SET Software Engineering Thailand Meeting: Functional Programming with Scala - Prequel

Meet-up, May 28, 2015, Launchpad, Bangkok
Roland Petrasch & Brian Topping
Welcome!

SET Meet-up
Programming with Scala
Agenda
SET — Software Engineering Thailand
Programming with Scala – Prequel
(speaker: Roland Petrasch)
Type Systems / Data Types
Functional Programming
Combination of OOP and FP
Programming with Scala -
Object-Oriented meets Functional
(Speaker: Brian Topping)
Discussion
Roland Petrasch28.5.2015, SET Meet-up
Programming with Scala - Prequel
Slide 2
Programming
Debugging
Debugging
Programming

SET — Software Engineering Thailand
The Interest Group from & (not only) for Developers
Open Group: Members, Sponsors and Organizers welcome
Next topics: HCI Patterns, Scrum, Model-Based UID, Agile Testing,
Software Architectures & Frameworks, S.O.L.I.D. & Co., UML ...
Contact
Roland Petrasch, Professor for Software Engineering
Thammasat University, Department of Computer Science,
Rangsit Campus, Pathum Thani
roland.petrasch@cs.tu.ac.th
roland.petrasch@gmail.com
Slide 3

Type system
Ruleset: assign types to variables, expressions, functions
Error prevention (type errors), e.g. float calculateThrust(Position p)
Other purposes: documentation, compiler optimization
Evolution
Type safety: type checking
During compile-time (static typing), e.g. C++, C#, Java, Scala
During run-time (dynamic typing), e.g. JavaScript, Ruby, Python
Combination, e.g. C# 4.0 (dynamic)
SET Meet-up
Slide 4
Untyped Primitive Data Types Abstract Data Types Classes …
Assembler Fortran, Pascal, C C++, Java, C#

JavaScript: dynamic typing, implicit type conversion, type inference
var five = '5', two = 2, sth;
sth = five * two; → 10
sth = five + two; → 52
sth = five + two * 5; → 510
sth = five + two * 5 + true; → 510true
sth = five + two * 5 + true / 10; → 5100.1
sth = (five + two * 5 + true) / 10; → NaN
Sth = 10 / 0; → Infinity
SET Meet-up
Slide 5

Type system
Type inference: automatically deduce the type at compile time
partly or completely, e.g. var i = 5; // inferred type: int
Type inference is not only a feature of dynamically typed languages
Static/dynamic typing and strong/weak typing are orthogonal:
Many discussions about strong and weak typing, no precise definition
Weak typing: use a value of one type as if it were a value of another type,
e.g. in C: char c = 'A'; c++; c=42;
or in JavaScript: var s = 'x' + 3; // get 'x3'
Strong typing
e.g. in Java: String s = "1" + ' ' * 2 + false; // 164false
int i = '3' * 2.2; // error
Importance of „strong” and semantically rich data types
SET Meet-up
Slide 6
To a C programmer
strong typing means
pressing the keys
harder

Data Types
Data Types and type checking is very (!) important
Example: float calculateThrust(Position p)
2 software components:
One is providing and another one is using
the calculateThrust function
SET Meet-up
Slide 7
Mars Lander by Brian McMullin
Source: Defense Intelligence
Agency, DoD, USA
ThrustContolUnit
NavigationSystem
calculateThrust(Position p)Call
Germany USA
The thrust is a float. A good data type? Yes? Okay? Right?
No: Sematics (unit) not clear: Can be „newton” (metric) or
„pound-force” (non metric) → sematically poor in this case

Semantically rich data types and error prevention
„NASA lost a $125 million Mars orbiter because one
engineering team used metric units while another
used English units for a key spacecraft operation.“
(http://edition.cnn.com/TECH/space/9909/30/mars.metric/)
Example 2.0:
Thrust calculateThrust(Position p)
A new data type Thrust is needed
class Thrust {
private Unit unit;
private Value value;
public float getValue(Unit unit) …
}
SET Meet-up
Slide 8
Mars Polar Lander
Source: NASA

Functional programming
Declarative programming paradigm
Evaluation of functions, avoids changing-state & mutable data,
eliminate side-effects
Functions: output depends only on input → error prevention
Example for side-effect in Java
int i=0;
if (true != false || ++i==0) { }
System.out.println(i); // i is 0
Based on lambda calculus for simple semantics for computation
Anonymous Functions (→ closures), no explicit names
Functions of a single input
Example (λx.x+1) 4 → 4+1 → 5
-function
-expression computaton
SET Meet-up
Slide 9


Functional programming
λ-Example 2.0
(λx.x+2)((λy.y*10)4)
SET Meet-up
Slide 10
Alonzo Church (1903–1995),
Photo: Wikipedia
"Answer to the Ultimate Question of
Life, the Universe, and Everything“
(The Hitchhiker's Guide to the Galaxy)

Combination of object-oriented and functional paradigms
OO and FP have similarities, e.g.
Expressive power (semantics)
Encapsulation of small parts (reduction of complexity)
Data types
...
OOP: High in cohesion (class consists of data and functions) and
low in coupling (classes should be independent as far as possible)
FP: Data is only loosely coupled to functions, functions hide their
implementation, immutability, no side-effects
Multi-paradigm languages like Scala combine the features from
object-oriented and functional paradigms
SET Meet-up
Slide 11

Combination of Object-oriented and Functional (object-functional)
OOP & FP are orthogonal concepts, i.e. they are not contradictory, e.g.
encapsulation, inheritance, polymorphism, abstration do not imply mutability
SET Meet-up
Slide 12
calcValue
IntegerValue
Class
calcValue
OO: Every value is an object; attributes & behavior of an object are
described by a class; objects are instances of a class)
FP: Every function is a value; and every value is an object
→ so every function is an object

This was the little prequel,
thank you for your attention
SET Meet-up
Brian Topping
Entrepreneurial, results-oriented architect
Developer and leader with > 20 years
experience
Companies: Apple, Oracle, Nokia, AT&T,
Intel and Bloomberg.
Dozens of programming & scripting
languages, OS, and IDE

SET Software Engineering Thailand Meeting: Functional Programming with Scala - Prequel

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