COP4020 Programming Languages

1. COP4020Programming Languages Functional Programming Prof. Xin Yuan

2. Topics • Functional programming with Scheme • Learn by examples COP4020 Spring 2014

3. Defining Global Names • A global name is defined with the “define” special form(define namevalue) • Usually the values are functions (lambda abstractions) • Examples: • (define my-name ''foo'') • (define determiners '(''a'' ''an'' ''the'')) • (define sqr (lambda (x) (* x x))) • (define twice (lambda (f a) (f (f a)))) • (twice sqr 3)  ((lambda (f a) (f (f a))) (lambda (x) (* x x)) 3)  …  81 COP4020 Spring 2014

4. Defining Global Names • A global name defined with a lambda express in essence creates a named function. • This allows recursion – one of the most important/useful concept in scheme. • (define fab (lambda (x) (if (= x 0) 0 (if (=x 1) 1 (+ (fab (- x 1)) (fab (- x 2))))))) • Define a function that computes the sum of all elements in a list. • Define a function that put an element at the end of a list COP4020 Spring 2014

5. Load program from file • Load function • (load filename) load the program from a file • (load “myscheme”): load the program in “myscheme.scm” • After load, one has access to all global names in the program. COP4020 Spring 2014

6. I/O • (display x) prints value of x and returns an unspecified value • (display "Hello World!")Displays: "Hello World!" • (display (+ 2 3))Displays: 5 • (newline) advances to a new line • (read) returns a value from standard input • (if (member (read) '(6 3 5 9)) "You guessed it!" "No luck")Enter: 5Displays: You guessed it! • (read-line) returns the string in a line (without the “\n”). COP4020 Spring 2014

7. I/O with files Open file for reading: (open-input-file filename) Open file for writing: (open-output-file filename) Read one character from a file: (read-char file) Read a scheme object from file: (read file) Check the current character in a file without consuming the character: (peek-char file) Check end-of-file: (eof-object? (peek-char file)) Write one character to a file: (write-char char file) Write a scheme object to file: (write object file) Close file: (close-port file) COP4020 Spring 2014

8. Blocks • (begin x1x2 … xn) sequences a series of expressions xi, evaluates them, and returns the value of the last one xn • Examples: • (begin   (display "Hello World!")   (newline) ) COP4020 Spring 2014

9. Example 1: define factorial function: fact(n) = 1*2*3*…*n COP4020 Spring 2014

10. Example 1: define factorial function: fact(n) = 1*2*3*…*n • Recursive factorial:(define fact   (lambda (n)     (if (zero? n) 1 (* n (fact (- n 1))))   ) ) • (fact 2)  (if (zero? 2) 1 (* 2 (fact (- 2 1))))  (* 2 (fact 1))  (* 2 (if (zero? 1) 1 (* 1 (fact (- 1 1)))))  (* 2 (* 1 (fact 0)))  (* 2 (* 1 (if (zero? 0) 1 (* 0 (fact (- 0 1))))  (* 2 (* 1 1))  2 COP4020 Spring 2014

11. Example 2 • Sum the elements of a list(define sum   (lambda (lst)      (if (null? lst)       0       (+ (car lst) (sum (cdr lst)))    )   ) ) • (sum '(1 2 3))  (+ 1 (sum (2 3)) (+ 1 (+ 2 (sum (3)))) (+ 1 (+ 2 (+ 3 (sum ())))) (+ 1 (+ 2 (+ 3 0))) COP4020 Spring 2014

12. Example 3 • Generate a list of n copies of x(define fill (lambda (n x) ??? )) COP4020 Spring 2014

13. Example 3 • Generate a list of n copies of x(define fill (lambda (n x) (if (= n 0) () (cons x (fill (- n 1) x))) )) • (fill 2 'a)  (cons a (fill 1 a))  (cons a (cons a (fill 0 a)))  (cons a (cons a ()))  (a a) COP4020 Spring 2014

14. Example 4 • Replace x with y in list xs(define subst (lambda (x y xs) ??? )) COP4020 Spring 2014

15. Example 4 • Replace x with y in list xs(define subst (lambda (x y xs) (cond ((null? xs) ()) ((eq? (car xs) x) (cons y (subst x y (cdr xs)))) (else (cons (car xs) (subst x y (cdr xs)))) ) )) COP4020 Spring 2014