This document provides an overview of functional programming concepts for object-oriented programmers. It discusses the fundamentals of FP including immutability, purity, first-class and higher-order functions, closures, and recursion. It provides examples of these concepts in languages like Lisp, F#, C#, and JavaScript. The document also compares OO and FP concepts and discusses derived FP concepts like partial application, lazy evaluation, and pattern matching.

2. About me
 Name: Nikolay Mozgovoy (FB, LinkedIn)
 Developer in Sigma Software since 2013
and mentor since 2016
 Teacher in KhAI (Department Of Computer
Systems, Networks And Cybersecurity)
 GGJ Ukraine prizewinner (2016, 2017)
 GGJ Site organizer (Kharkiv)
 Media/Global hack weekend competitor
 MSCSD: App builder
 Caught Lisp in 2016

3. FP for OO adepts

4. Talk overview
 Functional programming fundamentals
 History
 Relevance
 Basic concepts
 Derived concepts

5. Paradigms in time
 Imperative
 Machine (1940s)
 Procedural (1960s – FORTRAN, ALGOL, COBOL, BASIC)
 Structured (1966-1968 ‘Go To Statement Considered Harmful’ by Dijkstra)
 Object-oriented (1967 - Simula, 1972 - Smalltalk)
 Event-driven (1970s)
 Declarative
 Functional (1958 - LISP)
 Logic (1972 - Prolog)
 Domain-specific (1974 - SQL)
 Functional reactive (1997-2000 Paul Hudak)

6. Lambda calculus
 Formal system in mathematical logic for expressing
computation based on function abstraction and
application using variable binding and substitution
 Turing complete
 Developed by Alonzo Church in the 1930s
 Notation:
 λx. t[x] – function that do anything with x
 λx. x^2 – x squaring function same as y=x^2
 (λx y. x + y) 1 2 = (λy. 1 + y) 2 = 1 + 2 – carrying

7. Functional Languages
 LISP - 1958
 Scheme - 1970
 Racket - 1994
 Common Lisp - 1984
 Clojure - 2007
 ML (Meta Language) - 1973
 Ocaml -1996
 F# - 2005
 Erlang - 1986
 Haskell - 1990
 Scala - 2004

8. CPU cores per year
0
4
8
12
16
20
24
28
32
2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018
Intel AMD

9. GPU cores per year
0
512
1024
1536
2048
2560
3072
3584
4096
4608
5120
2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018
CUDA Cores (Nvidia) Stream Processors (AMD)

10. RAM price ($ per Mbyte)
0.00
0.20
0.40
0.60
0.80
1.00
1.20
2000 2002 2004 2006 2008 2010 2012 2014 2016

11. Imperative code & parallel execution
BigInteger[] arr = Enumerable.Range(0, 10000)
.Select(x => new BigInteger(x)).ToArray();
BigInteger sum = 0;
foreach (var n in arr) sum += n; // OK
Parallel.For(0, arr.Length, n => sum += n); // Wrong result
// And if we apply lock it would degradate to de-facto single-threaded code
arr.AsParallel().Aggregate((a, b) => a + b); // OK

12. Functional code & parallel execution
1
3
2 43
7
10
(a, b) => a + b (a, b) => a + b
(a, b) => a + b

13. Basic OO concepts
 Encapsulation
 Inheritance
 Polymorphism
 Abstraction

14. Basic FP concepts
 Immutability
 Purity
 First-class &
Higher-order functions
 Recursion

15. Basic OO concepts vs Basic FP concepts
 Encapsulation
 Inheritance
 Polymorphism
 Abstraction
 Immutability
 Purity
 First-class &
Higher-order functions
 Recursion

16. The failure of state
public void ProcessPackage(Stream package)
{
SaveMetadataInDb(package);
package.Seek(0, SeekOrigin.Begin); // !!!
/* ... */
SaveContentInSearchEngine(package);
/* ... */
LogOperation(package);
}

