2017-11-26

brief overview

a paradigm is a typical example or pattern of something; a pattern or model

the programming paradigms we are going to look at are:

procedural

object-oriented

functional

logic

common languages usually support a mix and possibly all programming paradigms. some languages are designed to favor one paradigm. for example java with object-oriented programming

some ways in which languages make it difficult to use other paradigms are:

for example the need to create classes for creating functions

unsupported features, for example first-class functions

only domain- and problem-specific built-ins

complicated syntax

series of commands that modify a programs state

imperative commands structured into subroutines

structure a program into objects with behaviour and data. or: subroutines and data stored in records

everything that is not imperative. or: describe "what" and not exactly "how"

treat computation as the evaluation of mathematical functions

express facts and rules and ask the system

the order of expressions is usually very relevant. this can be problematic for parallelism and the understanding of code

is relatively few abstractions away from the machine (von neumann achitecture), this makes manual low-level performance optimisations easier to make

avoids having to deal with goto by using subroutines

avoids having to deal with subroutines that modify global state by encapsulating data in classes and objects

ideally no side-effects (does not mutate anything outside of the function)

order of computation is less relevant as there is less hidden shared state. this is beneficial for parallelisation because it makes it simpler to evaluate function calls independently on separate processors

referential transparency (same input leads to same result, function and specific arguments equal one specific result, function calls an variables can be substituted with results)

more predictable (formal verification, automatic optimisations)

state changes are always explicit via return values and function arguments

logic and control

leaves much of the "how", the choice of necessary computations, to the system

the factorial function implemented in different styles. each example shows definition then application

the factorial function calculates the product of all positive integers less than or equal to n

factorial(5) = 5 * 4 * 3 * 2 = 120

factorial = (n) -> result = 1 while n >= 1 result = result * n n = n - 1 result

factorial 5

class Factorial result: null calculate: (n) -> @result = 1 while n >= 1 @result = @result * n n = n - 1

f = new Factorial f.calculate 5 f.result

factorial = (n) -> if n <= 1 then 1 else n * factorial(n - 1)

factorial 5

factorial = (n) -> [1..n].reduce (result, n) -> result * n

factorial(0,1). factorial(N,F) :- N > 0, N1 is N - 1, factorial(N1,F1), F is N * F1.

?- factorial(5, X).