Concepts of Programming Languages

Concepts of Programming
Languages
Eelco Visser

Delft
TI1220
University of
Technology Concepts of Programming Languages
Challenge the future

Overview
today’s lecture

A new course

• from IN1605-B to TI1220

What is this course about?

• why study concepts of programming languages?

How is this course organized?

• lectures and assignments

Introduction to Scala

• writing and running Scala programs

TI1220 - Introduction 2

I
A New Course

TI 1220 - Lecture 1: Introduction 3

A New Course
the only constant is change

IN1605-B: Algoritmen en Programmeertalen:
Programmeertalen

• 4 ECTS

• Haskell + C

• Peter Kluijt

TI1220: Concepten van Programmeertalen

• 6 ECTS

• Scala + C + JavaScript

• Eelco Visser


Under New Management
Eelco Visser

Lecturer

• Dr. Eelco Visser

• UHD in Software Engineering Research Group

• http://eelcovisser.org

• Contact: e.visser@tudelft.nl

• first ask student assistants

• send questions by email

• appointment if necessary


My Research
software language design & engineering

Spoofax Language Workbench

• design and implement languages & IDEs

• from high-level declarative specification

Domain-specific language design

• WebDSL: a DSL for web programming

• Mobl: a DSL for mobile web applications

Interaction design

• researchr.org: building a digital library


Examination Old Course

You can still sit for exams of IN1605-B

• Provided you have finished the lab (practicum)

Sittings

• Together with exam for this course (TI1220)

• April 11, 2010

• June 22, 2010


Feedback Appreciated

An invitation to Murphy

• New content

• New software

• New lectures, assignments, exams

• New lecturer

• New assistants (mostly)

Please let us know how it goes

• report problems early

• also let us know what goes right :)


II
Concepts of Programming Languages


Computers Process Data

data computer data


Programmable Computers

data computer data

program


Programming Language

data computer data

L
program


Turing Machines
computability

Turing/Church Thesis

• Every effective computation can be carried out by a
Turing machine*

Corollary

• All (Turing Complete) programming languages can
express all effective computations

• Why bother with new programming languages?

(*) see IN2505


History of Programming Languages
timeline

History of Programming Languages
1954 1960 1965 1970 1975 1980 1985 1990 1995 2000 2001 2002 2003 2004

1986 1990 1990 1991 1991 1993 1994 1995 1996 1996 1997 1997 2000 2001 2001 2003 2003 2004
For more than half of the fifty years computer programmers have been This timeline includes fifty of the more than 2500 documented
writing code, O’Reilly has provided developers with comprehensive, programming languages. It is based on an original diagram created

www.oreilly.com
in-depth technical information. We’ve kept pace with rapidly changing by Éric Lévénez (www.levenez.com), augmented with suggestions
technologies as new languages have emerged, developed, and from O’Reilly authors, friends, and conference attendees.
matured. Whether you want to learn something new or need
For information and discussion on this poster,
answers to tough technical questions, you’ll find what you need
go to www.oreilly.com/go/languageposter.
in O’Reilly books and on the O’Reilly Network.
©2004 O’Reilly Media, Inc. O’Reilly logo is a registered trademark of O’Reilly Media, Inc. All other trademarks are property of their respective owners. part#30417

This timeline includes 50 of the more than 2500 documented programming languages.


Programming Languages in TI

TI1200: Object-Oriented Programming

• Java

TI1400: Computer Systems

• Assembly

TI1500: Web- and Database Technology

• HTML, PHP, SQL, JavaScript

TI1600: Multi-Agent Systems

• Prolog, GOAL


“A programming language is low level when its
programs require attention to the irrelevant”
Alan J. Perlis. Epigrams on Programming. SIGPLAN Notices, 17(9):7-13, 1982.


