invitation to scala
DESCRIPTION
These are the slides for the talk I gave in Harvard IQSS's "tech talk" series. The commands and files for the demo ("REPL") part of the lecture can be obtained here: http://www.mbarsinai.com/blog/2013/08/04/invitation-to-scala/.TRANSCRIPT
Introducing ScalaIQSS Tech-Talk, 2013-06-27Michael Bar-Sinai
Sunday, 4 August, 13
Based in part on Scala for the impatient, Scala for java programmers tutorial at Typesafe.com, and Prof. Mayer Goldberg advanced programming class in Ben-Gurion university of the Negev, Israel
Sunday, 4 August, 13
In a Few BulletsDeveloped in 2001, by Martin Odersky, EPFL Professor
Runs on the JVM (also, CLR)
Statically typed and with concise syntax
Object oriented and functional
Clean and elegant design
Strong collection framework
Expression-only, DSL friendly, advanced features
Sunday, 4 August, 13
In a Few BulletsDeveloped in 2001, by Martin Odersky, EPFL Professor
Runs on the JVM (also, CLR)
Statically typed and with concise syntax
Object oriented and functional
Clean and elegant design
Strong collection framework
Expression-only, DSL friendly, advanced features
Sunday, 4 August, 13
In a Few BulletsDeveloped in 2001, by Martin Odersky, EPFL Professor
Runs on the JVM (also, CLR)
Statically typed and with concise syntax
Object oriented and functional
Clean and elegant design
Strong collection framework
Expression-only, DSL friendly, advanced features
Sunday, 4 August, 13
Sunday, 4 August, 13
What is a...
Computation?
Sunday, 4 August, 13
What is a computation?PROCEDURAL
Finite automaton working on an infinite tape
...0 1 110101110
Sunday, 4 August, 13
What is a computation?PROCEDURAL FUNCTIONAL
Finite automaton working on an infinite tape
...0 1 110101110
Data flowing through a program
f(x) /2 Rf(6) 2 R
Sunday, 4 August, 13
REPLSEE ACCOMPANYING FILES
Sunday, 4 August, 13
List OperationsRun some binary operator on the list items and an intermediate results
fold, reduce, scan
Concurrent: aggregate,reduce
\ndlrow olleH
Sunday, 4 August, 13
List OperationsRun some binary operator on the list items and an intermediate results
fold, reduce, scan
Concurrent: aggregate,reduce
\ndlrow olleH
Sunday, 4 August, 13
Expressions vs. StatementsSTATEMENT
if ( map contains x ) { map(x) = computeNewX(map(x))} else { map(x) = computeNewX( 0 )}
Sunday, 4 August, 13
Expressions vs. StatementsSTATEMENT
EXPRESSION
if ( map contains x ) { map(x) = computeNewX(map(x))} else { map(x) = computeNewX( 0 )}
map(x) = if ( map contains x ) computeNewX( map(x) ) else computeNewX( 0 )
Sunday, 4 August, 13
Expressions vs. StatementsSTATEMENT
EXPRESSION
if ( map contains x ) { map(x) = computeNewX(map(x))} else { map(x) = computeNewX( 0 )}
map(x) = if ( map contains x ) computeNewX( map(x) ) else computeNewX( 0 )
ACTUAL SCALA CODEmap(x) = computeNewX( map.getOrElse(x,0) )
Sunday, 4 August, 13
Class Syntax
Classes have a primary constructor that is part of the class definition
Clients can’t distinguish between getters and direct field access
Better privacy control
Sunday, 4 August, 13
Classesclass Person( var name:String, val id:Int ) {! def greet = "Hello, my name is %s".format(name)}
class Person2( name:String, val id:Int ) {! def greet = "Hello, my name is %s".format(name)}
class Person3( name:String, val id:Int ) {! val greet = "Hello, my name is %s".format(name)}
Sunday, 4 August, 13
Classes (cont.)
