want something like "static" variables in C/C++. CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  15. Closures (continued) • One use of closures is to implement 'static' variables safely • Closure -- Combination of a function and environment • Environment can include values of variables • > (let ((number-of-hits 0)) • (defun increment-hits () (incf number-of-hits))) • INCREMENT-HITS • number-of-hits is a free variable • It is a lexical variable (has scope) • A function that refers to a free lexical variable is called a closure • Multiple functions can share the same environment • > (let ((number-of-hits 0)) • (defun increment-hits() (incf number-of-hits)) • (defun reset-hit-counter() (setf number-of-hits 0)) CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  16. Closures (cont) • Example 1 • > (defun make-adder (n) ; returns function to add n to x • #'(lambda (x) (+ x n))) • > (setf add3 (make-adder 3)) ; returns function to add 3 • #<Interpreted function C0EBF6> • > (funcall add3 2) ; Call function add3 with argument 2 • 5 • > (setf (symbol-function 'add3-b) (make-adder 3)) • > (add3-b 5) • 8 • Example 2 - Returns an "opposite" function • > (defun our-complement (f) • #(lambda (&rest args) • (not (apply f args)))) • > (mapcar (our-complement #'oddp) '(1 2 3 4)) • (NIL T NIL T) • Tip of the iceberg in terms of possibilities CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  17. Trace • Format • (trace <procedure-name>) • Example • > (defun factorial (x) (if (<= x 1) 1 (factorial (- x 1)))) • > (trace factorial) • Causes entry, exit, parameter, and return values to be printed • For EACH procedure being traced • > (factorial 3) • ; 1> FACTORIAL called with arg: 3 • ; | 2> FACTORIAL called with arg: | 2 • ; | | 3> FACTORIAL called with arg: | | 1 • ; | |3< FACTORIAL returns value: | | 1 • ; | 2< FACTORIAL returns value: | 2 • ; 1< FACTORIAL returns value: 6 • 6 • Untrace stops tracing a procedure: (untrace <procedure-name>) CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  18. Print, Read • Print • Example • > (print '(A B)) • Evaluates a single argument • Can be a constant (ex. 100), a variable (x), or any single expression • Prints its value on a new line • Returns the value printed • Read - reads a single expression • Example: • > (read)20 23 (A B) C • 20 ;; Note: Only the 20 is read; rest is ignored • > (progn (print 'Enter-temperature) (read)) • Enter-temperature 20 • 20 CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  19. Format • Format • Allows more elegant printing • > (progn (print "Enter temperature: ") (read)) • "Enter temperature: " 32 • 32 • > (progn (format t "~%Enter temperature: ") (read)) • Enter temperature: 32 • 32 • The second parameter (t) is the output buffer (T=stdout) • The character “~” signifies that a control character follows • The character “%” signifies a newline (Lisp: “~%” C: “\n”) • The characters “~a” tells Lisp to substitute the next value • printf ("The value is ( %d, %d )", x, y); /* A C stmt */ • > (format t "The value is ( ~a, ~a )" x y) ;; Lisp way • > (format t "The value is ( ~10a, ~a )" x y) ; Can get fancy CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  20. Streams • Write output to a file (e.g. knowledge base) • Prototype of with-open-file • (with-open-file (<stream name> • <file specification> • :direction <:input or :output>) • …) • Example • > (setf fact-database • '((It is raining) • (It is pouring) • (The old man is snoring))) • > (with-open-file (my-file ”myfile.lsp” :direction :output) • (print fact-database my-file)) CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  21. My-Trace-Load • Redefine the built-in Lisp “Load” function • Example of Built In function • > (load "a.lsp") • Additional requirements • Want to print each expression that is read in. • Want to print the value returned by the expression • Definition • (defun my-trace-load (filename &aux next-expr next-result) • (with-open-file (f filename :direction :input) • (do ((next-expr (read f nil) (read f nil))) • ((not next-expr)) • (format t "~%~%Evaluating '~a'" next-expr) • (setf next-result (eval next-expr)) • (format t "~% Returned: '~a'" next-result)))) CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  22. Read-Line, Read-Char • Read-Line • Reads an individual line (terminated by a carriage return) • Returns it as a character string • > (read-line)Hello World • “Hello World” • Read-Char • Reads an individual character • Returns it as a character • > (read-char)x • #\x ;; This is Lisp notation for the character ‘x’ CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  23. Symbols • We've used symbols as names for things • A symbol is much more than just a name • Includes: