scala programming language - uni-bielefeld.degsauthof/docs/scala.pdf · scala programming language...

ScalaProgrammingLanguage

GeorgSauthoff

Introduction

Features

Conclusion

Scala Programming Language

Georg [email protected]

Universität BielefeldAG Praktische Informatik

June 1, 2010

mailto:[email protected]


GeorgSauthoff

Introduction

Features

Conclusion

Scala

Multi-paradigm general purpose programming language byMartin Oderskysince 2003

functionalobject orientedimperative

eager evaluation (lazy annotations available)type inference (in a limited fashion)rich operator overloadingfor the JVMintegrates well with Java modules

GeorgSauthoff

Introduction

Features

Conclusion

Example

1 o b j e c t Fac1 {2 de f f a c ( i : B i g I n t ) : B i g I n t = i match {3 case _ i f i == 1 => i4 case _ => i ∗ f a c ( i − 1)5 }6 de f main ( a r g s : Ar ray [ S t r i n g ] ) =7 p r i n t l n ( f a c ( I n t e g e r p a r s e I n t ( a r g s ( 0 ) ) ) )8 }

9 > module Main10 > where11 > import System ( getArgs )12

13 > f a c : : I n tege r −> I n tege r14 > f a c 1 = 115 > f a c i = i ∗ f a c ( i −1)16

17 > main = do18 > ( x :_) <− getArgs19 > putStrLn $ show $ f a c ( ( read x ) : : I n tege r )

GeorgSauthoff

Introduction

Features

Conclusion

Example

1 o b j e c t Fac1 {2 de f f a c ( i : B i g I n t ) : B i g I n t = i match {3 case _ i f i == 1 => i4 case _ => i ∗ f a c ( i − 1)5 }6 de f main ( a r g s : Ar ray [ S t r i n g ] ) =7 p r i n t l n ( f a c ( I n t e g e r p a r s e I n t ( a r g s ( 0 ) ) ) )8 }9 > module Main10 > where11 > import System ( getArgs )12

13 > f a c : : I n tege r −> I n tege r14 > f a c 1 = 115 > f a c i = i ∗ f a c ( i −1)16

17 > main = do18 > ( x :_) <− getArgs19 > putStrLn $ show $ f a c ( ( read x ) : : I n tege r )


GeorgSauthoff

Introduction

Features

Conclusion

Syntax

No offside rule;{}. in some contexts optionaltype declarations optional if type inference is possible(limited)

GeorgSauthoff

Introduction

Features

Conclusion

Lambda functions

1 o b j e c t Reduce {2 de f mult ( l : Ar ray [ I n t ] ) : I n t =3 l r e d u c e L e f t { _ ∗ _ }4 // l . r e d u c e L e f t ( ( i , j ) => i ∗ j )5

6 de f main ( a r g s : Ar ray [ S t r i n g ] ) =7 p r i n t l n ( mult ( a r g s map { I n t e g e r p a r s e I n t _ } ) )8 }

9 > module Main10 > where11

12 > import System ( getArgs )13

14 > mult = f o l d l (∗ ) 115

16 > main = do17 > l <− getArgs18 > putStrLn $ show $ mult $ map (\ x −> ( read x ) : : I n tege r ) l

GeorgSauthoff

Introduction

Features

Conclusion

Lambda functions

1 o b j e c t Reduce {2 de f mult ( l : Ar ray [ I n t ] ) : I n t =3 l r e d u c e L e f t { _ ∗ _ }4 // l . r e d u c e L e f t ( ( i , j ) => i ∗ j )5

6 de f main ( a r g s : Ar ray [ S t r i n g ] ) =7 p r i n t l n ( mult ( a r g s map { I n t e g e r p a r s e I n t _ } ) )8 }9 > module Main10 > where11

12 > import System ( getArgs )13

14 > mult = f o l d l (∗ ) 115

16 > main = do17 > l <− getArgs18 > putStrLn $ show $ mult $ map (\ x −> ( read x ) : : I n tege r ) l


GeorgSauthoff

Introduction

Features

Conclusion

Lambda functions

reduce, foldr, map etc. are all member functions ofcollections like array, list ...in comparison to Haskell or C++ STL not very orthogonalhowever, they are implemented only once in the traitIterable

