# Chapter 15. Functional Programming Languages

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1 Chapter 15 Functional Programming Languages

3 Chapter 15 Topics Introduction Mathematical Functions Fundamentals of Functional Programming Languages The First Functional Programming Language: LISP Introduction to Scheme COMMON LISP ML Haskell Applications of Functional Languages Comparison of Functional and Imperative Languages Copyright 2009 Addison-Wesley. All rights reserved. 1-3

4 Introduction The design of the imperative languages is based directly on the von Neumann architecture Efficiency is the primary concern, rather than the suitability of the language for software development The design of the functional languages is based on mathematical functions A solid theoretical basis that is also closer to the user, but relatively unconcerned with the architecture of the machines on which programs will run Copyright 2009 Addison-Wesley. All rights reserved. 1-4

5 Mathematical Functions A mathematical function is a mapping of members of one set, called the domain set, to another set, called the range set A lambda expression specifies the parameter(s) and the mapping of a function in the following form λ(x) x * x * x for the function cube (x) = x * x * x Copyright 2009 Addison-Wesley. All rights reserved. 1-5

6 Lambda Expressions Lambda expressions describe nameless functions Lambda expressions are applied to parameter(s) by placing the parameter(s) after the expression e.g., (λ(x) x * x * x)(2) which evaluates to 8 Copyright 2009 Addison-Wesley. All rights reserved. 1-6

7 Functional Forms A higher-order function, or functional form, is one that either takes functions as parameters or yields a function as its result, or both Copyright 2009 Addison-Wesley. All rights reserved. 1-7

8 Function Composition A functional form that takes two functions as parameters and yields a function whose value is the first actual parameter function applied to the application of the second Form: h f g which means h (x) f ( g ( x)) For f (x) x + 2 and g (x) 3 * x, h f g yields (3 * x)+ 2 Copyright 2009 Addison-Wesley. All rights reserved. 1-8

9 Apply-to-all A functional form that takes a single function as a parameter and yields a list of values obtained by applying the given function to each element of a list of parameters Form: α For h (x) x * x α( h, (2, 3, 4)) yields (4, 9, 16) Copyright 2009 Addison-Wesley. All rights reserved. 1-9

10 Fundamentals of Functional Programming Languages The objective of the design of a FPL is to mimic mathematical functions to the greatest extent possible The basic process of computation is fundamentally different in a FPL than in an imperative language In an imperative language, operations are done and the results are stored in variables for later use Management of variables is a constant concern and source of complexity for imperative programming In an FPL, variables are not necessary, as is the case in mathematics Copyright 2009 Addison-Wesley. All rights reserved. 1-10

11 Fundamentals of Functional Programming Languages - continued Referential Transparency - In an FPL, the evaluation of a function always produces the same result given the same parameters Tail Recursion Writing recursive functions that can be automatically converted to iteration Copyright 2009 Addison-Wesley. All rights reserved. 1-11

12 LISP Data Types and Structures Data object types: originally only atoms and lists List form: parenthesized collections of sublists and/or atoms e.g., (A B (C D) E) Originally, LISP was a typeless language LISP lists are stored internally as singlelinked lists Copyright 2009 Addison-Wesley. All rights reserved. 1-12

13 LISP Interpretation Lambda notation is used to specify functions and function definitions. Function applications and data have the same form. e.g., If the list (A B C) is interpreted as data it is a simple list of three atoms, A, B, and C If it is interpreted as a function application, it means that the function named A is applied to the two parameters, B and C The first LISP interpreter appeared only as a demonstration of the universality of the computational capabilities of the notation Copyright 2009 Addison-Wesley. All rights reserved. 1-13

14 Origins of Scheme A mid-1970s dialect of LISP, designed to be a cleaner, more modern, and simpler version than the contemporary dialects of LISP Uses only static scoping Functions are first-class entities They can be the values of expressions and elements of lists They can be assigned to variables and passed as parameters Copyright 2009 Addison-Wesley. All rights reserved. 1-14

15 Evaluation Parameters are evaluated, in no particular order The values of the parameters are substituted into the function body The function body is evaluated The value of the last expression in the body is the value of the function Copyright 2009 Addison-Wesley. All rights reserved. 1-15

16 Primitive Functions Arithmetic: +, -, *, /, ABS, SQRT, REMAINDER, MIN, MAX e.g., (+ 5 2) yields 7 QUOTE - takes one parameter; returns the parameter without evaluation QUOTE is required because the Scheme interpreter, named EVAL, always evaluates parameters to function applications before applying the function. QUOTE is used to avoid parameter evaluation when it is not appropriate QUOTE can be abbreviated with the apostrophe prefix operator '(A B) is equivalent to (QUOTE (A B)) Copyright 2009 Addison-Wesley. All rights reserved. 1-16