17. Immutability
 Impossibility to change the state of the object once it was created
 Immutable objects are simpler to test, and use
 Truly immutable objects are always thread-safe
 They help to avoid temporal coupling
 Their usage is side-effect free (no defensive copies)
 Identity mutability problem is avoided
 They always have failure atomicity
 They are much easier to cache
 They prevent NULL references, which are bad
 I the Lord do not change (Malachi 3:6)

18. Immutability example
; LISP
(define animals (list 'dog 'cat 'fish))
(cons 'horse animals) ; returns a new list of animals, not changing existing one
// F#
let animals = ["dog";"cat";"rat"]
let moreAnimals = List.append animals ["horse"] // returns a new list
// C#
var animals = new ReadOnlyCollection<string>(new[] { "dog", "cat", "fish" });
var moreAnimals = animals.Append("horse"); // returns new Enumerable

19. The sin of impurity
public async Task HookNextBuild(IProject project, Task buildFinished) {
var buildStarted = SyncEvents.VsBuildStarted(this.VsPid).ToTask(); // IPC
await Task.WhenAny(buildStarted, buildFinished).ConfigureAwait(false);
if (this.recentAnalysisCompletion != null) {
await this.recentAnalysisCompletion;
this.recentAnalysisCompletion = null;
this.recentCompletionTime = DateTime.Now; // Time
}
var analysisCompletion = await this.LaunchAnalysisAsync(project);
this.recentAnalysisCompletion = analysisCompletion;
}

20. Purity
 The property of functions not to have side-effects and any dependence from
external state or time
 Function is considered pure when:
 The function always evaluates the same result value given the same
argument value(s)
 Evaluation of the result does not cause any side effect
 Application:
 Simplifies testing
 Simplifies parallel computing
 Allows memoization

21. Purity example
// in F#
let inc x = x + 1 // pure function
let mutable i = 0
let badInc () = i <- i + 1 // impure function
; in LISP
(define (inc x) (+ x 1)) ; pure function
(define i 1)
(define (bad-inc!) (set! i (+ i 1)))
// in C#
Math.Pow(2, 8); // pure function
Path.GetTempFileName() ; // impure function
DateTime.Now; // impure function

22. First-class and Higher-order functions
 First-class functions is a property of programming language to treat functions
as a standard datatype
 Function is considered higher-order function when it does at least one of the
following:
 takes one or more functions as arguments
 returns a function as its result

23. First-class and Higher-order functions
// C#
// toUpper is first-class function
Func<string, string> toUpper = str => str.ToUpper();
// Select is higher-order function
new[] { "dog", "cat" }.Select(toUpper);
// F#
// fun x -> -x is first-class function & Seq.sortBy is higher-order function
[22;31;1;5;7] |> Seq.sortBy(fun x -> -x)
// >> is a higher-order function, it returns a composition of the functions
(add1 >> add1) 1 // 3

24. Recursion
 Recursion is idiomatic way to perform iterations or lopping in functional
programming languages
 Tail recursion usually can be recognized and optimized by a compiler
 Mutual recursion may require trampoline to handle it
 To iterate is human, to recurse, divine (L. Peter Deutsch)

25. Recursion example (LISP)
; mutual recursion
(define (r1 i)
(if (< i 0) 1 (r2 (- i 1))))
(define (r2 i)
(if (< i 0) 1 (r1 (- i 1))))
(r1 1000000)
; tail recursion
(define (factorial x)
(if (< x 2)
1
(* x (factorial (- x 1)))))

26. Recursion example (F#)
// mutual recursion
let rec r1 i =
if i < 0
then 1
else r2(i - 1)
and r2 i =
if i < 0
then 1
else r1(i - 1)
// tail recursion
let rec factorial x =
if x < 2
then 1
else x * factorial(x - 1)

27. Derived FP concepts
 Closures
 Partial application
 Lazy evaluation
First-class &
Higher-order functions

28. Closures
 A data structure containing a lambda expression, and an environment to be
used when that lambda expression is applied to arguments
 Closures are used to:
 bind together data and behavior (just like objects in OOP)
 emulate state
 emulate object system
function
environment
in which
function got
created
closure
object