GeorgSauthoff

Introduction

Features

Conclusion

Example

1 > module Main2 > where3 > import System4

5 > sep = f o l d r (\ b a −> i f b == ’ , ’ then [ ] : a6 e l s e ( b : ( head a ) ) : t a i l a ) [ [ ] ]7

8 > main = do9 > ( a : [ ] ) <− getArgs10 > putStrLn $ show $ sep a

11 i n t main ( i n t argc , char ∗∗ a rgv )12 {13 vec to r <s t r i n g > v ( 1 ) ;14 f o r e a c h ( char c , a rgv [ 1 ] )15 i f ( c == ’ , ’ ) v . r e s i z e ( v . s i z e ( )+1) ;16 e l s e v . back ( ) . push_back ( c ) ;17 f o r e a c h ( s t r i n g &s , v ) cout << s << ’ ’ ;18 cout << ’ \n ’ ; re tu rn 0 ;19 }

GeorgSauthoff

Introduction

Features

Conclusion

Example

1 > module Main2 > where3 > import System4

5 > sep = f o l d r (\ b a −> i f b == ’ , ’ then [ ] : a6 e l s e ( b : ( head a ) ) : t a i l a ) [ [ ] ]7

8 > main = do9 > ( a : [ ] ) <− getArgs10 > putStrLn $ show $ sep a11 i n t main ( i n t argc , char ∗∗ a rgv )12 {13 vec to r <s t r i n g > v ( 1 ) ;14 f o r e a c h ( char c , a rgv [ 1 ] )15 i f ( c == ’ , ’ ) v . r e s i z e ( v . s i z e ( )+1) ;16 e l s e v . back ( ) . push_back ( c ) ;17 f o r e a c h ( s t r i n g &s , v ) cout << s << ’ ’ ;18 cout << ’ \n ’ ; re tu rn 0 ;19 }


GeorgSauthoff

Introduction

Features

Conclusion

Example

1 o b j e c t Sep {2

3 // S i g n a t u r e ( Cur ry s yn tax ) :4 // f o l d R i g h t : (B) ( ( In t , B) => B)B5

6 de f sep ( s : S t r i n g ) : L i s t [ S t r i n g ] =7 s . f o l d R i g h t ( L i s t ( new S t r i n g ( " " ) ) )8 { (b , a ) => i f ( b == ’ , ’ ) ( new S t r i n g ) : : a9 e l s e ( b + ( a head ) ) : : ( a t a i l ) }10

11 de f main ( a r g s : Ar ray [ S t r i n g ] ) =12 p r i n t l n ( sep ( a r g s ( 0 ) ) )13

14 }


GeorgSauthoff

Introduction

Features

Conclusion

Compile times

Thinkpad X200, excluding pre-heating runs, in seconds

ghc 6.10.4 -O2 scalac 2.7.5

Fac1 0.6 2Sep 0.4 2.1


GeorgSauthoff

Introduction

Features

Conclusion

OOP

no multiple inheritancebut, traitsGenerics


GeorgSauthoff

Introduction

Features

Conclusion

Traits

don’t confuse with C++ traitslike interfaces, but may contain function bodiesno constructorsto implement unrelated/orthogonal aspectscan extend classes, traitsmay contain stateone class may ’implement’/’extend’ several traits . . .


GeorgSauthoff

Introduction

Features

Conclusion

Trait Example

1 t r a i t ChangeL i s t ene r { de f change }2 c l a s s Widget { }3 c l a s s Button extends Widget with ChangeL i s t ene r {4 de f change = p r i n t l n ( " changed ! " )5 }6 t r a i t ChangeSender {7 p r i v a t e va r l = L i s t [ ChangeL i s t ene r ] ( )8 de f addChangeL i s t ene r ( a : ChangeL i s t ene r ) : Un i t =9 l : := a10 de f emitChanges ( ) = l f o r e a c h ( f => f . change )11 }12 c l a s s Base { }13 c l a s s Pane l extends Base with ChangeSender { }14 o b j e c t T r a i t {15 de f main ( a r g s : Ar ray [ S t r i n g ] ) = {16 va r p = new Pane l ; va r b = new Button17 p . addChangeL i s t ene r ( b )18 p . emitChanges19 }20 }

