OOP Functional Objects

Functional Objects

Eelco Visser

Delft
http://twitter.com/TI1220
University of
Technology Concepts of Programming Languages
Challenge the future

Recap
getting started with Scala

Object-Oriented Programming in Scala

• Expressions
• Immutable and mutable variables (val, var)
• Function definitions
• Classes and objects
• Class members
• instance variables
• methods

• Singleton objects

B:6.1 TI 1220 - Lecture 2: Functional Objects 2

Overview
today’s lecture

Names

• Binding, name spaces

Functional objects

• Objects without mutable state

• Defining operators

Built-in Control Structures

• If, While, For, Exceptions

• Variable scope


I
Concept: Name Binding


Binding Variable Identifiers
name binding

binding
defining occurrence

val msg = "Hello, " + "world!"

println(msg)

applied occurrence

val variables cannot be rebound


Binding Variable Identifiers
name binding

binding
defining occurrence

var greeting = "Hello, world!"

greeting = "Leave me alone, world!"

rebinding
var variables can be rebound


Binding Function Identifiers
name binding

binding
defining occurrence

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

val maxWidth = widthOfLength(longestLine)

applied occurrence


Binding Class Identifiers
name binding
binding
defining occurrence

class ChecksumAccumulator {
var sum = 0
}

var acc = new ChecksumAccumulator
var csa = new ChecksumAccumulator

acc.sum = 3
csa = acc applied occurrence

TI 1220 - Lecture 2: Functional Objects 8

Rebinding vs Mutation
name binding

class ChecksumAccumulator {
var sum = 0
} binding
var acc = new ChecksumAccumulator
var csa = new ChecksumAccumulator
mutation
acc.sum = 3
csa = acc

rebinding


Name Spaces
name binding

object foo {
val foo : Int = 0
def foo(x : Int) = x + 1
}

object bar {
def bar() = foo.foo(foo.foo)
}

variables, functions, objects are in separate name spaces


Substitution
name binding

val msg = "Hello, " + "world!"

println(msg)

?

println("Hello, " + "world!")

can we replace applied occurrence with bound expression/value?


II
Functional Objects


Rational Numbers
mathematical laws

• Rational = Int x Int

• Notation: numerator/denominator

• Addition

• example: 1/2 + 2/3 = 3/6 + 4/6 = (3 + 4)/6 = 7/6

• general: n1/d1 + n2/d2 = (n1*d2 + n2*d1) / (d1*d2)

• Multiplication

• n1/d1 + n2/d2 = (n1 * n2) / (d1 * d2)

• Division

• n1/d1 / n2/d2 = n1/d2 * d2/n2


Constructing a Rational
class parameters

class Rational(n: Int, d: Int) {
println("Created " + n + "/" + d)
}

scala> new Rational(1, 2)
Created 1/2
res0: Rational = Rational@2d83e895


Immutable Object Trade-offs

Advantages

• easier reasoning

• pass around freely (no risk of undesired mutations)

• cannot be changed concurrently in two threads

• immutable object make safe hashtable keys

Disadvantages

• copying large object graphs vs in-place update


Reimplementing toString
overriding methods

override def toString = n + "/" + d
}

scala> val half = new Rational(1, 2)
half: Rational = 1/2


Checking Preconditions

require(d != 0)
}

scala> val half = new Rational(1, 0)
java.lang.IllegalArgumentException: requirement failed


Visibility of Class Parameters

require(d != 0)
def add(that: Rational): Rational =
new Rational(n * that.d + that.n * d, d * that.d)
}

$ fsc Rational.scala
Rational.scala:5: error: value d is not a member of Rational
^
Rational.scala:5: error: value d is not a member of Rational
^
two errors found


Adding Fields

require(d != 0)
val numer: Int = n
val denom: Int = d
override def toString = numer + "/" + denom
new Rational(
numer * that.denom + that.numer * denom,
denom * that.denom)
}

scala> new Rational(1,2) add new Rational(2,3)
res0: Rational = 7/6


Non-Functional Objects
destructive update
class ImperativeRational(n: Int, d: Int) {
require(d != 0)
var numer: Int = n
var denom: Int = d
def add(that: ImperativeRational) {
numer = numer * that.denom + that.numer * denom;
denom = denom * that.denom;
}
} scala> val half = new ImperativeRational(1, 2)
half: ImperativeRational = 1/2

scala> val twothirds = new ImperativeRational(2,3)
twothirds: ImperativeRational = 2/3

scala> half.add(twothirds)

scala> half
res1: ImperativeRational = 7/6


Self References
this

def lessThan(that: Rational) =
this.numer * that.denom < that.numer * this.denom

def max(that: Rational) =
if (this.lessThan(that)) that else this


Auxiliary Constructors

require(d != 0)
val numer = n
val denom = d
def this(n: Int) = this(n, 1) // auxiliary constructor
...
}

scala> new Rational(6)


