# Functional Languages. Hwansoo Han

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1 Functional Languages Hwansoo Han

2 Historical Origins Imperative and functional models Alan Turing, Alonzo Church, Stephen Kleene, Emil Post, etc. ~1930s Different formalizations of the notion of an algorithm or effective procedure, Based on automata, symbolic manipulation, recursive function definitions, and combinatorics A conjecture known as Church-Turing thesis If an algorithm (a process that terminates) exists, there is an equivalent Turing machine, -calculus, and recursion Equivalent computability among models 2

3 Turing Machine Turing s model of computing Pushdown automaton + an unbounded storage tape Turing machine computes in an imperative way By changing the values in cells of its tape like variables just as a high level imperative program computes by changing the values of variables 3 by Tom Dunne

4 Lambda Calculus Church s model of computing Notion of parameterized expressions With each parameter introduced by an occurrence of the letter - hence the notation s name. -calculus was the inspiration for functional programming Compute by substituting parameters into expressions, just as one computes in a high level functional program by passing arguments to functions Functional languages such as Lisp, Scheme, FP, ML, Miranda, and Haskell are attempts to realize Church's -calculus in practical form as a programming language 4

5 Programs as Mathematical Proofs To prove a proposition, mathematicians use a proof A program is a constructive proof for any given inputs Constructive proof one that shows how to obtain a mathematical object with some desired property Non-constructive proof one that merely shows that such an object must exist by contradiction, counting, reduction to known theorem whose proof is also non-constructive Logic programming is also tied to the notion of constructive proofs, but at a more abstract level: Logic programmer writes a set of axioms that allow the computer to discover a constructive proof for each particular set of inputs 5

6 Functional Programming Concepts Do everything by composing functions, which have No notion of internal state, and thus no side effects Features, many of which are often missing in imperative languages First-class function values and high-order functions First-class function can be a parameter, a return value, assigned to a variable High-order function takes a function as a parameter, and return a function as a result Extensive polymorphism List types and operators Structured function returns Fully general aggregates for structured objects Garbage collection 6

7 Recursion in Functional Languages How get anything done in a functional language? Recursion (especially tail recursion) takes the place of iteration In general, you can get the effect of a series of assignments x := 0... x := expr1... x := expr2... from f3(f2(f1(0))), where each f expects the value of x as an argument, f1 returns expr1, and f2 returns expr2 7

8 Recursion in FL (cont d) Recursion even does a nifty job of replacing a looping 8 x = 0; i = 1; j = 100; while (i < j) { x = x + i*j; i = i + 1; j = j - 1 } return x becomes f(0,1,100), where f(x, i, j) == if i < j then f (x+i*j, i+1, j-1) else x

9 Scheme a Lisp descendant Scheme is a particularly elegant Lisp Interpreter runs a read-eval-print loop Things typed are evaluated recursively once Anything in parentheses is a function call (unless quoted) Parentheses are not just grouping Adding a level of parentheses changes meaning (+ 3 4) (load my_scheme_prog ) evaluate from the loaded file ((+ 3 4)) ERROR eval: 7 is not a procedure 9

10 Scheme Values Boolean values: #t and #f Numbers Quoting (+ 3 4) 7 ; function (quote (+ 3 4)) (+ 3 4) ; quoted ( ) is not function (+ 3 4) (+ 3 4) ; is the same as quote Conditional Expression (if (< 2 3) 4 5) 4 (cond ((< 3 2) 1) ((< 4 3) 2) (else 3)) 3 10

11 Scheme (cont d) Lambda expressions - function (lambda (a b) (if (< a b) a b)) Nested scopes with let and letrec (for recursion) (let ( list_of_pairs ) the_rest ) ; list_of_pairs: var for the_rest (let ((a 3) (b 4) (square (lambda (x) (* x x))) (plus +)) (sqrt (plus (square a) (square b)))) 5.0 ; sqrt is a built-in func (let ((a 3)) ; scope of binding produced by let is for the 2 nd arg only (let ((a 4) (b a)) ; a takes the value of outer a names are visible at the end (+ a b))) 7 11

12 Scheme (cont d) Equivalence (if (= n 1) 1 3) (eq? a b) (eqv? a b) ; a and b refer to the same object? ; a and b are semantically equivalent? (in different locations) (equal? a b) ; a and b are the same recursive structure? List operators (car (2 3 4)) 2 (cdr (2 3 4)) (3 4) (cdr (2)) () (cons 2 (3 4)) (2 3 4) (cons 2 3) (2. 3) ; an improper list Type predicates (boolean? x) (char? x) (string? x) (symbol? x) (number? x) (pair? x) (list? x) (complex? x) (rational? x) (real? x) (integer? x)... 12

13 Scheme (cont d) Imperative stuff vector type To access an arbitrary element in a sequence Indexed by an integer, like an array, but elements can be heterogeneous types Assignments: (set! x 3) (set-car! l (c d)) (set-cdr! l (e)) Sequencing: (begin (display hi ) (display mom )) Iteration (for-each) I/O (read, display) 13

14 Evaluation Order Revisited Applicative order (as in Scheme) What you're used to in imperative languages Usually faster Normal order Like call-by-name don't evaluate arg until you need it Sometimes faster applicative order normal order (double (* 3 4)) (double (* 3 4)) (double 12) (+ (* 3 4) (* 3 4)) ( ) (+ 12 (* 3 4)) 24 ( ) 24 14

15 Strict vs. Non-strict Functions Strict function vs. non-strict function A strict function requires all arguments to be well-defined (successfully evaluated), so applicative order can be used Scheme, ML A non-strict function does not require all arguments to be well-defined (some may fail to be evaluated at run-time); it requires normal-order evaluation (lazy evaluation) Miranda, Haskel 15

16 Higher-Order Functions Higher-order functions Map Takes a function as argument or return a function as a result (map * (2 4 6) (3 5 7)) ( ) Reduce (folding) 16 (define fold (lambda (f i l) (if (null? l) i ; i is commonly the identity element for f (f (car l) (fold f i (cdr l)))))) (define total (lambda (l) (fold + 0 l))) (total ( )) 15 (define total-all (lambda (l) (map total l))) (total-all (( ) ( ) ( ))) ( )

17 Functional Programming: Pros Advantages Lack of side effects makes programs easier to understand Lack of explicit evaluation order (in some languages) offers possibility of parallel evaluation (e.g. MultiLisp) Lack of side effects and explicit evaluation order simplifies some things for a compiler Programs are often surprisingly short Language can be extremely small and yet powerful 17

18 Functional Programming: Cons Problems Difficult but not impossible! to implement efficiently on von Neumann machines Lots of copying of data through parameters Apparent need to create a whole new array in order to change one element Heavy use of pointers (space/time and locality problem) Frequent procedure calls Heavy space use for recursion Requires garbage collection Requires a different mode of thinking by the programmer Difficult to integrate I/O into purely functional model 18

19 Guile Scheme interpreter in Linux Install guile from the Interpreters 19 \$ guile guile> (+ 3 4) 7 guile> (map * ( ) ( )) ( ) guile> (load my_prog.scm ) ; load insert func. from file guile> (insert a (b c d)) guile> (quit) \$

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