Transcript Programming Language Paradigms: summary

Programming Language
Paradigms: summary
Object-oriented programming
• Objects are the fundamental building blocks
of a program.
• Interaction is structured as messages send to
receivers.
• Receivers implement methods in response
to messages.
Smalltalk
• Smalltalk is a pure OO language
– Everything is an object.
– All objects have a uniform communication
structure.
• Method invocation is done at runtime
– Dynamic binding
• Language, object system and development
environment tightly integrated.
Inheritance
• One of the most popular OO features is
inheritance.
– Can define new classes in terms of existing
classes.
– This creates a class hierarchy in which
subclasses take on the behavior of their parent
classes.
• A subclass can generate new behaviors by
– Defining new methods
– Overriding a parent method.
Polymorphism
• Polymorphism is the idea that an object or method
can perform the same operation on different data
types.
– Addition is polymorphic in most languages.
• Allows a programmer to write code without
knowing the types of the data involved.
• [listOfStuff sort] <- don’t need to know what this
is a list of.
Encapsulation
• Encapsulation provides a way of packing all the
structure and behavior of an object into a single
module.
– Interface: the public face of an object
– Implementation: How methods are carried out.
• Prevents users from misusing internal structure.
• Implementation can be changed without affecting
external programs.
• Promotes information hiding.
Objects and Messages
• Everything in Smalltalk is an object.
• You communicate with objects via
messages.
• The receiver of a message responds via the
execution of a method.
<object> <selector1>: arg1 <selector2>: arg2 …
myList at: 1 put: someObject
Program design
• Program design consists of constructing a
set of objects and determining how they
relate to each other.
– is-A relationship
– has-A relationship
• Suitable for complex, decomposable
applications.
– GUIs
– Distributed systems
Smalltalk vs. Java
• Similarities
– Integrated object system
– Garbage collection
– Single inheritance
• Differences
– Smalltalk has no non-object datatypes
– Smalltalk methods are weakly typed.
Smalltalk vs. C++
• Similarities
– Polymorphism
• Differences
– Multiple inheritance in C++
– No Object class in C++
• No typeof/getClass methods in C++
– Garbage collection
– C++ is strongly typed – most binding happens at
compile time.
– No integrated IDE/Object system
Functional Programming
• Functions are the basic building blocks.
• A function takes zero or more inputs and returns
an output
– Inputs are unchanged
• Bottom-up programming is the dominant style.
• More complex functions are constructed from
simpler functions.
• Leads to the construction of libraries and modules.
Lisp
• Read-eval-print loop
– Arguments are evaluated at runtime.
• Weakly typed.
• Symbolic programming – able to work with
either lists or symbols.
• Large number of built-in functions
Recursion
• Functional languages tend to use recursion.
• Divide problem into:
– Base case
– Recursive case.
• Tail recursion: recursive functions that
return the final value in the base case with
no other processing are tail recursive.
– Can be rewritten as iteration
Lists
• Lists are the primary data structure
– Car/first: data
– Cdr/rest – pointer to the next element in the list.
a
b
c
Applicative Programming
• Applicative programming is the idea of
applying a function to a list of objects.
• Functions are first-class objects.
• Serves the same function as iteration
– More terse and generalizable
• Mapcar, apply
OOP in Lisp
• CLOS – Common Lisp Object System
• Provides classes, methods
– Multiple inheritance, polymorphism
• Implemented as generic functions
– Methods are not associated specifically with a
class, as in Smalltalk.
Declarative Programming
• Clauses are the fundamental building block.
• Emphasis on relations between data.
• Built-in inference engine handles
procedural control.
• Implication used to direct search.
Declarative Programming
• Best suited for domains where relations
between data members are the important
factor.
– Expert systems
– Databases
– Natural Language
• Goal: abstract away from procedural
knowledge; focus on data.
Knowledge Representation in
Prolog
• Facts (unit clauses):
– likes(homer, beer).
<- ends with a period
• Constants are lower case.
• Names of relations are lower case.
• Conjunctions:
– likes(homer,beer).
– likes(homer,food).
• Statement is represented as separate Prolog clauses.
Knowledge Representation in
Prolog
• Rules
– does(lisa,homework) :- likes(lisa,school).
– Equivalent to:
• likes(lisa,school) ->does(lisa,homework)
– Read as: “Lisa does homework if she likes
school.” or …
– “To prove that Lisa does homework, prove that
she likes school.”
Overall themes:
Programs as Data
• In both Lisp and Prolog, we saw programs
that had the ability to dynamically generate
new code.
• This allows an application an incredible
amount of flexibility.
• Requirement: the structure of a program
must be accessible and easily modifiable.
Overall themes: Abstraction
• Abstraction is the key to large-scale
software development.
• Components are developed, tested, and then
treated as a black box.
– Used to build larger components.
• Key requirement: no side effects!
Overall themes:
Functions as objects
• All of the languages we’ve studied treat functions
as first-class objects.
• This means that functions can be passed into and
returned from functions.
• Allows highly generic methods to be created
– Sort, find, priority queue
• Implementation of a message can be determined at
run-time
Overall themes: Language Design
• Adapt language to fit your problem
• Reduce the programmer’s cognitive burden
• Enhance readability/maintainability
• Allows for short pieces of code with a
single well-defined purpose.
Overall Themes: Reuse
• Code reuse is one of the most important
goals of software engineering.
• One of the primary goals of OOP and
functional programming.
• If you find yourself building the same
function more than once, generalize.
– Improves readability
– Reduces effort