COMP313-11 Notes 1
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intro
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self + my languages + you + goals + assignment + video

what is a pl
abstractions in pl 
computational paradigms
language definition
language translation
language design

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what is a pl
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PL is a notational system for describing computation
in machine-readable and in human-readable form

3 key concepts:
- computation: turing machine (universal, (alonzo) church's hypothesis)
    turing-complete (general-purpose vs special-purpose, e.g SQL)
- machine-readable: fast translation and interpretation, context-free languages
- human-readable: communication, software engineering

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abstractions in pl 
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data abstractions and control abstractions, at multiple levels

bit patterns -> primitive types (int, double, ...)
more structure: arrays, structs, objects, classes / abstract data types
packages, modules, libraries

control:
basic statements and expressions:  x = x+3;   GOTO ... named variables
IF/SWITCH, loops (FOR WHILE ...)
functions procedures methods recursion
threads/processes

turing complete: integer variables + integer arithmetic
execute sequence of statements, need assignment, selection (if)
and looping (e.g. while)

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computational paradigms
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imperative/procedural  vs. declarative (?)
HOW vs. WHAT


object-oriented:  data+ops + inheritance
functional: everything is an expression, i.e. has a value
if strict then no side-effects ("mathematical" functions)

turing complete: integer variables + arithmetic functions,
plus define functions using existing functions, selection and recursion

gcd x y = if y == 0 then x else gcd y (x `mod` y)

logic, e.g.Prolog
gcd(X,0,X).
gcd(X,Y,Z) :- not(Y=0), U is X mod Y, gcd(Y,U,Z).

also "style-question", can use functional style in oo,
or write C-like Java, etc. 

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language definition
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syntax + semantics : structure + meaning

context-free grammars, e.g if statement like

<if-statement> ::= if ( <expression> ) <statement1> [ else <statement2> ]

semantics:

".. evaluate <expression> first, if it is not 0, then <statement1>
is executed, else (i.e. if <expression> is 0), then if present, 
<statement2> is executed

operational semantics, denotational semantics, axiomatic semantics

"procedural" semantics == defined by an implementation ...???

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language translation
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        source -+
                V
input -> interpreter -> output



[ source -> compiler -> "binary" ]+

executable code -+
                 V
       input ->  cpu -> output

 

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language design
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data and control abstractions for "complexity control"

efficiency 
readability
productivity


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questions
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why have different loops (for/while/do-while ..., continue/break/return...)
why some many turing-complete languages?
...