CS A331 Programming Language Concepts

Transcription

1 CS A331 Programming Language Concepts Lecture 10 Alternative Programming Languages (Functional LISP Declarative - PROLOG) March 24, 2014 Sam Siewert

2 Functional PL Concepts Based on Lambda Calculus Output is Math Function of Inputs with No Internal State, No Side Effects Lambda Calculus Also Used to Specify PL Semantics (Denotational Semantics) Nested Functions and Recursion Everything is a Function, Including Data (Much like Everything is and Object in OOP) Common Lisp Symbolic AI, Functional Core Numerous Lisp Variants E.g. Scheme sudo apt-get install scm Good so you can follow along in Chapter 10, PLP Haskell Purely Functional Sam Siewert 2

3 Lisp Basics Winston & Horn (setf friends '(dick jane sally)) friends (setf enemies '(troll grinch ghost)) (setf enemies (remove 'ghost enemies)) enemies (setf friends (cons 'ghost friends)) friends (defun newfriend (name) (setf enemies (remove name enemies)) (setf friends (cons name friends))) (first '(a b c)) (rest '(a b c)) (rest '(c)) (first (rest '(a b c))) (first '(rest (a b c))) (car '(a b c)) (cdr '(a b c)) (cadr '(a b c)) (second '(a b c)) (setf alist '( )) (setf alist '(a b c d e) blist '(x y z)) t nil pi (append alist blist) (length alist) (reverse alist) (setf sam '((height 6.1) (weight 210))) (assoc 'height sam) (setf nlist '( )) (max ) ; parallel let (setf x 'outside) (let ((x 'inside) (y x)) (list x y)) ; sequential let (setf x 'outside) (let* ((x 'inside) (y x)) (list x y)) () (expt 2 3) (cos 3) Sam Siewert 3

4 Lisp Predicates and Functions ; Conditional expression (defun prob-term (p) (cond ((> p 0.75) 'very-likely) ((> p 0.5) 'likely) ((> p 0.25) 'unlikely) (t 'very-unlikely))) ; Predicates and conditionals (equal '( ) '( )) (equal alist blist) (member 'e alist) (equal (lambda (x) (* x x)) (lambda (x) (* x x))) (eql (lambda (x) (* x x)) (lambda (x) (* x x))) (equal (lambda (x) (* x x)) (lambda (y) (* y y))) (= 4 4.0) (defun square (n) (* n n)) (mapcar #'square '(1 2 3)) (mapcar #'= '(1 2 3) '(3 2 1)) (funcall #'(lambda (param) (first param)) '(a b c d e f)) (setf trees '((maple shade) (apple fruit))) (member '(maple shade) trees) (member '(maple shade) trees :test #'equal) Sam Siewert 4

5 Lisp Recursion Designed for Recursion, Nesting and Mapping Functions to Data of Most any Type ; Recursion (defun fibonacci (N) "Compute the N'th Fibonacci number." (if (or (zerop N) (= N 1)) 1 (+ (fibonacci (- N 1)) (fibonacci (- N 2))))) (defun count-elements (list) (if (endp list) 0 (+ 1 (count-elements (rest list))))) (trace count-elements) (count-elements '( )) (defun count-atoms (list) (cond ((null list) 0) ((atom list) 1) (t (+ (count-atoms (first list)) (count-atoms (rest list)))))) (count-atoms '(sqrt (expt x 2) (expt y 2))) Sam Siewert 5

6 Lisp Introduction - Functional Learn by Example from Many Excellent Tutorials Here, here, here Try Examples for Scheme in PLP and here, here, >(defun fib (N) "Compute the N'th Fibonacci number." (if (or (zerop N) (= N 1)) 1 Simplify with let expressions (let ((F1 (fib (- N 1))) (F2 (fib (- N 2)))) (+ F1 F2)))) Note: use trace for debug FIB >(trace fib) (FIB) >(fib 3) 3 1> (FIB 3) 2> (FIB 2) 3> (FIB 1) <3 (FIB 1) 3> (FIB 0) <3 (FIB 1) <2 (FIB 2) 2> (FIB 1) <2 (FIB 1) <1 (FIB 3) Sam Siewert 6

