Comp Programming Languages Proposal 1 All humans possess a common logical structure which operates independently of language Proposal 2 Language determines what the individual perceives in the world and how he thinks about it.

“ A language that doesn’t affect the way you think about programming, is not worth knowing” The Whorfian Hypothesis

All computers are equivalent to a Turing Machine and therefore are equivalent to each other. Computer manufacturers do not say “our machine has some magic instruction which makes our computer more powerful than the competition”. They say “Our machines are faster, cheaper, and are more greener” All programs can be constructed using two registers and two instructions, so all programming languages are equivalent Language developers admit this; they do not say “our language has some magic feature which allows our language to do more than in any other language”. Instead, their advertising goes like this. “Our language is more user-friendly, our compilers are faster and they produce smaller executable files, and are more greener”

What languages do we know ?

Java Ada ALGOL 60 Assembler Basic FORTRAN Lisp Scheme Joy Haskell Clean COBOL Erlang Prolog FP F# Lua Occam Oz Pascal Python C++ SmallTalk C# C PHP Tcl

Criteria for Comparison Criteria CJavaasmSchemeProlog

Lang Paradigm

Programming Paradigms What is a “Paradigm” ? Important Paradigms Imperative (procedural) Declarative Functional Logic Object Oriented Event Driven

Imperative vs. Declarative (Functional) float x; float function square(float a) { float b; b = a x a; return b; } x = square(3); (define (square a) (x a a)) (square 3)

Matlab Lisp C Java C++ Event Driven ActionScript Prolog Functional Object Oriented Logic Imperative Assembler

Timeline

Scheme : Definitions and Expressions

Scheme : Functions (define (seven x) (* x 7))

Scheme : Functions (define (sum x y) (+ x y))

Scheme : Conditionals (1) (define (test x) (cond ( (> x 0)1) ( (= x 0)0) ( (< x 0)-1)))

Scheme : Conditionals (2) (define (test2 x) (if (< x 0)(- x) x))

Recursion See Recursion On page 269 in the index of Kernighan and Richie’s book The C Programming language recursion86,139,141,182,202,269 Recursion in Language: 5 th C BC Panini Sanskrit Grammar Rules 20 th C Chomsky theorizes that unlimited extension of English is possible through the use of recursion. … try Googling “Recursion”

There are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we now know we don’t know. But there are also unknown unknowns. These are things we do not know we don’t know US Defense Secretary Donald Rumsfeld on February 12, 2002

Little harmonic Labyrinth

Scheme : Recursion : The Factorial Function (1) How many ways can you get n people to sit in n chairs ? n = 3. (A,B,C)n = 4. (A,B,C,D)

Scheme : Recursion (define (sum n) (if (= n 1)1 (+ n (sum(- n 1))))) sum the numbers 1 to N (e,g, 1 to 4) = 4 + ( ) = (2+1) = (1) (sum 4) = 4 + (sum 3) =

Scheme : Recursion (2) The Factorial Function (define (fact n) (if (= n 1)1 (* n (fact (- n 1)))))

Functional Recursion // Imperative Iteration ( define (fact n) (if (= n 1)1 (* n (fact (- n 1)))))

I lost the number of my new mobile phone How can I find the number ? Get my girlfriend to phone every single mobile number. (OK I’ll cook supper!) Approximate solution: Say a mobile number has 10 digits (actually 11) and each digit can be 0 – 9 (10 possibilitities). So the solution is 10! How long will this take? Calculatore!

Fundamental Principles of Recursion A recursive function must call a simplified version of itself. This is guaranteed to “bottom out” (to halt). Any call to itself would produce an infinite regress. Don’t run this … ah go on then.. (define (fact n) (if (= n 1)1 (* n (fact n))))... and wait for the error message... or worse

Recursion in Language begin articleadjectivenoun end ornate noun begin ornate noun relative pronoun verb end preposition fancy noun verb fancy noun fancy noun fancy noun

Recursion in Language begin articleadjectivenoun end ornate noun begin ornate noun relative pronoun verb end preposition fancy noun verb fancy noun fancy noun fancy noun

Recursion in Language begin articleadjectivenoun end begin ornate noun relative pronoun verb end preposition fancy noun verb fancy noun fancy noun

Recursion is not Self-Reference A recursive function makes reference to a simplified version of itself: (I am) better than what (I was) I’m the humblest person I know I never make misteaks “I never make predictions. I never have and I never will” This sentence contains five words This sentence no verb

()A A + var A Compilers - Parsing

( ) + var A

=expression term var num ()expression Statement Expression Term

Prolog Syntax SheetFacts and Rules parent(abraham, isaac). Fact. Lower case for names. parent(isaac, esau). Fact. Lower case for names. grandfather(X,Y) :- parent(X,A),parent(A,Y). Rule. If X is the grandfather of Y then X is the parent of A and A is the parent of Y. For example, if george “X” is the parent of colin “A” and colin “A” is the parent of tristan “Y” then george “X” is the grandparent of Tristan “Y”. Queries ?- parent(abraham,X). The capital X designates an “output” variable which will be all sons of Abraham. ?- grandfather(abraham,X). The capital X designates an “output” variable which will be all grandchildren of Abraham.

lectures(colin,3063). lectures(colin,3079). lectures(pete,3078). lectures(richard,3062). lectures(richard,3040). studies(tom,3063). studies(tom,3062). studies(kate,3063). studies(kate,3040). studies(kate,3062).

?- lectures(colin,Mo),studies(kate,Mo). ?- lectures(colin,Mo),studies(X,Mo).

Monty Python Holy Grail witch(X) :- burns(X),female(X). burns(X) :- wooden(X). wooden(X) :- floats(X). wooden(woodBridge). stone(stoneBridge). floats(bread). floats(apple). floats(cherry). floats(X) :- sameweight(duck, X). female(girl). sameweight(duck,girl). ?- witch(girl)

Monty Python Holy Grail Yes [trace] 12 ?- witch(girl). Call: (7) witch(girl) ? creep Call: (8) burns(girl) ? creep Call: (9) wooden(girl) ? creep Call: (10) floats(girl) ? creep Call: (11) sameweight(duck, girl) ? creep Exit: (11) sameweight(duck, girl) ? creep Exit: (10) floats(girl) ? creep Exit: (9) wooden(girl) ? creep Exit: (8) burns(girl) ? creep Call: (8) female(girl) ? creep Exit: (8) female(girl) ? creep Exit: (7) witch(girl) ? creep Yes

Coda It is easier to write an incorrect program than to read a correct one. There are two ways to write error-free programs, only the third one works. It is easier to change the specification to fit the program than vice-versa Why did the Roman Empire collapse? What is the Latin for “office automation” ?