Understanding Computation
purpose of programming languages

Encoding computation

• Computer needs list of instructions to move its parts

Understanding computation

• Discuss concepts

• Formulate algorithms

• Reason about problem solutions

Programming languages are for people


Increasing the Level of Abstraction
concepts of programming languages
calc:
push eBP ; save old frame pointer
mov eBP,eSP ; get new frame pointer
sub eSP,localsize ; reserve place for locals
.
. ; perform calculations, leave result in AX
.
mov eSP,eBP ; free space for locals
pop eBP ; restore old frame pointer
ret paramsize ; free parameter space and return

push eAX ; pass some register result
push byte[eBP+20] ; pass some memory variable (FASM/TASM syntax)
push 3 ; pass some constant
call calc ; the returned result is now in eAX

http://en.wikipedia.org/wiki/Calling_convention

def f(x)={ ... } f(e1)

Programming Models

Language provides model of computation

• data & operations, composition & abstraction

Language targets application domain

• Systems programming

• Embedded systems

• Web programming

• Enterprise software


Programming Language DNA

Scope and binding

• declaration and binding of names, free and bound
occurrences, renaming, substitution

Data structures

• product and sum types, records, recursive types

Modularization and data abstraction

• modules, classes, higher-order functions, class-based
vs prototype inheritance

Syntax and interpretation


Why Study Concepts?

Design abstractions for new domains

• from library to language feature

• apply PL techniques (MapReduce)

Learn new computational models

• understand fundamental ideas underlying PLs

• experience with variety of models

• learn new languages in future

Choosing the right language


Goals of this Course

• Understand programming languages and programs in
terms of concepts that transcend individual languages

• Consider advantages and disadvantages in the use of
particular languages and concepts

• Analyze programs in terms of concepts

• Application of concepts in designing programs


Approach

Study three concrete languages

• Scala + C + JavaScript

• Learn to program in these languages

• Learn concepts by using them in programming

• Programming assignments about languages

Generalize

• What are the underlying ideas?

• Learn to learn new languages


Scala
functional programming

• Lecture 1: Getting Started with Scala

• Lecture 2: Functional Objects

• Lecture 3: Functions & Closures

• Lecture 4: List Programming (Recursion)

• Lecture 5: Trees & XML

• Lecture 6: Composition, Inheritance, Traits

• Lecture 7: Modular Programming

• Assignment: interpreter for tiny functional language


C
systems programming

• Lecture 8: Memory Management

• stack vs heap

• pointers

• Lecture 9: Implementing Objects

• function pointers

• simulating dynamic dispatch

• Assignment: implementing OO in C


JavaScript
web programming

Prototype-Based Object-Oriented Language

• Lecture 10: Prototypal Inheritance

• History: Smalltalk, Self (Squeak)

• Lecture 11: Asynchronous functions

• Lecture 12: DOM manipulation

• Lecture 13: Regular expressions

• Lecture 14: mobl: a domain-specific language for the
mobile web

• Assignment: playing with prototypes


Meta-Programming
implementation of programming languages

Other courses consider internals of languages

• IN2505: Fundamental Computer Science 2: Theory of Computation

• grammars and automata

• IN4303: Compiler Construction

• translation of high-level source to machine language

• IN4308: Model-Driven Software Development

• design of domain-specific software languages


III
Course Organization


Ingredients
course organization

This is a semester course

• Q3 + Q4

Lectures

• Tuesday morning

Practicum

• on Tuesday or Thursday afternoon

• Exercises + Graded Assignments

Exams


Exams
course organization

Exam 1

• Monday 11 April, 2011, 9:00-12:00

• E1: about material of Q3 (Scala)

Exam 2

• Wednesday, 22 June, 2011, 9:00-12:00

• If Exam 1 >= 6

• then E2a: material of Q4 (C+JavaScript)

• else E2b: material Q3+Q4 (Scala+C+JavaScript)


Exams
course organization

Exam 3 (resit)

• If Exam 2 < 6, even if Exam 1 >= 6

• Wednesday, 24 August, 2011, 9:00-12:00

• E3: material Q3+Q4 (Scala+C+JavaScript)

Note

• next year programming exams


Practicum
course organization

Scala (Q3)

• Exercises: hand in on paper to assistants at end of lab!