7 Lambda Calculus Two Major Theoretical Models of Computability (1930 s) Turing Machine (Abstract Machine PDA with Tape) Computable Functions Non-Computable Functions (Busy beaver) Alonzo Church s Hypothesis and Lambda Calculus Any Sufficient Formalism (Turing Machine, Lambda Calculus) is Complete, Equally Powerful and Anything Computed in One can Be Computed in the Other Both Pre-date Modern Von Neumann Digital Computers (1930 s Prior to WWII during time of Enigma) Age of Computational Machines E.g. Difference Engine Sam Siewert 7

8 Functional PL Features PLP Page 507 Lisp vs. Scheme 1. First-class function values and higher order functions 2. Polymorphism (dynamic bindings) Scheme Deep Binding 3. List types and operators 4. Structured functions returns (compared to single value) 5. Constructors for aggregates 6. Garbage collection (automatic memory management) Common Lisp >((lambda (x) (* x x)) 3) 9 >(sqrt 25) 5.0 Scheme Only (Why not Common Lisp?) > (let ((a 3) > (b 4) > (square (lambda (x) (* x x))) > (plus +)) > (sqrt (plus (square a) (square b)))) 5.0 Sam Siewert 8

9 Common Lisp vs. Scheme Scheme Only (see note on p. 511, A word of caution ) > (let ((a 3) > (b 4) > (square (lambda (x) (* x x))) > (plus +)) > (sqrt (plus (square a) (square b)))) 5.0 Common Lisp (But, this is different, so how do we do the same?) >(defun square (x) (* x x)) SQUARE Note: namespace square and plus >(defun plus (a b) (+ a b)) PLUS > Note: square and plus ok in let, but not used (let ((a 3) (b 4) (square (lambda (x) (* x x))) (plus +)) (sqrt (plus (square a) (square b)))) 5.0 Sam Siewert 9

10 Scheme Tools SCM SCM on Linux is Simple Command Line scm SCM version 5e5, Copyright (C) Free Software Foundation. ;loading /usr/share/slib/require ;done loading /usr/share/slib/require.scm ;loading /usr/share/slib/require ;done loading /usr/share/slib/require.scm ;loading /usr/lib/scm/link ;done loading /usr/lib/scm/link.scm ;loading /usr/lib/scm/transcen ;done loading /usr/lib/scm/transcen.scm > (load "map.scm") ;loading map.scm ;done loading map.scm #<unspecified> > (let ((a 3) > (b 4) > (square (lambda (x) (* x x))) > (plus +)) > (sqrt (plus (square a) (square b)))) 5.0 > Sam Siewert 10

11 Scheme Tools DrScheme (DrRacket) Nice GUI and IDE for Scheme Development and Test (No LISP support) Sam Siewert 11

12 Lisp Tools - Linux CLISP with Emacs Editor or Simple Shell Interpreter Sam Siewert 12

13 Lisp Equivalent Let* with funcall ;(defun square (x) (* x x)) ;(defun plus (a b) (+ a b)) (let* ((a 3) (b 4) (square #'(lambda (x) (* x x))) (plus #'+)) (sqrt (funcall plus (funcall square a) (funcall square b)))) Common Lisp Can Be Functional, But Not So Easy 1. Let* causes sequential bindings (not required, but interesting) 2. # refers to the function object and defers evaluation (required) 3. funcall actually invokes the function object (required) Now the Let expression is much more like Scheme Sam Siewert 13