class Person4( aName:String, anId:Int ) {! private val id = anId! private[this] var pName = aName
! def name = pName! def name_=( newName:String ) { pName = newName }
! override def toString = "[Person4 id:%d name:%s]".format(id,name)}
Sunday, 4 August, 13
Multiple InheritanceWould have been nice if it worked
It doesn’t
Java allowed only multipleinheritance of interfacesJDK8 would update this, slightly
Scala simulates multiple inheritance using type linearization
Would have been nice if it worked...it mostly does
A
B1 B2
C
A
B1 B2
C
Sunday, 4 August, 13
TraitsAlmost like class:
Can have fields, protocols and behavior (implementations)
Can’t have constructor parameters
Can require minimal interface from implementing classes
Class can extend as many as needed
Types are generated at declaration point
Sunday, 4 August, 13
Objectsand the absence of static
Replace the static parts in java
Manual declaration of a runtime singletons
Classes can have “companion objects” that have the same name
Good place for utility methods or special “apply” methods
App trait allows script-like behavior
Sunday, 4 August, 13
Pattern Matching
Control structure that allows
switching
type inquiry
variable de-composition
Specialized Classes optimized for this
Sunday, 4 August, 13
Simple Pattern Matching
def toFuzzyStringInt( i:Int ) = i match {! case 0 => "Nada"! case 1 => "One"! case 2 => "A Pair"! case 12 => "a dozen"! case _ if i<0 => "Less that zero"! case _ => "%,d".format(i)}
Sunday, 4 August, 13
Switch by Type
def prettyPrint( a:Any ) = a match {! case i:Int => "%,d".format(i)! case s:String => "[%s]".format(s)! case sym:Symbol => ":%s".format(sym)! case _ => a.toString}
Sunday, 4 August, 13
DecompositionFirst, meet the case class:
Regular class, but with immutable declared fields, toString, equals and hashCode automatically defined
sealed abstract class Treecase class Sum( l:Tree, r:Tree ) extends Treecase class Var( n:String ) extends Treecase class Con( v:Int ) extends Tree
Sunday, 4 August, 13
Decomposition
def evalTree( t:Tree, e:Map[String,Int] ): Int = t match {! case Sum(l,r) => evalTree(l, e) + evalTree(r,e)! case Var( n ) => e(n)! case Con( i ) => i}
Allows downcasting, accessing sub-classes fields and varying actions based on the class of the parameter, in a
single syntactical maneuver
Sunday, 4 August, 13
... In the real world
(merged,original) match {case ( Pass(_), ( _, _) ) => truecase ( Unfixable(_), ( _, _) ) => falsecase ( Fixable(_,t,_), (t1,t2) ) => f.ld<ld(t1)+ld(t2)
}
Given two strings, we need to decide whether it is more likely that they are two separate words or one broken word
Sunday, 4 August, 13
... In the real world
(merged,original) match {case ( Pass(_), ( _, _) ) => truecase ( Unfixable(_), ( _, _) ) => falsecase ( Fixable(_,t,_), (t1,t2) ) => f.ld<ld(t1)+ld(t2)
}
ere are two ways of constructing a software design: One way is to make it so simple that there are obviously no de"ciencies, and the other way is to make it so complicated that there are no obvious de"ciencies. e "rst method is far more difficult.
-- C. A. R. Hoare, Turing Award lecture, 1980
Given two strings, we need to decide whether it is more likely that they are two separate words or one broken word
Sunday, 4 August, 13
Option[T]And the death of the NullPointerException
Indicates possibly missing values
Has two implementations: None and Some(t)
“Collection of at most one item”
Convention more than a language feature
Sunday, 4 August, 13
Functions as valuesCreating new functions from existing ones
Powerful tool, but can get messy
Allows for DSL creation
def bind1( f:(Int,Int)=>Int, v:Int ) =
(a:Int)=>f(a,v)
Sunday, 4 August, 13
Not CoveredCreation of DSLs
Implicit conversions
Continuations
Frameworks
XML
Parsers/Combinators
Macros
Annotations
Genericity
Type System
Actors
Regular Expressions
Extractors
... many more
Sunday, 4 August, 13
Pointers
www.scala-lang.org
Typesafe.com
Scala for the Impatient, by Cay Horstmann (http://horstmann.com/scala/)First chapters available after registering with at Typesafe’s site
(google)
Sunday, 4 August, 13
Introducing ScalaIQSS Tech-Talk, 2013-06-27Michael Bar-Sinai
THANKS
Sunday, 4 August, 13