name, package, value, function, plist (Property List) • A symbol is a "substantial object" • Some functions for manipulating symbols • symbol-name, symbol-plist, intern,… • Details are in Chapter 8 of textbook CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  24. Symbol Names • By default Lisp symbol names are upper case • > (symbol-name 'abc) • "ABC" • Can use "|" to delimit symbol names • > (list '|abc|) ; no conversion • "abc" • > (list '|Lisp 1.5| '|| '|abc| '|ABC|) • (|Lisp 1.5| || |abc| ABC) ; Note that only ABC doesn't have delimiter (default) • Intern creates new symbols • > (set (intern '|5.1/5)|) 999) • 999 • > |5.1/5)| • 999 CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  25. Packages • Package • A name space for symbols • Large programs use multiple packages • > (defpackage "MY-APPLICATION" ;; Creates new package • (:use "COMMON-LISP" "MY-UTILITIES") ;; Other packages • (:nicknames "APP") • (:export "WIN" "LOSE" "DRAW")) ;; Symbols exported • #<The MY-APPLICATION package> • > (in-package my-application) ;; Sets this to be default • New symbols are (by default) created in default package • The default package, when Lisp is started is COMMON-LISP-USER • The COMMON-LISP packages is automatically used CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  26. Numbers • Number crunching is one of Lisp's strengths • Many data types, automatically converted from one to another • Many numeric functions • Example: Factorial program was easier in Lisp (no overflow!) • Four distinct types • Types: Integer, Floating Point Number, Ratio, and Complex Number • Examples: 100, 123.45, 3/2, #c(a b) ; #c(a b) = a+bi • Predicates: integerp, floatp, ratiop, complexp • Basic rules for automatic conversion • If function receives floating point #'s, generally returns floating point #'s • If a ratio divides evenly (for example, 4/2), it will be converted to integer • If complex # has an imaginary part of zero, converted to a real CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  27. Subset of Type Hierarchy Lisp Expression Return Value (setf a 12); 12 (type a 'bit); NIL (type a 'fixnum); T (type a 'integer); T (integerp a); T (rationalp a); T (realp a); T (numberp a); T CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  28. More on Numbers • Conversion • (float) (truncate) (floor) (ceiling) (round) … • > (mapcar #'float '(1 2/3 .5)) • (1.0 0.6667 0.5) • Comparison • Use = to compare for equality (also, <,<=, >, >=, /=) • > (= 4 4.0) • T • Other Misc. functions • Max • Min • (expt x n) = X^n • (exp x) = E^x • (log x n) = logn X; N is optional (default = natural log) • sqrt, sin, cos, tan CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  29. Eval • Eval - Evaluates an expression and returns its value • > (eval '(+ 1 2 3)) • 6 • Top-level is also called the read-eval-print loop • Reads expression, evaluates it, prints value, loops back • > (defun our-toplevel () • (loop • (format t "%~> ") • (print (eval (read)))) • Eval • Inefficient - slower than running compiled code • Expression is evaluated without a lexical context CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  30. Macros • Macros • A common way to write programs that write programs • Defmacro is used to define macros • Example: A macro to set its argument to NIL • > (defmacro nil! (x) • (list 'setf x nil)) • > (nil! A) ;; Expands to (setf A NIL), is then evaluated • NIL • macro-expand-1 Generates macro expansion (not eval'ed) • > (macroexpand-1 '(nil! A)) • (SETF A NIL) • T CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  31. Macros • Typical procedure call (defun) • Evaluate arguments • Call procedure • Bind arguments to variables inside the procedure • Macro procedure (defmacro) • Macros do not evaluate their arguments • When a macro is evaluated, an intermediate form is produced • The intermediate form is evaluated, producing a value CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  32. Example: Manually Define Pop • Review of the built in operation “Pop”: • Implements Stack data structure “Pop” operation • > (setf a '(1 2 3 4 5)) • > (pop a) • 1 • > a • (2 3 4 5) • How would you emulate this using other functions? • Attempt 1: Remove the element “1” from A • > (setf a (rest a)) • (2 3 4 5) ;; A is set correctly to (2 3 4 5), but we want “1” to be returned • Attempt 2: Remove first element AND return it • > (prog1 (first a) (setf a (rest a))) • Attempt 3: Write a Lisp expression that generates above expression • > (list 'prog1 • (list 'first a) • (list 'setf a (list 'rest a))) CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  33. Our-Pop, using Macro • Convert Lisp expression into a macro (Our-Pop) • > (defmacro our-pop (stack) • (list 'prog1 • (list 'first