• Assignment 1 (graded)

C (Q4)


JavaScript (Q4)


Graded assignments should be submitted as code


Final Grade
course organization
val H = all exercises handed in

val A = 0.4*A1 + 0.3*A2 + 0.3*A3

val E2 = if(E1 >= 6) (E1 + E2a)/2

else E2b

val E = if(E2 >= 6) E2 else E3

val G = if(H && Ai >= 6 && E >= 6)

0.4*A + 0.6E

else min(E, 5)


Practicum Groups
course organization

Tuesday: Group 1

• mentor groups 1 to 6

Thursday: Group 2

• mentor groups 7 to 12


Practicum Assistants
course organization

Tuesday Thursday

Maartje de Jonge Sander van der Burg

Laurent Verweijen Bart van Vuuren

Vlad Vergu Peter Pul

Joost Heijkoop


coffee break


IV
The Scala Programming Language

Term Rewriting 37

Programming Language Schools
imperative vs functional programming

Imperative Programming (Algol, C, Pascal)

• programming with side effects

• structured control-flow

Object-Oriented Programming (C++, Java)

• imperative programming with data abstraction

Functional Programming (ML, Haskell)

• functions as first-class citizens

• immutable data (pure)


Pizza
history

POPL 1997 Philip Wadler, Martin Odersky, Gilad Bracha, Dave Stoutamire (source: http://lamp.epﬂ.ch/pizza/gj/)

• "Pizza into Java: Translating theory into practice", Martin Odersky and
Philip Wadler. Proc. 24th ACM Symposium on Principles of Programming
Languages, Paris, France, January 1997.

Extending Java with Functional Concepts

• Parametric polymorphism (generics)

• First-class functions (function pointers)

• Algebraic data types (class cases and pattern matching)

Continued in GJ and Java Generics


Scala
history

A Scalable Language

• object-oriented + functional programming

Properties

• compatible (combines with Java)

• concise (sugar + abstractions)

• high-level

• statically typed

Designed by Martin Odersky et al. at EPFL


Scala in Industry
history

Scala at Twitter

• Ruby-on-Rails did not scale

• Replaced back-end processing (API) with Scala

BankSimple

• Back-end services in Scala or Clojure

Alex Payne

• Programming Scala by Wampler & Payne


Programming in Scala
Book

A comprehensive step-by-step guide

by Martin Odersky, Lex Spoon, and Bill Venners

http://www.artima.com/shop/programming_in_scala

Available as e-book (PDF)

Lectures are based on book; not everything in book is on slides; reading
book is required; see Literature slides at end of each lecture for required
reading!


Installing Scala

Download Interpreter & Compiler

• Make your own installation (assignment 1)

• http://www.scala-lang.org/downloads

Editors

• Emacs, Vi, <your favorite editor>

Integrated Development Environments

• Plugins for Eclipse, IntelliJ, NetBeans

B:2.1 TI1220 - Introduction 43

The Scala Interpreter

$ scala
Welcome to Scala version 2.8.0.final (Java HotSpot(TM)
64-Bit Server VM, Java 1.6.0_22).
Type in expressions to have them evaluated.
Type :help for more information..


V
Variables and Functions

B:2 Term Rewriting 45

Evaluating Expressions
repl: read, eval, print loop

scala> 1 + 2
res0: Int = 3

scala> res0 * 3
res1: Int = 9

scala> println("Hello, world!")
Hello, world!


Variable Declaration and Use
binding values to names

scala> val msg = "Hello, world!"
msg: java.lang.String = Hello, world!

scala> val msg2: java.lang.String =
| "Hello again, world!"
msg2: java.lang.String = Hello again, world!

scala> val msg3: String = "Hello yet again, world!"
msg3: String = Hello yet again, world!

scala> println(msg)
Hello, world!


Immutable and Mutable Variables
changing value bound to name

scala> msg = "Goodbye cruel world!"
<console>:6: error: reassignment to val
msg = "Goodbye cruel world!"

scala> var greeting = "Hello, world!"
greeting: java.lang.String = Hello, world!

scala> greeting = "Leave me alone, world!"
greeting: java.lang.String = Leave me alone, world!


Function Definition and Call
binding functions to names

scala> def max(x: Int, y: Int): Int = {
if (x > y) x
else y
}
max: (x: Int,y: Int)Int

scala> max(3, 5)
res2: Int = 5

scala> def max2(x: Int, y: Int) = if (x > y) x else y
max: (x: Int,y: Int)Int


Unit
the type of side effects

scala> def greet() = println("Hello, world!")
greet: ()Unit

scala> greet()
Hello, world!


Scala Scripts

//file: hello.scala
println("Hello, world, from a script!")

$ scala hello.scala
Hello, world, from a script!