14 Many Options Lisp IDE and Editors GNU Common Lisp for Windows (per lab instructions) CLISP sudo apt-get install clisp sudo apt-get install clisp-doc sudo apt-get install slime In Emacs, do M-x run-ilisp, Dialect: clisp Can Use Emacs, Vi, etc. and add Syntax Features sudo apt-get install ilisp Tricky to set up if you re new to Emacs Can Use Eclipse Tricky to use if you re new to Eclipse I have never used Cusp and Dendelion Can Just use Nano, Vi, etc. and Tough it Out! Sam Siewert 14

15 Lisp and Scheme on Linux - Summary PLP Prefers to Use Scheme Examples For PLP book, use SCM or DrScheme instead of GCM/CLISP Both are Fine, But Have +/- A Point of Much Discussion & Many Prefer Scheme Efficiency Issues E.g. Lazy Evaluation of Expressions Common Lisp Though is Portable, Comprehensive and Lots of Code to Re-Use Including CLOS Modify /etc/default/gcl for profiling and ANSI support DEFAULT_GCL_ANSI="y" DEFAULT_GCL_PROF="y" Sam Siewert 15

16 Declarative Programming Language Concepts Program Axioms Runtime derives Constructive Proof for Inputs (Query) Based on Logic Rules and Facts Query Rule and Fact Base with a Predicate Forward Chaining Derives conclusions from assertions Backward Chaining Find Assertions that Support a Hypothesis The Cut! Controls Backtracking Semantic Web Automated Theorem Proving Science and Medicine Hypothesis, Facts, Rules Sam Siewert 16

17 Prolog Introduction - Declarative RULES %% Demo coming from %% likes(sam,food) :- indian(food), mild(food). likes(sam,food) :- chinese(food). likes(sam,food) :- italian(food). likes(sam,chips). FACTS indian(curry). indian(dahl). indian(tandoori). indian(kurma). mild(dahl). mild(tandoori). mild(kurma). chinese(chow_mein). chinese(chop_suey). chinese(sweet_and_sour). italian(pizza). italian(spaghetti). Sam Siewert 17

18 Load a file?- [likes]. Prolog Interactive Session % likes compiled 0.01 sec, 4,792 bytes true. Simple Forward Chaining?- likes(sam,x). X = dahl. List all rules for Food Sam Likes?- listing(likes). likes(sam, A) :- indian(a), mild(a). likes(sam, A) :- chinese(a). likes(sam, A) :- italian(a). likes(sam, chips). true. Hypothesis?- likes(sam,pizza). true.?- likes(sam,dahl). true.?- likes(sam,cauliflower). false.?- likes(sam,curry). false.?- listing(indian). indian(curry). indian(dahl). indian(tandoori). indian(kurma). true. Sam Siewert 18

19 Turn on tracing?- trace. Prolog Interactive Session Predicate & Hypothesis [trace]?- likes(sam,dahl). [trace]?- Call: (6) likes(sam, dahl)? creep Call: (7) indian(dahl)? creep Exit: (7) indian(dahl)? creep Simple Forward Chaining Call: (7) mild(dahl)? creep [trace]?- likes(sam,x). Exit: (7) mild(dahl)? creep Call: (6) likes(sam, _G387)? creep Exit: (6) likes(sam, dahl)? creep Call: (7) indian(_g387)? creep true Exit: (7) indian(curry)? creep Call: (7) mild(curry)? creep [trace]?- likes(sam,beet). Fail: (7) mild(curry)? creep Call: (6) likes(sam, beet)? creep Redo: (7) indian(_g387)? creep Call: (7) indian(beet)? creep Exit: (7) indian(dahl)? creep Fail: (7) indian(beet)? creep Call: (7) mild(dahl)? creep Redo: (6) likes(sam, beet)? creep Exit: (7) mild(dahl)? creep Call: (7) chinese(beet)? creep Exit: (6) likes(sam, dahl)? creep Fail: (7) chinese(beet)? creep X = dahl. Redo: (6) likes(sam, beet)? creep Call: (7) italian(beet)? creep Fail: (7) italian(beet)? creep Redo: (6) likes(sam, beet)? creep Fail: (6) likes(sam, beet)? creep false. Sam Siewert 19