stack) • (list 'setf stack (list 'rest stack)))) • Note similarity to Defun • Example Call • > (OUR-POP a) • Notes • The parameter “A” is NOT evaluated • “A” is substituted for stack wherever the variable stack appears • (list 'prog1 • (list 'first a) • (list 'setf a (list 'rest a))) • Intermediate form is generated • (prog1 (first a) (setf a (rest a))) • Intermediate form is evaluated • A is set to (rest A); the first element is returned CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  34. Our-Pop using Defun • Why doesn’t this (defun) work the same way? • > (defun our-pop (stack) • (prog1 (first stack) (setf stack (rest stack)))) • > (setf a '(1 2 3 4 5)) • > (our-pop a) • 1 • > a • (1 2 3 4 5) • Reason: Lisp passes parameters “by-value” • The value of A is COPIED into the variable “stack” • Any changes to the variable “stack” are done to the COPY, and NOT the original variable A • When the function returns, the original value of A is unchanged CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  35. Significance of Eval Steps • Macro evaluation has several steps (as noted) • The parameter “A” is NOT evaluated • “A” is substituted for stack wherever the variable stack appears • Intermediate form is generated • Intermediate form is evaluated • Note that A is evaluated at step 4 above (not step 1) • Why does this matter? • Answer: For the same reason that it matters in C/C++ macros • You may not want arguments evaluated at all • Or, you may want them evaluated multiple times • Macros give this flexibility CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  36. Backquotes • Significance of Evaluation Steps (cont) • Consider • > (defmacro our-if-macro (conditional then-part else-part) • (list 'if conditional then-part else-part)) • > (defun our-if-fun (conditional then-part else-part) • (if conditional then-part else-part)) • > (if (= 1 2) (print "Equal") (print "Not Equal")) • Lisp evaluates all parameters of OUR-IF-FUN before function is called • Backquote Mechanism • Forward quotes: Entire next expression is not evaluated • > (defun temp () (setf a '(a b c d e))) • Backquote: Next expression is not evaluated (with exceptions) • > (defun temp () (setf a `(a b c d e))) • > (defun temp (x) (setf a `(a b c d e ,x)) • The “,x” expression is evaluated; the value of X is used. CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  37. Backquotes (cont.) • Exceptions - Backquote evaluates the following • ,variable - Evaluates the value of the variable • > (setf x '(h i j)) • > (setf a `(a b c ,x e f)) • (A B C (H I J) E F) • ,@variable - Splices the elements of a list • > (setf a `(a b c ,@x e f)) • (A B C H I J E F) • Backquotes simplify macro development • > (defmacro our-if-macro (conditional then-part else-part) • (list 'if conditional then-part else-part)) ;; old way • > (defmacro our-if-macro (conditional then-part else-part) • `(if ,conditional ,then-part ,else-part)) CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  38. Backquotes simplify macros • Original version of our-pop • > (defmacro our-pop (stack) • (list 'prog1 • (list 'first stack) • (list 'setf stack (list 'rest stack)))) • Our-pop redefined using backquotes • > (defmacro our-pop (stack) • `(prog1 (first ,stack) (setf ,stack (rest ,stack)))) • Syntax is much closer to the intermediate form • Macros can be defined with following parameters • Optional (&optional) • Rest (&rest) • Key (&key) CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  39. Macro Design • Macro: take a number n, evaluate its body n times • Example call • > (ntimes 10 (princ ".")) • ………. • NIL • First attempt (incorrect) • > (defmacro ntimes (n &rest body) • `(do ((x 0 (+ x 1))) • ((>= x ,n)) • ,@body)) • For example call, within body of macro • n bound to 10 • body bound to ((princ ".")) • (do ((x 0 (+ x 1))) • ((>= x 10)) • (princ ".")) • Macro works for example. Why is it incorrect? CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  40. Inadvertent Variable Capture • Consider • Initialize x to 10, increment it 5 times • > (let ((x 10)) • (ntimes 5 (setf x (+ x 1))) • x) • 10 • Expected value: 15 • Why? Look at expansion • > (let ((x 10)) • (do ((x 0 (+ x 1))) • ((>= x 5)) • (setf x (+ x 1))) • x) • X is used in let and in the iteration • setf increments iteration variable! CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  41. Gensym Gives New Symbol • Gensym • Generates a new (uninterned) symbol • > (defmacro ntimes (n &rest body) ; Still incorrect though • (let ((g (gensym))) • `(do ((,g 0 (+ ,g 1))) • ((>= ,g ,n)) • ,@body))) • The value of symbol G is a newly generated symbol • How does this avoid the problem? • What if the call has a variable G? • Look at the expansion of • (let ((x 