GeorgSauthoff

Introduction

Features

Conclusion

C++ Trait

1 #i nc l ude <ios t r eam >2

3 t emp la t e <typename T>4 s t r u c t Number {5 enum { X = 0 } ;6 } ;7

8 t emp la t e <> s t r u c t Number { enum { X = 23 } ; } ;9

10 t emp la t e <typename T> s t r u c t Number<T∗> { enum { X = 42 } ; } ;11

12 t emp la t e <typename T>13 vo id p r i n t ( const T &t ) { s td : : cout << Number<T>: :X << ’ \n ’ ; }14

15 i n t main ( ) {16 i n t ∗ i ; p r i n t ( i ) ;17 double a ; p r i n t ( a ) ;18 re tu rn 0 ;19 }

GeorgSauthoff

Introduction

Features

Conclusion

Verbatim XML support as language feature

1 o b j e c t Xml {2

3 v a l x = " bar "4

5 v a l doc = <xml>6 <e n t r y date="1">foo </ent ry >7 <e n t r y date="2">{ x }</ent ry >8 </xml>9

10 de f main ( a r g s : Ar ray [ S t r i n g ] ) = {11 p r i n t l n ( doc )12 v a l e s = doc \\ " e n t r y "13 p r i n t l n ( e s )14 v a l ds = doc \\ " e n t r y " f i n d { ( x ) =>15 ( x . a t t r i b u t e ( " date " ) != None16 && x . a t t r i b u t e ( " date " ) . ge t == "1" ) }17 p r i n t l n ( ds )18 }19 }


GeorgSauthoff

Introduction

Features

Conclusion

Case classes

1 a b s t r a c t c l a s s Tree2

3 case c l a s s Lea f ( s : S t r i n g ) extends Tree4 case c l a s s Br ( l : Tree , r : Tree ) extends Tree5

6 o b j e c t Case {7

8 de f depth ( a : Tree ) : I n t =9 a match {10 case Lea f ( s ) => 111 case Br ( l , r ) =>12 1 + ( Math max( depth ( l ) , depth ( r ) ) )13 }14

15 de f main ( a r g s : Ar ray [ S t r i n g ] ) =16 p r i n t l n ( depth (17 Br ( Br ( Lea f ( " a " ) , Br ( Lea f ( "b" ) ,18 Lea f ( " c " ) ) ) , Lea f ( "d" ) ) ) )19 }


GeorgSauthoff

Introduction

Features

Conclusion

Generics

1 a b s t r a c t c l a s s Tree [T]2 case c l a s s Lea f [T ] ( s : T) extends Tree [T]3 case c l a s s Br [T ] ( l : Tree [T] , r : Tree [T ] ) extends Tree [T]4

5 c l a s s He lpe r [T] {6 de f depth ( a : Tree [T ] ) : I n t =7 a match {8 case Lea f ( s ) => 19 case Br ( l , r ) =>10 1 + ( Math max( depth ( l ) , depth ( r ) ) )11 }12 }13 o b j e c t Gen {14 de f main ( a r g s : Ar ray [ S t r i n g ] ) = {15 v a l h = new He lpe r [ S t r i n g ]16 p r i n t l n ( h . depth (17 Br ( Br ( Lea f ( " a " ) , Br ( Lea f ( "b" ) ,18 Lea f ( " c " ) ) ) , Lea f ( "d" ) ) ) )19 }20 }


GeorgSauthoff

Introduction

Features

Conclusion

Inheritance vs. Pattern Matching

1 a b s t r a c t c l a s s Tree { de f depth : I n t }2

3 case c l a s s Lea f ( s : S t r i n g ) extends Tree {4 de f depth : I n t = 15 }6 case c l a s s Br ( l : Tree , r : Tree ) extends Tree {7 de f depth : I n t = 1 + ( Math max( l . depth , r . depth ) )8 }9

10 o b j e c t Depth {11

12 de f main ( a r g s : Ar ray [ S t r i n g ] ) =13 p r i n t l n ( (14 Br ( Br ( Lea f ( " a " ) , Br ( Lea f ( "b" ) ,15 Lea f ( " c " ) ) ) , Lea f ( "d" ) ) )16 . depth )17 }