17 Function Definition: LAMBDA Lambda Expressions Form is based on λ notation e.g., (LAMBDA (x) (* x x) x is called a bound variable Lambda expressions can be applied e.g., ((LAMBDA (x) (* x x)) 7) Copyright 2009 Addison-Wesley. All rights reserved. 1-17

18 Special Form Function: DEFINE A Function for Constructing Functions DEFINE - Two forms: 1. To bind a symbol to an expression e.g., (DEFINE pi ) Example use: (DEFINE two_pi (* 2 pi)) 2. To bind names to lambda expressions e.g., (DEFINE (square x) (* x x)) Example use: (square 5) - The evaluation process for DEFINE is different! The first parameter is never evaluated. The second parameter is evaluated and bound to the first parameter. Copyright 2009 Addison-Wesley. All rights reserved. 1-18

20 Numeric Predicate Functions #T is true and #F is false (sometimes () is used for false) =, <>, >, <, >=, <= EVEN?, ODD?, ZERO?, NEGATIVE? Copyright 2009 Addison-Wesley. All rights reserved. 1-20

21 Control Flow: IF Selection- the special form, IF (IF predicate then_exp else_exp) e.g., (IF (<> count 0) (/ sum count) 0) Copyright 2009 Addison-Wesley. All rights reserved. 1-21

22 Control Flow: COND Multiple Selection - the special form, COND General form: (COND (predicate_1 expr {expr}) (predicate_1 expr {expr})... (predicate_1 expr {expr}) (ELSE expr {expr})) Returns the value of the last expression in the first pair whose predicate evaluates to true Copyright 2009 Addison-Wesley. All rights reserved. 1-22

23 Example of COND (DEFINE (compare x y) (COND ((> x y) x is greater than y ) ((< x y) y is greater than x ) (ELSE x and y are equal ) ) ) Copyright 2009 Addison-Wesley. All rights reserved. 1-23

24 List Functions: CONS and LIST CONS takes two parameters, the first of which can be either an atom or a list and the second of which is a list; returns a new list that includes the first parameter as its first element and the second parameter as the remainder of its result e.g., (CONS 'A '(B C)) returns (A B C) LIST takes any number of parameters; returns a list with the parameters as elements Copyright 2009 Addison-Wesley. All rights reserved. 1-24

25 List Functions: CAR and CDR CAR takes a list parameter; returns the first element of that list e.g., (CAR '(A B C)) yields A (CAR '((A B) C D)) yields (A B) CDR takes a list parameter; returns the list after removing its first element e.g., (CDR '(A B C)) yields (B C) (CDR '((A B) C D)) yields (C D) Copyright 2009 Addison-Wesley. All rights reserved. 1-25

26 Predicate Function: EQ? EQ? takes two symbolic parameters; it returns #T if both parameters are atoms and the two are the same; otherwise #F e.g., (EQ? 'A 'A) yields #T (EQ? 'A 'B) yields #F Note that if EQ? is called with list parameters, the result is not reliable Also EQ? does not work for numeric atoms Copyright 2009 Addison-Wesley. All rights reserved. 1-26

27 Predicate Functions: LIST? and NULL? LIST? takes one parameter; it returns #T if the parameter is a list; otherwise #F NULL? takes one parameter; it returns #T if the parameter is the empty list; otherwise #F Note that NULL? returns #T if the parameter is() Copyright 2009 Addison-Wesley. All rights reserved. 1-27

28 Example Scheme Function: member member takes an atom and a simple list; returns #T if the atom is in the list; #F otherwise DEFINE (member atm lis) (COND ((NULL? lis) #F) ((EQ? atm (CAR lis)) #T) ((ELSE (member atm (CDR lis))) )) Copyright 2009 Addison-Wesley. All rights reserved. 1-28

29 Example Scheme Function: equalsimp equalsimp takes two simple lists as parameters; returns #T if the two simple lists are equal; #F otherwise (DEFINE (equalsimp lis1 lis2) (COND ((NULL? lis1) (NULL? lis2)) ((NULL? lis2) #F) ((EQ? (CAR lis1) (CAR lis2)) (equalsimp(cdr lis1)(cdr lis2))) (ELSE #F) )) Copyright 2009 Addison-Wesley. All rights reserved. 1-29

30 Example Scheme Function: equal equal takes two general lists as parameters; returns #T if the two lists are equal; #F otherwise (DEFINE (equal lis1 lis2) (COND ((NOT (LIST? lis1))(eq? lis1 lis2)) ((NOT (LIST? lis2)) #F) ((NULL? lis1) (NULL? lis2)) ((NULL? lis2) #F) ((equal (CAR lis1) (CAR lis2)) (equal (CDR lis1) (CDR lis2))) (ELSE #F) )) Copyright 2009 Addison-Wesley. All rights reserved. 1-30

