A talk on functional programming principles, patterns and examples for people who come from object oriented languages

1. Functional Programming
Principles & Patterns

2. Agenda
● Introduction to Functional Programming
● Principles
● Concepts
● Patterns
● Q & A
Code-examples are written in Java and JavaScript

3. Introduction to Functional Programming
● Benefits
○ Declarative (what vs. how)
○ Scalability / Concurrency
○ Testability
○ Composability & Modularity
○ Ability to reason about programs / Simplicity
○ ...
○ In our case: Broadening our Horizon
● Use-cases
○ Mathematical programming
○ Distributed Systems
○ High Concurrency
○ GUI programming (new!)
○ ...

4. Functional Programming Languages
● Common Lisp
● Haskell
● SML
● Clojure
● Scala
● Erlang
● (Java)
● (JavaScript)
● ...

5. Concepts
● Declarative vs. Imperative
● Pure Functions
● Higher Order Functions (+Lambdas)
● Immutability
● Recursion
● Laziness
● map/filter/reduce/zip/flatMap
● Currying

6. Declarative vs. Imperative
● Defining WHAT to do vs. HOW to do it
● Expressive
● Improve / Optimize underlying algorithms
● Eliminate Side-effects as much as possible

7. Declarative vs. Imperative - Example

8. Concepts
● Declarative vs. Imperative
● Pure Functions
● Higher Order Functions (+Lambdas)
● Immutability
● Recursion
● Laziness
● map/filter/reduce/zip/flatMap
● Currying

9. Pure Functions
● No side-effects
● Referential Transparency (caching, ...)
● Thread-safe
● Compiler-Optimizations
● ...

10. Concepts
● Declarative vs. Imperative
● Pure Functions
● Higher Order Functions (+Lambdas)
● Immutability
● Recursion
● Laziness
● map/filter/reduce/zip/flatMap
● Currying

11. Higher Order Functions
● Critical concept in FP
● Functions can take functions as arguments
● Functions can return functions
● Enables functional composition

12. Higher Order Functions - Example #1

13. Higher Order Functions - Example #2

14. Concepts
● Declarative vs. Imperative
● Pure Functions
● Higher Order Functions (+Lambdas)
● Immutability
● Recursion
● Laziness
● map/filter/reduce/zip/flatMap
● Currying

15. Immutability
● Persistent Data Structures
● Every time a data structure would be mutated, a new one
is returned instead
● Simplifies state management and mutation tracking
● Enables optimizations
● Works well with pure functions
● Memory overhead is mitigated by sharing (Tries)

16. Concepts
● Declarative vs. Imperative
● Pure Functions
● Higher Order Functions (+Lambdas)
● Immutability
● Recursion
● Laziness
● map/filter/reduce/zip/flatMap
● Currying

17. Recursion
● Divide and Conquer
● Can provide elegant solutions for complex problems
● Not as complicated to do, if one knows how ;)

18. Recursion #1

19. Concepts
● Declarative vs. Imperative
● Pure Functions
● Higher Order Functions (+Lambdas)
● Immutability
● Recursion
● Laziness
● map/filter/reduce/zip/flatMap
● Currying

20. Laziness
● Evaluation is deferred to the last possible moment
● Avoids needless calculations
● Working with infinite data structures
● Works very well with declarative programming

21. Laziness Example

22. Concepts
● Declarative vs. Imperative
● Pure Functions
● Higher Order Functions (+Lambdas)
● Immutability
● Recursion
● Laziness
● map/filter/reduce/zip/flatMap
● Currying

23. map / Filter / Reduce / Zip
● Backbone of functional programming

24. map

25. Filter

26. Reduce

27. Zip

28. Concepts
● Declarative vs. Imperative
● Pure Functions
● Higher Order Functions (+Lambdas)
● Immutability
● Recursion
● Laziness
● map/filter/reduce/zip/flatMap
● Currying

29. Currying
● Concept by Haskell Curry
● Translating a function that takes multiple arguments into
a sequence of functions which all take 1 argument
● e.g.: add(a, b) AND add(a)(b)
● Improves reusability and composition
● In some languages (Haskell, F#) functions are curried by
default

30. Currying - Example #1

31. Currying - Example #2

32. Currying - Example #3

33. Patterns
● GOF-Patterns in Functional Style
● Memoization
● Advanced Currying
● Immutability in practice
● Maybe / Optional
● Outlook: Advanced Topics

34. GOF Patterns vs. Functional Programming

35. GOF - Decorator (wrapper function)

36. GOF - Strategy (higher order functions)

37. Memoization (Caching Technique - Java)

38. Memoization (Caching Technique - JS)

39. Immutability in practice
● Data Structures don’t necessarily need to be persistent
in nature
● Use map / filter / reduce
● Examples in JS:

40. Maybe / Optional
● Pattern for handling absent (e.g.: Null) values
● Avoid annoying if (x==null) checks
● Great for declarative API’s
● Your business logic doesn’t need to deal with null
values, it can just use Maybe()’s
○ At the end, when the actual value is needed, the Maybe can be
evaluated and the null can be handled

41. Maybe / Optional

42. Advanced Topics
● Monoids / Functors / Applicatives
● Algebraic Structures in general
● Monads
● Transducers
● ...

43. Q & A

45. Resources #1
● https://vimeo.com/113588389
● http://looprecur.com/blog/
● https://www.youtube.com/playlist?list=PLK_hdtAJ4KqX0JOs_KMAmUNTNMRYhWEaC
● http://www.infoq.com/presentations/Simple-Made-Easy
● http://www.infoq.com/presentations/Clojure-Design-Patterns
● https://www.manning.com/books/functional-programming-in-java
● https://dzone.com/articles/functional-programming-java-8
● http://www.vasinov.com/blog/16-months-of-functional-programming/
● https://www.reddit.com/r/functionalprogramming/
● https://github.com/jhusain/learnrxjava/
● http://reactivex.io/learnrx/
● http://blog.jhades.org/java-8-how-to-use-optional/

46. Resources #2
● http://javascriptair.com/episodes/2015-12-30/
● http://www.amazon.de/Purely-Functional-Data-Structures-Okasaki/dp/0521663504/ref=sr_1_1?ie=UTF8&qid=1452875910&sr=8-1&k
eywords=functional+data+structures
● http://www.amazon.de/Functional-Thinking-Paradigm-Over-Syntax/dp/1449365515/ref=sr_1_1?ie=UTF8&qid=1452875871&sr=8-1&k
eywords=functional+thinking
● http://www.amazon.de/Becoming-Functional-Joshua-Backfield/dp/1449368174/ref=sr_1_1?ie=UTF8&qid=1452875879&sr=8-1&keyw
ords=becoming+functional
● http://www.amazon.de/Functional-JavaScript-Introducing-Programming-Underscore-js/dp/1449360726/ref=sr_1_1?ie=UTF8&qid=145
2875887&sr=8-1&keywords=functional+javascript
● https://github.com/MostlyAdequate/mostly-adequate-guide