Programming Paradigms

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Programming Paradigms
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A Question....

• What is your favourite programming language and
  why?

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Problem Solving

• Is there always only one way to think about a
  problem?
• Lateral thinking
• Sometimes a solution that is difficult to see
  becomes obvious when we think about the problem
  differently
• Are there different ways of thinking about
  programming?

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Paradigms

• What is a paradigm?
• Thomas S. Kuhn The Structure of Scientific
  Revolutions
• Text defn a model or mental framework for
  representing or thinking about something

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Paradigms (cont)

• A paradigm offers a particular way of approaching
  an idea
• It embodies certain assumptions about what is and
  is not possible
• It entails what are the appropriate questions to
  ask about the subject at hand

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Programming Paradigms

• Offer different ways of approaching programming
  problems
• Do we think of a problem in terms of
• Functions?
• Accessing memory locations?
• Objects?
• Logical assertions?

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But First...

• To explore each paradigm, we will solve the same
  problem in different ways
• The problem we will use is the GCD or greatest
  common divisor problem
• The GCD of two numbers u and v is the largest
  number which divides u and v evenly

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The GCD Problem

• The GCD of two numbers u and v is
• The gcd of u and v is u if v 0
• If v ! 0, then the gcd of u and v is the gcd of
  v and u modulus v

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The Imperative/Procedural Paradigm

• Largely based upon the Von Neumann model of
  computation
• Sequential execution of instructions
• Use of variables to represent memory locations
• Use of assignment to change the values of
  variables

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The Imperative/Procedural Paradigm (cont)

• Computation is described as a sequence of
  instructions
• Primary feature is a sequence of statements which
  represent commands or imperatives

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The Object-Oriented Programming Paradigm

• Somewhat of an extension or refinement of the
  procedural paradigm
• Fundamental unit of computation is the object

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The Object-Oriented Programming Paradigm

• Each object has a set of properties associated
  with it
• Ex size, height, colour
• Each object also has a set of operations or
  methods associated with it.
• Ex walk, talk, speak, draw

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The Object-Oriented Programming Paradigm

• Computation by message passing between objects
• Essentially computers in the small
• Primary goal to create reusable software
  components

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The PIE of OOP

• In the words of Homer Simpson Mmmm, pie....
• Not the same thing....

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The PIE of OOP

• Polymorphism from the Greek for many forms
• Inheritance allows us to define objects in
  terms of parent objects
• Encapsulation each object is self-contained

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The Functional Programming Paradigm

• A much different approach to programming
• Basic unit of computation is the function
• Thus our program is just the repeated application
  of functions to known data.
• Benefit much easier to reason about our programs
  (no side effects)

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The Functional Programming Paradigm

• Typically
• No loops (recursion is heavily used instead)
• No variables (only bindings)

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A Side Note About Side Effects

• When a function changes its external environment
  as a result of evaluating itself
• Ex
• Modification of a global variable
• Output

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Referential Transparency

• A key concept in functional programming
• Essentially means a function will always return
  the same value given the same arguments
• Or more succinctly NO SIDE EFFECTS

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Functions Are First Class Values

• Can pass functions as arguments to other
  functions.
• Sometimes called higher order programming or
  higher order functions

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The Logic Programming Paradigm

• A COMPLETELY different approach to programming
• Rather than specify how a problem is to be
  solved, we describe what a solution will look
  like and let the inference engine figure out how
  to solve the problem

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The Logic Programming Paradigm

• As a result, sometimes called declarative
  programming (as opposed to imperative)
• Fundamental units of computation are facts and
  clauses (or rules).
• The inference engine uses formal logic to find
  solutions to our problems

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The GCD Problem

• The GCD of two numbers u and v is
• The gcd of u and v is u if v 0
• The gcd of u and v is x if v ! 0 and y is u
  modulus v and x is the gcd of v and y

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A Big Question...

• How do we select the best paradigm for the
  problem at hand, a priori (beforehand)?
• Rules of thumb
• Mathematical problem gt functional programming
• Modeling gt OOP