31 Example Scheme Function: append append takes two lists as parameters; returns the first parameter list with the elements of the second parameter list appended at the end (DEFINE (append lis1 lis2) )) (COND ((NULL? lis1) lis2) (ELSE (CONS (CAR lis1) (append (CDR lis1) lis2))) Copyright 2009 Addison-Wesley. All rights reserved. 1-31

32 Example Scheme Function: LET General form: (LET ( (name_1 expression_1) (name_2 expression_2)... (name_n expression_n)) body ) Evaluate all expressions, then bind the values to the names; evaluate the body Copyright 2009 Addison-Wesley. All rights reserved. 1-32

33 LET Example (DEFINE (quadratic_roots a b c) (LET ( (root_part_over_2a (/ (SQRT (- (* b b) (* 4 a c)))(* 2 a))) (minus_b_over_2a (/ (- 0 b) (* 2 a))) (DISPLAY (+ minus_b_over_2a root_part_over_2a)) (NEWLINE) (DISPLAY (- minus_b_over_2a root_part_over_2a)) )) Copyright 2009 Addison-Wesley. All rights reserved. 1-33

34 Tail Recursion in Scheme Definition: A function is tail recursive if its recursive call is the last operation in the function A tail recursive function can be automatically converted by a compiler to use iteration, making it faster Scheme language definition requires that Scheme language systems convert all tail recursive functions to use iteration Copyright 2009 Addison-Wesley. All rights reserved. 1-34

35 Tail Recursion in Scheme - continued Example of rewriting a function to make it tail recursive, using helper a function Original: (DEFINE (factorial n) Tail recursive: (IF (= n 0) 1 (* n (factorial (- n 1))) )) (DEFINE (facthelper n factpartial) (IF (= n 0) factpartial facthelper((- n 1) (* n factpartial))) )) (DEFINE (factorial n) (facthelper n 1)) Copyright 2009 Addison-Wesley. All rights reserved. 1-35

36 Scheme Functional Forms Composition The previous examples have used it (CDR (CDR '(A B C))) returns (C) Apply to All - one form in Scheme is mapcar Applies the given function to all elements of the given list; (DEFINE (mapcar fun lis) )) (COND ((NULL? lis) ()) (ELSE (CONS (fun (CAR lis)) (mapcar fun (CDR lis)))) Copyright 2009 Addison-Wesley. All rights reserved. 1-36

37 Functions That Build Code It is possible in Scheme to define a function that builds Scheme code and requests its interpretation This is possible because the interpreter is a user-available function, EVAL Copyright 2009 Addison-Wesley. All rights reserved. 1-37

38 Adding a List of Numbers ((DEFINE (adder lis) (COND ((NULL? lis) 0) (ELSE (EVAL (CONS '+ lis))) )) The parameter is a list of numbers to be added; adder inserts a + operator and evaluates the resulting list Use CONS to insert the atom + into the list of numbers. Be sure that + is quoted to prevent evaluation Submit the new list to EVAL for evaluation Copyright 2009 Addison-Wesley. All rights reserved. 1-38

39 COMMON LISP A combination of many of the features of the popular dialects of LISP around in the early 1980s A large and complex language--the opposite of Scheme Features include: records arrays complex numbers character strings powerful I/O capabilities packages with access control iterative control statements Copyright 2009 Addison-Wesley. All rights reserved. 1-39

40 ML A static-scoped functional language with syntax that is closer to Pascal than to LISP Uses type declarations, but also does type inferencing to determine the types of undeclared variables It is strongly typed (whereas Scheme is essentially typeless) and has no type coercions Includes exception handling and a module facility for implementing abstract data types Includes lists and list operations Copyright 2009 Addison-Wesley. All rights reserved. 1-40

41 ML Specifics Function declaration form: fun name (parameters) = body; e.g., fun cube (x : int) = x * x * x; - The type could be attached to return value, as in fun cube (x) : int = x * x * x; - With no type specified, it would default to int (the default for numeric values) - User-defined overloaded functions are not allowed, so if we wanted a cube function for real parameters, it would need to have a different name - There are no type coercions in ML Copyright 2009 Addison-Wesley. All rights reserved. 1-41

42 ML Specifics (continued) ML selection if expression then then_expression else else_expression where the first expression must evaluate to a Boolean value Pattern matching is used to allow a function to operate on different parameter forms fun fact(0) = 1 fact(n : int) : int = n * fact(n 1) Copyright 2009 Addison-Wesley. All rights reserved. 1-42