Private Fields and Methods

require(d != 0)
private val g = gcd(n.abs, d.abs)
val numer = n / g
val denom = d / g
...
private def gcd(a: Int, b: Int): Int =
if (b == 0) a else gcd(b, a % b)
}

scala> new Rational(6,42)


Defining Operators
use functions as infix operators

new Rational(numer * that.denom + that.numer * denom,
denom * that.denom)

scala> new Rational(1,2).add(new Rational(2,3))
scala> new Rational(1,2) add new Rational(2,3)


Invoking Operators
operator call is method call


Defining Operators
operator identifiers

def +(that: Rational): Rational =
new Rational(numer * that.denom + that.numer * denom,
denom * that.denom)

def *(that: Rational): Rational =
new Rational(numer * that.numer, denom * that.denom)

scala> val d = a + b * c
d: Rational = 11/14
scala> val d = a.+(b.*(c))
d: Rational = 11/14
scala> val d = a * b + c
d: Rational = 16/21
scala> val d = (a.*(b)).+(c)
d: Rational = 16/21


Identifiers
lexical syntax

Alphanumeric identifier

• identifier: [$A-Za-z_][$A-Za-z_0-9]* ($ reserved for Scala compiler)

• camel-case convention: toString, HashSet

Operator identifier

• Unicode set of mathematical symbols(Sm) or other symbols(So), or to
the 7-bit ASCII characters that are not letters, digits, parentheses,
square brackets, curly braces, single or double quote, or an underscore,
period,semi-colon, comma, or back tick character.

Literal Identifier

• arbitrary string enclosed in back ticks (` . . . `).


Method Overloading
for different argument types


def *(i: Int): Rational =
new Rational(numer * i, denom)

scala> val c = new Rational(3,7)
c: Rational = 3/7

scala> c * 2


Method Overloading
does not apply to this



scala> 2 * c
<console>:7: error: overloaded method value * with alternatives: (Double)
Double <and> (Float)Float <and> (Long)Long <and> (Int)Int <and> (Char)Int
<and> (Short)Int <and> (Byte)Int cannot be applied to (Rational)
2 * c
^


Implicit Conversions



implicit def intToRational(x: Int) = new Rational(x)

scala> 2 * c


Summary
functional objects

Immutable objects

• class parameters

• immutable fields (val)

• methods don’t change object, but return value

Natural, concise notation

• methods as infix operators, operator identifiers

• method overloading

• implicit conversion


Exam Question
How many Rational objects are created while executing:

require(d != 0)
val numer: Int = n
val denom: Int = d
def +(that: Rational): Rational =
new Rational(
numer * that.denom + that.numer * denom,
denom * that.denom)
}
var half = new Rational(1,2)
half = half + half + half

(a) 1
(b) 2
(c) 3
(d) 4

TI1220 - Introduction 32

coffee break


II
Built-in Control Structures

B:6 TI 1220 - Lecture 2: Functional Objects 34

Control-Flow Structures

Control-Flow Structures

• ordering execution

• choice, iteration, exception

Functional Control Structures

• can be used as expression

• return a value

Minimal set of built-in control structures

• control abstraction using function literals (next week)


If Expressions
conditional choice

var filename = "default.txt"
if (!args.isEmpty) imperative style
filename = args(0)

val filename =
if (!args.isEmpty) args(0) functional style
else "default.txt"


Equational Reasoning
replace equals by equals

val filename =
if (!args.isEmpty) args(0)
else "default.txt"
println(filename)

println(if (!args.isEmpty) args(0) else "default.txt")


Equational Reasoning
replace equals by equals

val x = e; f(x) <=> f(e)

if e has no side effects


While Loops
iteration

def gcdLoop(x: Long, y: Long): Long = {
var a = x
var b = y
while (a != 0) {
val temp = a
a = b % a
b = temp
}
b
}


