Functional programming vs Object Oriented programming

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I ve been mainly exposed to OO programming so far and am looking forward to learning a functional language  My questions are     When do you choose functional programming over object-oriented   What are the typical problem definitions where functional programming is a better choice

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If you re in a heavily concurrent environment  then pure functional programming is useful   The lack of mutable state makes concurrency almost trivial   See Erlang  In a multiparadigm language  you may want to model some things functionally if the existence of mutable state is must an implementation detail  and thus FP is a good model for the problem domain   For example  see list comprehensions in Python or std range in the D programming language   These are inspired by functional programming

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Object Oriented Programming offers    Encapsulation  to   control mutation of internal state limit coupling to internal representation  Subtyping  allowing    substitution of compatible types  polymorphism  a crude means of sharing implementation between classes  implementation inheritance     Functional Programming  in Haskell or even in Scala  can allow substitution through more general mechanism of type classes  Mutable internal state is either discouraged or forbidden  Encapsulation of internal representation can also be achieved  See Haskell vs OOP for a good comparison   Norman s assertion that  Adding a new kind of thing to a functional program may require editing many function definitions to add a new case   depends on how well the functional code has employed type classes  If Pattern Matching on a particular Abstract Data Type is spread throughout a codebase  you will indeed suffer from this problem  but it is perhaps a poor design to start with   EDITED Removed reference to implicit conversions when discussing type classes  In Scala  type classes are encoded with implicit parameters  not conversions  although implicit conversions are another means to acheiving substitution of compatible types

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When do you choose functional programming over object oriented    When you anticipate a different kind of software evolution    Object-oriented languages are good when you have a fixed set of operations on things  and as your code evolves  you primarily add new things   This can be accomplished by adding new classes which implement existing methods  and the existing classes are left alone  Functional languages are good when you have a fixed set of  things  and as your code evolves  you primarily add new operations on existing things  This can be accomplished by adding new functions which compute with existing data types  and the existing functions are left alone    When evolution goes the wrong way  you have problems    Adding a new operation to an object-oriented program may require editing many class definitions to add a new method  Adding a new kind of thing to a functional program may require editing many function definitions to add a new case    This problem has been well known for many years  in 1998  Phil Wadler dubbed it the  expression problem    Although some researchers think that the expression problem can be addressed with such language features as mixins  a widely accepted solution has yet to hit the mainstream      What are the typical problem definitions where functional programming is a better choice    Functional languages excel at manipulating symbolic data in tree form   A nbsp favorite example is compilers  where source and intermediate languages change seldom  mostly the same things   but compiler writers are always adding new translations and code improvements or optimizations  new operations on things    Compilation and translation more generally are  killer apps  for functional languages

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You don t necessarily have to choose between the two paradigms  You can write software with an OO architecture using many functional concepts  FP and OOP are orthogonal in nature  Take for example C   You could say it s mostly OOP  but there are many FP concepts and constructs  If you consider Linq  the most important constructs that permit Linq to exist are functional in nature  lambda expressions  Another example  F   You could say it s mostly FP  but there are many OOP concepts and constructs available  You can define classes  abstract classes  interfaces  deal with inheritance  You can even use mutability when it makes your code clearer or when it dramatically increases performance  Many modern languages are multi-paradigm  Recommended readings As I m in the same boat  OOP background  learning FP   I d suggest you some readings I ve really appreciated   Functional Programming for Everyday  NET Development  by Jeremy Miller  A great article  although poorly formatted  showing many techniques and practical  real-world examples of FP on C    Real-World Functional Programming  by Tomas Petricek  A great book that deals mainly with FP concepts  trying to explain what they are  when they should be used  There are many examples in both F  and C   Also  Petricek s blog is a great source of information

[oop] Functional programming vs Object Oriented programming

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