Scala Scripts
with arguments

//file: helloarg.scala
// Say hello to the first argument
println("Hello, "+ args(0) +"!")

$ scala helloarg.scala delft
Hello, delft!


Control-Flow
imperative style
// file: printargs.scala
var i = 0
while (i < args.length) {
if (i != 0)
print(" ")
print(args(i))
i += 1
}
println()

$ scala printargs.scala imperative style
imperative style


Control-Flow
functional style

// file: printargsfun.scala
args.foreach(arg => println(arg))

$ scala printargsfun.scala first class functions
first
class
functions


Control-Flow
functional style

for (arg <- args)
println(arg)


Functional Style

Functional style

• functions are values

• operations map input values to output values

• instead of changing values in place (side effects)

A balanced attitude

• Prefer vals, immutable objects, and methods without side effects.

• Use vars, mutable objects, and methods with side effects when you have
a specific need and justification for them.


$ scala linecounts.scala linecounts.scala
22 | import scala.io.Source
0 |
55 | def widthOfLength(s: String) = s.length.toString.length
0 |
22 | if (args.length > 0) {
54 | val lines = Source.fromFile(args(0)).getLines.toList
37 | val longestLine = lines.reduceLeft(
49 | (a, b) => if (a.length > b.length) a else b
3 | )
43 | val maxWidth = widthOfLength(longestLine)
23 | for (line <- lines) {
50 | val numSpaces = maxWidth - widthOfLength(line)
33 | val padding = " " * numSpaces
47 | println(padding + line.length +" | "+ line)
3 | }
1 | }
4 | else
46 | Console.err.println("Please enter filename")


Print Lines
charcounts

import scala.io.Source
if (args.length > 0) {
for (line <- Source.fromFile(args(0)).getLines)
println(line.length + " | " + line)
} else
Console.err.println("Please enter filename")

$ scala countchars1.scala countchars1.scala
22 | import scala.io.Source
22 | if (args.length > 0) {
49 | for (line <- Source.fromFile(args(0)).getLines)
37 | println(line.length + " " + line)
6 | } else
46 | Console.err.println("Please enter filename")


Width of Longest Line
charcounts

def widthOfLength(s: String) = s.length.toString.length

val lines = Source.fromFile(args(0)).getLines.toList

val longestLine = lines.reduceLeft(
(a, b) => if (a.length > b.length) a else b
)

val maxWidth = widthOfLength(longestLine)


Print Lines with Length
charcounts

for (line <- lines) {
val numSpaces = maxWidth - widthOfLength(line)

val padding = " " * numSpaces

println(padding + line.length +" | "+ line)
}


import scala.io.Source

def widthOfLength(s: String) = s.length.toString.length

if (args.length > 0) {
val lines = Source.fromFile(args(0)).getLines.toList
val longestLine = lines.reduceLeft(
(a, b) => if (a.length > b.length) a else b
)
val maxWidth = widthOfLength(longestLine)
for (line <- lines) {
val numSpaces = maxWidth - widthOfLength(line)
val padding = " " * numSpaces
println(padding + line.length +" | "+ line)
}
}
else
Console.err.println("Please enter filename")


VI
Classes and Objects

B:2.3 Term Rewriting 62

Class: Object Blueprint
declaration and instantiation

class ChecksumAccumulator {}

val acc = new ChecksumAccumulator
val csa = new ChecksumAccumulator


Fields (Instance Variables)
object state

class ChecksumAccumulator {
var sum = 0
}


acc.sum = 3


Reference vs Content
object state

var sum = 0
}


acc.sum = 3

// Won’t compile, because acc is a val
acc = new ChecksumAccumulator


Data Hiding
object state

private var sum = 0
}

acc.sum = 5 // Won’t compile, because sum is private


Methods
object behavior

private var sum = 0
def add(b: Byte): Unit = {
sum += b
}
def checksum(): Int = {
return ~(sum & 0xFF) + 1
}
}


Methods
parameters are vals

def add(b: Byte): Unit = {
b = 1 // This won’t compile, because b is a val
sum += b
}


Methods
sugar

// file: ChecksumAccumulator.scala
private var sum = 0
def add(b: Byte) { sum += b }
def checksum(): Int = ~(sum & 0xFF) + 1
}


Singleton Objects
defining ‘static’ methods
import scala.collection.mutable.Map
object ChecksumAccumulator {
private val cache = Map[String, Int]()
def calculate(s: String): Int =
if (cache.contains(s))
cache(s)
else {
for (c <- s)
acc.add(c.toByte)
val cs = acc.checksum()
cache += (s -> cs)
cs
}
}


Using Singleton Objects
methods and state

ChecksumAccumulator.calculate("Every value is an object.")