Do-While Loops
iteration

var line = ""
do {
line = readLine()
println("Read: " + line)
} while (line != "")


Assignment has Type Unit

var line = ""
while ((line = readLine()) != "") // This doesn’t work!
println("Read: " + line)


Iteration vs Recursion

def gcd(x: Long, y: Long): Long =
if (y == 0) x else gcd(y, x % y)

def gcdLoop(x: Long, y: Long): Long = {
var a = x
var b = y
while (a != 0) {
val temp = a
a = b % a
b = temp
}
b
}


For Expressions
functional iteration

val filesHere = (new java.io.File(".")).listFiles
for (file <- filesHere)
println(file)


Ranges
iterating over sequence of numbers

scala> for (i <- 1 to 4)
| println("Iteration " + i)
Iteration 1
Iteration 2
Iteration 3
Iteration 4

// Not common in Scala...
for (i <- 0 to filesHere.length - 1)
println(filesHere(i))


Filtering
iterating over subset of a collection

for (file <- filesHere)
if (file.getName.endsWith(".scala"))
println(file)

for (file <- filesHere if file.getName.endsWith(".scala"))
println(file)


Multiple Filters

for (
file <- filesHere if file.isFile;
if file.getName.endsWith(".scala")
) println(file)


Nested Iteration

def fileLines(file: java.io.File) =
scala.io.Source.fromFile(file).getLines.toList

def grep(pattern: String) =
for (
file <- filesHere if file.getName.endsWith(".scala");
line <- fileLines(file) if line.trim.matches(pattern)
) println(file + ": " + line.trim)

grep(".*gcd.*")


Mid-Stream Variable Binding

def grep(pattern: String) =
for {
file <- filesHere
line <- fileLines(file)
trimmed = line.trim
if trimmed.matches(pattern)
} println(file + ": " + trimmed)
grep(".*gcd.*")


Producing a New Collection
yield

def scalaFiles =
for {
file <- filesHere
} yield file


Composing filters

val forLineLengths =
for {
file <- filesHere
line <- fileLines(file)
trimmed = line.trim
if trimmed.matches(".*for.*")
} yield trimmed.length


Throwing Exceptions
exception handling

val half =
if (n % 2 == 0)
n / 2
else
throw new RuntimeException("n must be even")


Catching Exceptions
exception handling

import java.io.FileReader
import java.io.FileNotFoundException
import java.io.IOException
try {
val f = new FileReader("input.txt")
// Use and close file
} catch {
case ex: FileNotFoundException => // Handle missing file
case ex: IOException => // Handle other I/O error
}


Finally
exception handling

import java.io.FileReader
val file = new FileReader("input.txt")
try {
// Use the file
} finally {
file.close() // Be sure to close the file
}


Yielding a Value
exception handling

import java.net.URL
import java.net.MalformedURLException
def urlFor(path: String) =
try {
new URL(path)
} catch {
case e: MalformedURLException =>
new URL("http://www.scalalang.org")
}


Match Expressions
choosing between actions

val firstArg = if (args.length > 0) args(0) else ""
firstArg match {
case "salt" => println("pepper")
case "chips" => println("salsa")
case "eggs" => println("bacon")
case _ => println("huh?")
}


Match Expressions
choosing between values

val firstArg = if (!args.isEmpty) args(0) else ""
val friend =
firstArg match {
case "salt" => "pepper"
case "chips" => "salsa"
case "eggs" => "bacon"
case _ => "huh?"
}
println(friend)


Break and Continue in Java
not in Scala

int i = 0; // This is Java
boolean foundIt = false;
while (i < args.length) {
if (args[i].startsWith("")) {
i = i + 1;
continue;
}
if (args[i].endsWith(".scala")) {
foundIt = true;
break;
}
i = i + 1;
}


Replace Break/Continue with If ...

var i = 0
var foundIt = false
while (i < args.length && !foundIt) {
if (!args(i).startsWith("")) {
if (args(i).endsWith(".scala"))
foundIt = true
}
i = i + 1
}


... or with Recursion

def searchFrom(i: Int): Int =
if (i >= args.length) 1
else if (args(i).startsWith("")) searchFrom(i + 1)
else if (args(i).endsWith(".scala")) i
else searchFrom(i + 1)
val i = searchFrom(0)


