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

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