10)) • (ntimes 5 (setf x (+ x 1))) • x) CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  42. Expansion of ntimes (2) • Substitute for N and BODY • > (let ((x 10)) • (let ((g (gensym))) • `(do ((,g 0 (+ ,g 1))) • ((>= ,g ,5)) • ,@((setf x (+ x 1))))) • x) • Generate intermediate form • > (let ((x 10)) • (do ((#:G34 0 (+ #:G34 1))) • ((>= #:G34 5)) • (setf x (+ x 1))) • x) • Evaluate • 15 • This works for our example … but ... CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  43. Multiple Evaluation • What happens when we want to debug… • > (let ((x 10) (niteration 3)) • (ntimes niteration (setf x (+ x 1))) • x) • 13 • To debug, insert a print expression • > (let ((x 10) (niteration 3)) • (ntimes (print niteration) (setf x (+ x 1))) • x) • 3 • 3 • 3 • 3 • 13 • The argument is evaluated multiple times • Apparent when argument causes side-effects • What if the argument was a (setf …) • Non-intuitive: Expect argument to be evaluated only once. CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  44. Avoiding Multiple Evaluation • Solution is to copy value when you get into macro • Use copy when needed within macro • > (defmacro ntimes (n &rest body) • (let ((g (gensym)) • (h (gensym))) • `(let ((,h ,n)) • (do ((,g 0 (+ ,g 1))) • ((>= ,g ,h)) • ,@body)))) • This is correct • > (let ((x 10) (niteration 3)) • (ntimes (print niteration) (setf x (+ x 1))) • x) • 3 • 13 CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  45. Expansion of Ntimes (Final) • Pprint is a useful function for Pretty PRINTing expressions • > (pprint (macroexpand-1 • '(ntimes (print niteration) (setf x (+ x 1))))) • (LET ((#:G93 (PRINT NITERATION))) • (DO ((#:G92 0 (+ #:G92 1))) ((>= #:G92 #:G93)) (SETF X (+ X 1)))) • Problems of Multiple Evaluation and Inadvertent Variable Capture • Examples of errors that can occur when working with macros • Errors are common in Lisp as well as languages like C/C++ CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  46. Multiple Evaluation in Pop • Looking back at our Pop macro • It suffers from multiple evaluation • We can't use same technique, though • Need to get the first AND do a setf to change the value • > (defmacro our-pop (stack) • `(prog1 (first ,stack) (setf ,stack (rest ,stack)))) • Textbook solution avoids multiple evaluation • Page 301 • Uses a function get-setf-expansion to get to inner details of setf CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  47. Case Study: Expert Systems • Overview of using Lisp for • Symbolic Pattern Matching • Rule Based Expert Systems and Forward Chaining • Backward Chaining and PROLOG • Motivational example • Given: • A set of Facts • A set of Rules • Desired result • Answer complex questions and queries CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  48. Smart Animal Guessing • Facts about an animal named “Joey” • F1. (Joey’s mother has feathers) • F2. (Joey does not fly) • F3. (Joey swims) • F4. (Joey is black and white) • F5. (Joey lives in Antarctica) • Rules about animals in general • R1. If (animal X has feathers) THEN (animal X is a bird) • R2. If (animal X is a bird) and (animal X swims) and (animal X does not fly) and (animal X is black and white) THEN (animal is a penguin) • R3. If (animal X’s mother Z) THEN (animal X Z) • Example: if (animal X’s mother has feathers) • then (animal X has feathers) • R4. If (animal X Z) THEN (animal’s mother Z) • Notes • By combining the facts and rules, we can deduce that Joey is a penguin, and that the Joey’s mother is a penguin. CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  49. Symbolic Pattern Matching • Symbolic pattern matching example • Match F1 with the IF part of R1 • F1. (Joey’s mother has feathers) • R1. If (animal X has feathers) THEN (animal X is a bird) • The expression (Joey’s mother has feathers) matches the pattern (animal X has feathers). • The association (animal X = Joey’s mother) is implied • In general • Symbolic pattern matching • matching an ordinary expression (e.g. fact) to a pattern expression • Unification: more advanced version of pattern matching • match two pattern expressions to see if they can be made identical • Find all substitutions that lead to this CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

  50. Rule Based Expert System • Rule Based Expert Systems • Once the pattern matching step is done, then we know that Rule R1 can be combined with fact F1 • F1. (Joey’s mother has feathers) • R1. If (animal X has feathers) THEN (animal X is a bird) • The association (animal X = Joey’s mother), along with the second part of the rule (animal X is a bird) leads to a derivedfact: • (Joey’s mother is a bird) CSCI 2210 - Programming in Lisp; Instructor: Alok Mehta; 4.ppt

More Related