29. Closure example in F# (simple and artificial)
// creates a function that raises y to a power of x
let makePowerFn x =
(fun y -> pown y x) // this lambda is a closure itself (because of reference to x)
let pow2 = makePowerFn 2
pow2 3 // 9
let pow3 = makePowerFn 3
pow3 3 // 27

30. Closure example: pure recursive data structures
type Seq = { value: int; next: (unit -> Seq) }
let rec initSeq next value : Seq = {
value = value; // return current value
next = (fun () -> initSeq next (next value)) // create a new sequence with the next value
}
let pow2Seq = initSeq (fun x -> x * 2) 2 // sequence of powers of 2
pow2Seq.value // 2
pow2Seq.next().next().value // 8
// doesn’t it look like enumerator.MoveNext().MoveNext().Current in C#?

31. Closure example: trampoline (in JavaScript)
function factorial (n) {
return n < 2
? 1
: n * factorial(n -1);
}
factorial(3); // 6
/* maximum call stack
size exceeded */
factorial(100000);
function trampoline (fn) {
while (
typeof fn === 'function') {
fn = fn();
}
return fn;
}
function factorial (n, acc = 1) {
return function () {
return (n < 2)
? acc
: factorial(n - 1, n * acc);
}
}
/* no errors */
trampoline(factorial(100000));

32. Partial application
 Process of breaking a single function into a multiple ones of smaller arity
 Often conflated/mixed with currying witch is a process of breaking a function of
n arguments to a chain of n functions of 1 argument each
 Motivation is that very often the functions obtained by supplying some but not
all of the required arguments
 papply: (((a × b) → c) × a) → (b → c) = λ(f, x). λy. f (x, y)
 curry: ((a × b) → c) → (a → (b → c)) = λf. λx. λy. f (x, y)
 uncurry: (a → (b → c)) → ((a × b) → c) = λf. λ(x, y). f x y

33. Partial application example (simple & artificial)
// F#
let add1 x = x + 1
// traditional definition vs application
let add1 = (+) 1
let sortAsc = List.sortWith
(fun x y -> x - y)
let sortDsc = List.sortWith
(fun x y -> y - x)
// C#
Func<string, Claim> newClaim =
value => new Claim(
"group", value, "String", “sigma.software ");
/*…*/
var groups = RetrieveGroups();
// { "Management", "Accountants“ };
var groupClaims = groups.Select(newClaim);

34. Partial application (C#)
public class Package {
private readonly IContentReader contentReader;
private readonly IChecksumGenerator checksumGenerator;
private readonly IChecksumFormatter checksumFormatter;
public Package(
string name,
IContentReader contentReader,
IChecksumGenerator checksumGenerator,
IChecksumFormatter checksumFormatter) {
this.Name = name;
this.contentReader = contentReader;
this.checksumGenerator = checksumGenerator;
this.checksumFormatter = checksumFormatter; }
public string Name { get; }
public string GetChecksumStr(Encoding encoding) =>
this.checksumFormatter.Format(
this.checksumGenerator.Generate(contentReader.Read()),
encoding);
}
public interface IContentReader {
Stream Read();
}
public interface IChecksumGenerator {
byte[] Generate(Stream dataStream);
}
public interface IChecksumFormatter {
string Format(
byte[] checksum,
Encoding encoding);
}

35. Partial application (F#)
type Package = {
name: string;
getChecksumStr: (Encoding -> string) }
let initPackage
(name: string)
(content: (unit -> Stream))
(generateChecksum: ((unit -> Stream) -> byte[]))
(formatChecksum:(byte[] -> Encoding -> string)) : Package = {
name = name;
getChecksumStr = content |> generateChecksum |> formatChecksum
}