43 ML Specifics (continued) Lists Literal lists are specified in brackets [3, 5, 7] [] is the empty list CONS is the binary infix operator, :: 4 :: [3, 5, 7], which evaluates to [4, 3, 5, 7] CAR is the unary operator hd CDR is the unary operator tl fun length([]) = 0 length(h :: t) = 1 + length(t); fun append([], lis2) = lis2 append(h :: t, lis2) = h :: append(t, lis2); Copyright 2009 Addison-Wesley. All rights reserved. 1-43

44 ML Specifics (continued) The val statement binds a name to a value (similar to DEFINE in Scheme) val distance = time * speed; - As is the case with DEFINE, val is nothing like an assignment statement in an imperative language Copyright 2009 Addison-Wesley. All rights reserved. 1-44

45 Haskell Similar to ML (syntax, static scoped, strongly typed, type inferencing, pattern matching) Different from ML (and most other functional languages) in that it is purely functional (e.g., no variables, no assignment statements, and no side effects of any kind) Syntax differences from ML fact 0 = 1 fact n = n * fact (n 1) fib 0 = 1 fib 1 = 1 fib (n + 2) = fib (n + 1) + fib n Copyright 2009 Addison-Wesley. All rights reserved. 1-45

46 Function Definitions with Different Parameter Ranges fact n n == 0 = 1 n > 0 = n * fact(n 1) sub n n < 10 = 0 n > 100 = 2 otherwise = 1 square x = x * x - Works for any numeric type of x Copyright 2009 Addison-Wesley. All rights reserved. 1-46

47 Lists List notation: Put elements in brackets e.g., directions = ["north", "south", "east", "west"] Length: # e.g., #directions is 4 Arithmetic series with the.. operator e.g., [2, 4..10] is [2, 4, 6, 8, 10] Catenation is with ++ e.g., [1, 3] ++ [5, 7] results in [1, 3, 5, 7] CONS, CAR, CDR via the colon operator (as in Prolog) e.g., 1:[3, 5, 7] results in [1, 3, 5, 7] Copyright 2009 Addison-Wesley. All rights reserved. 1-47

49 List Comprehension Set notation List of the squares of the first 20 positive integers: [n * n n [1..20]] All of the factors of its given parameter: factors n = [i i [1..n div 2], n mod i == 0] Copyright 2009 Addison-Wesley. All rights reserved. 1-49

50 Quicksort sort [] = [] sort (a:x) = sort [b b x; b <= a] ++ [a] ++ sort [b b x; b > a] Copyright 2009 Addison-Wesley. All rights reserved. 1-50

51 Lazy Evaluation A language is strict if it requires all actual parameters to be fully evaluated A language is nonstrict if it does not have the strict requirement Nonstrict languages are more efficient and allow some interesting capabilities infinite lists Lazy evaluation - Only compute those values that are necessary Positive numbers positives = [0..] Determining if 16 is a square number member [] b = False member(a:x) b=(a == b) member x b squares = [n * n n [0..]] member squares 16 Copyright 2009 Addison-Wesley. All rights reserved. 1-51

52 Member Revisited The member function could be written as: member [] b = False member(a:x) b=(a == b) member x b However, this would only work if the parameter to squares was a perfect square; if not, it will keep generating them forever. The following version will always work: member2 (m:x) n m < n = member2 x n m == n = True otherwise = False Copyright 2009 Addison-Wesley. All rights reserved. 1-52

53 Applications of Functional Languages APL is used for throw-away programs LISP is used for artificial intelligence Knowledge representation Machine learning Natural language processing Modeling of speech and vision Scheme is used to teach introductory programming at some universities Copyright 2009 Addison-Wesley. All rights reserved. 1-53

54 Comparing Functional and Imperative Languages Imperative Languages: Efficient execution Complex semantics Complex syntax Concurrency is programmer designed Functional Languages: Simple semantics Simple syntax Inefficient execution Programs can automatically be made concurrent Copyright 2009 Addison-Wesley. All rights reserved. 1-54

55 Summary Functional programming languages use function application, conditional expressions, recursion, and functional forms to control program execution instead of imperative features such as variables and assignments LISP began as a purely functional language and later included imperative features Scheme is a relatively simple dialect of LISP that uses static scoping exclusively COMMON LISP is a large LISP-based language ML is a static-scoped and strongly typed functional language which includes type inference, exception handling, and a variety of data structures and abstract data types Haskell is a lazy functional language supporting infinite lists and set comprehension. Purely functional languages have advantages over imperative alternatives, but their lower efficiency on existing machine architectures has prevented them from enjoying widespread use Copyright 2009 Addison-Wesley. All rights reserved. 1-55

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