comp3104 2010-11 programming languages proposal 1 all humans possess a common logical structure...
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Comp3104 2010-11 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
FORTRANLisp
Scheme
Joy
Haskell
Clean
COBOL
Erlang
Prolog
FP
F#Lua
Occam
Oz
Pascal
Python
C++
SmallTalk
C#
C
PHP
Tcl
Criteria for ComparisonCriteria
C Java
asm
Schem
e
Prolog
LangLang
LangLang
Lang
Lang
Lang
Lang
Lang
LangLang
Lang
Paradigm
Paradigm
Paradigm
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))
RecursionRecursion See Recursion
On page 269 in the index of Kernighan and Richie’s book The C Programming language
recursion 86,139,141,182,202,269
Recursion in Language: 5thC BC Panini Sanskrit Grammar Rules
20thC 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 + 3 + 2 + 1= 4 + (3 + 2 + 1)= 4 + 3 + (2+1)= 4 + 3 + 2 + (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 article adjective noun end
ornate noun
beginornatenoun
relativepronoun
verb
end
preposition
fancynoun
verb
fancynoun
fancynoun
fancy noun
Recursion in Language
begin article adjective noun end
ornate noun
beginornatenoun
relativepronoun
verb
end
preposition
fancynoun
verb
fancynoun
fancynoun
fancy noun
Recursion in Language
begin article adjective noun end
beginornatenoun
relativepronoun
verb
end
preposition
fancynoun
verb
fancynoun
fancynoun
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
var = expression
term
+
-
term
+
-
var
num
( )expression
Statement
Expression
Term
Prolog Syntax Sheet Facts 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” ?
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