36. Lazy evaluation
 Lazy evaluation is a way of computing in which values are not calculated until
they are required
 Lazy evaluation can be accomplished:
 By language itself (Haskell)
 Using special data structures
 The opposite of lazy evaluation called eager evaluation
 Application:
 Infinite data structures
 Memory footprint optimization
 Startup speed optimization
 Often combined with memorization

37. Lazy evaluation example (simple & artificial - F#)
// F#
let lazyVal = lazy (10 * 10)
lazyVal // Lazy<int> Value is not created.
lazyVal.Force() // 100
let nat = Seq.initInfinite(fun n -> n + 1)
// seq<int> = seq [1; 2; 3; 4; ...]
Seq.skip 10 nat |> Seq.take 10
// seq<int> = seq [11; 12; 13; 14; ...]

38. Lazy evaluation example (F#)
type Package = {
name: string;
getChecksumStr: (Encoding -> string) }
let initPackage
(name: string)
(content: (unit -> Stream))
(generateChecksum: ((unit -> Stream) -> byte[]))
(formatChecksum:(byte[] -> Encoding -> string)) : Package =
let checksum = lazy (content |> generateChecksum)
{
name = name;
getChecksumStr = checksum.Value |> formatChecksum
}

39. Pattern matching
 Language feature that allows to choose the following instruction set based on
the match of given data with one of the declared patterns:
 Constants
 Predicates (functions that return Boolean values)
 Data type
 Anything else supported by a particular language

40. Pattern matching example
// F#
let ReadFromFile reader : StreamReader) =
match reader.ReadLine() with
| null -> printfn "n"; false
| line -> printfn "%s" line;
true
// C#
public static double ComputeArea (object shape)
{
switch (shape)
{
case Square s:
return s.Side * s.Side;
case Circle c:
return c.Radius * c.Radius * Math.PI;
case Rectangle r:
return r.Height * r.Length;
default: throw new ArgumentException();
}
}

41. Concepts comparison
Goal OOP FP
Binding data & behavior Class -> Object Closure
Passing dependencies Constructor Partial application
Decoupling from concrete
implementation
Interface Function signature
Controlling execution flow Switch statement Pattern matching

42. Summary
 Fundamentals concepts:
 Immutability
 Simplifies reasoning & testing; Increases scalability & reliability;
 Purity
 Simplifies reasoning and testing; Increases scalability & reliability;
 First-class, higher-order functions
 Closures
 Functional way to bind data and behavior
 Partial application
 Functional way to have constructors
 Lazy evaluation
 Speed ups initialization

43. What’s next?
 State management
 The problem of state
 Approaches of managing the state and side-effects
 Continuations
 Promises
 Monads
 FRP

44. Books
 Structure and interpretation of computer programs
 MIT course 6.037
 Introduction to Functional Programming
 Cambridge CS department
 Hackers and painters
 Google Play Books
 Amazon

45. Articles
 Rich Hickey – Simple made Easy
 https://github.com/matthiasn/talk-
transcripts/blob/master/Hickey_Rich/SimpleMadeEasy.md
 Vladimir Khorikov - Functional C#
 https://github.com/vkhorikov/CSharpFunctionalExtensions
 Robert Martin – FP vs OO
 http://blog.cleancoder.com/uncle-bob/2014/11/24/FPvsOO.html
 Why Functional Programming matters
 https://www.cs.kent.ac.uk/people/staff/dat/miranda/whyfp90.pdf
 Me - LISP: back to the future (a tribute to 60th anniversary):
 https://sigma.software/about/media/lisp-back-future-tribute-60th-
anniversary

46. Credits
 Penrose triangle © Tobias R. – Metoc [CC BY-SA 2.5] / Wikimedia Commons
 Six petal lotus © Christopher J. Fynn [CC BY-SA 3.0] / Wikimedia Commons
 Two intersecting Archimedean spirals of opposite directions © Nevit Dilmen
[GFDL, CC-BY-SA-3.0 or CC BY-SA 2.5] / Wikimedia Commons
 Objects should be immutable © Yegor Bugayenko
 Scheme: An interpreter for extended lambda calculus © Sussman and Steele