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. : arg1 : 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). Constants are lower case. Names of relations are lower case. Conjunctions: –likes(homer,beer). –likes(homer,food). Statement is represented as separate Prolog clauses. <- ends with a period

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