Intro to Functional Programming

FUNCTIONAL
PROGRAMMING
Hugo Firth
@hugo_ﬁrth
1

WHAT I’LL BE TALKING ABOUT
2
▫︎Comparison of diﬀerent paradigms
▫︎What is functional programming?
▫︎Purity
▫︎Laziness
▫︎Concurrency

WHAT I WON’T BE TALKING ABOUT
3
▫︎Functors
▫︎Monads
▫︎Category theory
▫︎Type theory

CLOJURE
5
▫︎Lisp (funny syntax)
▫︎Runs on the JVM
▫︎No classes, just functions
▫︎Supports the functional paradigm

DIFFERENT PARADIGMS
6
▫︎Machine code
▫︎Procedural
▫︎Object Oriented
▫︎Functional
▫︎Multi-paradigm
Many others such as logic and symbolic
programming

DIFFERENT PARADIGMS
7
▫︎Machine code
▫︎Procedural
▫︎Object Oriented
▫︎Functional
▫︎Multi-paradigm
Many others such as logic and symbolic
programming

PROCEDURAL
BASIC and C are the most notable
languages here
Gave us basic looping constructs and
subroutines
Why?
To abstract away from GOTO
statements
8

OBJECT
ORIENTED
Many languages here Java, C#, C++,
etc.
 
Gave us Classes and Objects
 
Why?
To abstract away from global state
9

FUNCTIONAL
Increasingly large number of
languages including Clojure, Scala and
Haskell
Gave us Functions
 
Why?
Remove all state
10

DEFINITION
A style of building the structure and
elements of computer programs, that treats
computation as the evaluation of
mathematical functions and avoids state
and mutable data
12http://en.wikipedia.org/wiki/Functional_programming

PURITY?
14
Same arguments in == same result
No side eﬀects!
Ever!

WHY?
15
Easier to design, test and understand
Referential transparency  
-> optimisations  
-> laziness

IMPURE
17
function calculateTax(amount) {
var taxRate = db.getTaxRate();
return taxRate / amount * 100;
}

IMPURE
18
function calculateTax(amount, taxRate) {
db.saveTaxRate(taxRate);
}

PURE
19
function calculateTax(amount, taxRate) {
}

PURE
20
var tax = calculateTax(100, 10);
// tax = 10

WHAT DOESN’T THIS LOOK LIKE?
21
for (int i = 0; i < count; i++) {
//do something
}

22
while(someCondition) {
//do something
}

23
foreach(var something in somethings) {
//do something
}

HIGHER ORDER FUNCTIONS
A higher order function is one that takes one
or more functions as input, or returns a
function.
25

IMPURE
26
var numbers = [1, 2, 3, 4, 5];
for(var i = 0; i < numbers.length; i++) {
numbers[i] = numbers[i] * 2;
}
//numbers => [2, 4, 6, 8, 10]

IMPURE
27
var numbers = [1, 2, 3, 4, 5];
var destination = [];
for(var i = 0; i < numbers.length; i++) {
var result = numbers[i] * 2;
destination.push(result);
}
//destination => [2, 4, 6, 8, 10]

MAP (HIGHER ORDER FUNCTION)
Map is the name of a higher-order function
that applies a given function to each
element of a list, returning a list of results
28

FILTER (HIGHER ORDER FUNCTION)
Filter is a higher-order function that
processes a collection to produce a new
collection containing exactly those elements
of the original collection for which a given
predicate returns true
29

FOLD / REDUCE / AGGREGATE
A family of higher-order functions that
analyse a recursive data structure and
recombine through use of a given combining
operation the results of recursively
processing its constituent parts, building up
a return value
30

HIGHER ORDER FUNCTIONS
31
Recursion and higher order functions are
the two constructs that allow us to do
anything in a functional language

WHAT IS THIS DOING!?!
33
var n = 1;
var num_elements = 0;
var sum_of_first_10 = 0;
while (num_elements < 10) {
if (n^2 % 5 == 0) {
sum_of_first_10 += n;
num_elements += 1;
}
n += 1;
}
//sum_of_first_10 => 225

WHY?
35
Readability - describe what you want! Not
how to get it

WHY?
36
Abstract away concepts of iteration,
transformation, ﬁltering, and accumulation.
Write functions to deal with elements not a
sequence.

WHY?
37
Maintainability - Small independently
testable functions

RANGE
Return inﬁnite sequence of numbers
39

REPEAT
Returns a lazy inﬁnite sequence of supplied
items
40

CYCLE
Returns a lazy inﬁnite sequence of items in a
collection
41

WHY?
43
Code is not evaluated until you need it.
Code that doesn’t need to be evaluated
won’t be.

IMMUTABLE CLASS
45
public class Dollars {
private double value;
public Dollars(double value) {
this.value = value;
}
public double getValue() {
return this.value;
}
}

46
this.value = value;
}
return this.value;
}
public void add(double value) {
this.value += value;
}
}
MUTABLE CLASS

47
IMMUTABLE CLASS
this.value = value;
}
return this.value;
}
public Dollars add(double value) {
return new
Dollars(this.value + value);
}
}

48
IMMUTABLE COLLECTIONS
(THEY’RE HARD)

49
Empty collection:
[ ]
//or
null

50
1 element collection:
var element = x;
//and
var restOfCollection =
EMPTY_COLLECTION;

51
var element = y;
//and
collectionX;

52
var element = z;
//and
collectionY;

53
IN PICTURES

54
EMPTY
COLLECTION
[ ]

55
EMPTY
COLLECTION
X
[ X ]

56
EMPTY
COLLECTION
X Y
[ X, Y ]

57
EMPTY
COLLECTION
X Y Z
[ X, Y, Z ]

58
LOOKS FAMILIAR RIGHT?

59
EMPTY
COLLECTION
X Y Z
Z2
[ X, Y, Z ]
[ X, Y, Z2 ]

WHY?
61
Pass objects and collections to other
functions with out fear of them changing

WHY?
64
Concurrency is hugely important with multi-
core processors
Simplicity - data sharing across threads and
processes is hard if it’s mutable

YOU CAN DO THIS IN ANY LANGUAGE!!!!
▫︎Try to write pure small pure methods/functions
▫︎Avoid state wherever you can
▫︎Separate impurities when they can’t be removed
▫︎Create immutable classes (value objects)
▫︎Look at simpler methods for concurrency
65

Intro to Functional Programming

More Related Content

What's hot (20)

Viewers also liked (20)

Similar to Intro to Functional Programming (20)

Recently uploaded (20)

Intro to Functional Programming