Scala Application

// file: Summer.scala
import ChecksumAccumulator.calculate
object Summer {
def main(args: Array[String]) {
for (arg <- args)
println(arg + ": " + calculate(arg))
}
}

$ scalac ChecksumAccumulator.scala Summer.scala
$ scala Summer of love
of: -213
love: -182


VII
Summary

Term Rewriting 73

Summary
lessons learned

Programming languages are for people

• to reason about solutions for problems

Declaration and use

• binding values to names, referring to values using
names

Immutable values

• cannot be changed once created (no side effects)

Objects

• data structures with methods


Exam Question

What happens when we execute the following code:

val greetStrings = new Array[String](3)
greetStrings(0) = "Hello"
greetStrings(1) = ", "
greetStrings(2) = "world!n"

(a) Error: greetStrings is an immutable variable that cannot
be assigned to
(b) Error: Array is a immutable data structure that cannot
be assigned to
(c) No error: greetStrings is a mutable variable that can be
assigned to
(d) No error: Array is a mutable data structure that can be
assigned to


Exercises Week 1
getting started with Scala

Set up your Scala environment

• download and install Scala interpreter and compiler

Scala basics

• evaluating expressions

• defining functions

• running scripts

• making applications

• hand in on paper to student assistants!


Literature
required reading


• Chapter 1: A Scalable Language

• Chapter 2: First Steps in Scala

• Chapter 3: Next Steps in Scala

• Chapter 4: Classes and Objects

• Chapter 5: Basic Types and Operations


Literature
http://researchr.org/bibliography/ti1220


Outlook
coming next

Functional programming

• Lecture 2: Functional Objects
• Chapters 6, 7

• Lecture 3: Functions & Closures
• Chapters 8, 9

• Lecture 4: List Programming
• Chapters 15, 16, 17

Lab Week 1

• Getting started with Scala


IX
Notes

Term Rewriting 80

Pictures
copyrights
Slide 1:
Kinetica by Paul Downey, Some rights reserved
Slide 2:
McPhillips’ Map of the City of Winnipeg by Manitoba Historical Maps, some rights reserved
Slide 4:
The Blue Hour by Andreas Wonisch, Some rights reserved
Slide 5:
by Eelco Visser, Some rights reserved
Slide 8:
I’m listening by Melvin Gaal, some rights reserved
Slide 10:
Bombe detail by Garret Coakley, Some rights reserved
Slide 13:
Alan Turing by oalfonso, some rights reserved
Slide 14:
History of Programming Languages by O’Reilly
Slide:
ABZ-DNA by dulhunk, some rights reserved
Slide 32, 34:
Envelopes by benchilada, some rights reserved
Slide 33:
Report card by Carosaurus, some rights reserved
Slide 36:
paper coffee cup by Shutterstock, some rights reserved


Pictures
copyrights
Slide 40:
Dinner at Roussillon (Martin Odersky) by Miles Sabin, some rights reserved
Slide 41:
Portrait: Alex Payne by Dave Fayram, some rights reserved
Slide 43:
“Plumbing Nightmare” by Natalie Wilkie, some rights reserved
Slide 72:
HIV: The Moleskine Summary by Niels Olson
Slide 75, 76:
remember to thank all the books you haven’t read over the past three years
by Natalia Osiatynska, Some rights reserved
Slide 7, 73:
Stupid Exam by Remi Carreiro, Some rights reserved
Slide 74:
Practice makes perfect by Simon Probert
Slide 10:
Bombe detail by Garret Coakley, Some rights reserved
Slide 29:
1.12.2011 <recipes> 362/365 by Phil Roeder, Some rights reserved
Slide 22,23:
China Tibet Himalaya by Bernhard Goldbach, Some rights reserved
Slide 17:
Thinking at Hell’s gate by innoxiuss, Some rights reserved


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