Variable Scope
almost identical to Java

Variables are declared in a scope

• { var j = 1; ... }

Inner scopes shadow variables in outer scopes

• { var j = 1; { var j = 2; ... } }


Multiplication Table

$ scala printmultitable.scala
1 2 3 4 5 6 7 8 9 10
2 4 6 8 10 12 14 16 18 20
3 6 9 12 15 18 21 24 27 30
4 8 12 16 20 24 28 32 36 40
5 10 15 20 25 30 35 40 45 50
6 12 18 24 30 36 42 48 54 60
7 14 21 28 35 42 49 56 63 70
8 16 24 32 40 48 56 64 72 80
9 18 27 36 45 54 63 72 81 90
10 20 30 40 50 60 70 80 90 100


def printMultiTable() {
var i = 1
// only i in scope here
while (i <= 10) {
var j = 1
// both i and j in scope here
while (j <= 10) {
val prod = (i * j).toString
// i, j, and prod in scope here
var k = prod.length
// i, j, prod, and k in scope here
while (k < 4) {
print(" ")
k += 1
}
print(prod)
j += 1
}
// i and j still in scope; prod and k out of scope
println()
i += 1
}
// i still in scope; j, prod, and k out of scope
}


Refactoring Imperative-Style Code
// Returns a row as a sequence
def makeRowSeq(row: Int) =
for (col <- 1 to 10) yield {
val prod = (row * col).toString
val padding = " " * (4 - prod.length)
padding + prod
}

// Returns a row as a string
def makeRow(row: Int) = makeRowSeq(row).mkString

// Returns table as a string with one row per line
def multiTable() = {
val tableSeq = // a sequence of row strings
for (row <- 1 to 10)
yield makeRow(row)
tableSeq.mkString("n")
}


VII
Summary

Term Rewriting 64

Summary
lessons learned

Functional Objects

• immutable objects

• operations create new objects

Functional Control Structures

• return a value

• have a type

• can be used as expressions


Literature
required reading

Programming in Scala

• Chapter 6: Functional Objects

• Chapter 7: Built-in Control Structures


Exercises Week 2
complex numbers

Scala Test

• using unit testing framework to define executable tests

Complex numbers

• define class to represent complex numbers

• define tests

Note: all assignments should be done individually!


http://www.scala-lang.org/api/current/index.html


Scala Style Guide
formatting rules

indentation: 2 spaces

http://www.codecommit.com/scala-style-guide.pdf


Outlook
coming next

Lecture 3: Functions & Closures
• Chapters 8, 9
Lecture 4: List Programming
• Chapters 15, 16, 17
Lecture 5: Trees
• Chapters 26, 22, 23

Lab Week 2

• Complex numbers in Scala


Exam Question (Week 1)

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



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



What is the return type of makeRowSeq:

def makeRowSeq(row: Int) =
for (col <- 1 to 10) yield {
val prod = (row * col).toString
val padding = " " * (4 - prod.length)
padding + prod
}

(a) String
(b) Int
(c) IndexedSeq[String]
(d) IndexedSeq[Int]


Pictures
copyrights
Slide 1:
A Plumpish Proportion by SSG Robert Stewart some rights reserved
Slide 3:
McPhillips’ Map of the City of Winnipeg by Manitoba Historical Maps, some rights reserved

Slide 19:
Envelopes by benchilada, some rights reserved
Slide 20:
Report card by Carosaurus, some rights reserved
Slide 30:
Sun is Shining by el patojo, some rights reserved


Pictures
copyrights
Slide:
Dinner at Roussillon (Martin Odersky) by Miles Sabin, some rights reserved
Slide:
Portrait: Alex Payne by Dave Fayram, some rights reserved
Slide:
“Plumbing Nightmare” by Natalie Wilkie, some rights reserved
Slide:
HIV: The Moleskine Summary by Niels Olson
Slide:
remember to thank all the books you haven’t read over the past three years
by Natalia Osiatynska, Some rights reserved
Slide:
Stupid Exam by Remi Carreiro, Some rights reserved
Slide:
Practice makes perfect by Simon Probert

http://www.flickr.com/photos/garrettc/3747802654/ Bombe detail by Garret Coakley, Some rights
reserved


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