GeorgSauthoff

Introduction

Features

Conclusion

Companion Classes

1 a b s t r a c t c l a s s Tree { de f depth : I n t }2

3 c l a s s Lea f ( s : S t r i n g ) extends Tree {4 de f depth : I n t = 15 }6 o b j e c t Lea f { de f app l y ( s : S t r i n g ) = new Lea f ( s ) }7 c l a s s Br ( l : Tree , r : Tree ) extends Tree {8 de f depth : I n t = 1 + ( Math max( l . depth , r . depth ) )9 }10 o b j e c t Br { de f app l y ( l : Tree , r : Tree ) = new Br ( l , r ) }11

12 o b j e c t Depth2 {13

14 de f main ( a r g s : Ar ray [ S t r i n g ] ) =15 p r i n t l n ( (16 Br ( Br ( Lea f ( " a " ) , Br ( Lea f ( "b" ) ,17 Lea f ( " c " ) ) ) , Lea f ( "d" ) ) )18 . depth )19 }

GeorgSauthoff

Introduction

Features

Conclusion

Companion Classes and Pattern Matching

1 o b j e c t Even { de f unapp ly ( x : I n t ) : Boolean = x % 2 == 0 }2 o b j e c t Odd { de f unapp ly ( x : I n t ) : Boolean = x % 2 != 0 }3 o b j e c t Nega t i v e {4 de f unapp ly ( x : I n t ) : Opt ion [ ( I n t , I n t ) ] =5 i f ( x<0) Some (42 , 23) e l s e None6 }7

8 o b j e c t Companion {9

10 de f main ( a r g s : Ar ray [ S t r i n g ] ) =11 {12 v a l x : I n t = I n t e g e r p a r s e I n t ( a r g s ( 0 ) )13 x match {14 case Negat i v e ( a , b ) => p r i n t l n ( " Answer i s : " + a )15 case Even ( ) => p r i n t l n ( " foo " )16 case Odd ( ) => p r i n t l n ( " bar " )17 }18 }19 }


GeorgSauthoff

Introduction

Features

Conclusion

Abstract Types

1 c l a s s Base [U] {2 // type U3 type T = { de f ge t :U }4

5 de f p r i n t ( x :T) = { p r i n t l n ( x . ge t ) }6 }7

8 c l a s s He lpe r { de f ge t = " foo " ; d e f bar = " bar " }9

10 o b j e c t A b s t r a c t {11 de f main ( a r g s : Ar ray [ S t r i n g ] ) = {12 // v a l x = new Base { type U = S t r i n g }13 v a l x = new Base [ S t r i n g ]14 x . p r i n t ( new He lpe r )15 }16 }

GeorgSauthoff

Introduction

Features

Conclusion

List comprehensions

1 > module Main2 > where3 > impor t System ( getArgs )4

5 > permute x = [ ( i , j ) | i <− [ 0 . . x ] , j <− [ i . . x ] ]6

7 > main = do8 > ( x :_) <− getArgs9 > putSt rLn $ show $ permute $ ( ( r ead x ) : : I n t )

10 o b j e c t Comprehend {11 de f permute ( n : I n t ) =12 f o r ( i <− 0 u n t i l n+1; j <− i u n t i l n+1)13 y i e l d P a i r ( i , j )14

15 de f main ( a r g s : Ar ray [ S t r i n g ] ) =16 {17 v a l x = I n t e g e r p a r s e I n t ( a r g s ( 0 ) )18 p r i n t l n ( permute ( x ) )19 }20 }

GeorgSauthoff

Introduction

Features

Conclusion

List comprehensions


5 > permute x = [ ( i , j ) | i <− [ 0 . . x ] , j <− [ i . . x ] ]6

7 > main = do8 > ( x :_) <− getArgs9 > putSt rLn $ show $ permute $ ( ( r ead x ) : : I n t )10 o b j e c t Comprehend {11 de f permute ( n : I n t ) =12 f o r ( i <− 0 u n t i l n+1; j <− i u n t i l n+1)13 y i e l d P a i r ( i , j )14

15 de f main ( a r g s : Ar ray [ S t r i n g ] ) =16 {17 v a l x = I n t e g e r p a r s e I n t ( a r g s ( 0 ) )18 p r i n t l n ( permute ( x ) )19 }20 }

GeorgSauthoff

Introduction

Features

Conclusion

List comprehensions


5 > permute x = [ ( i , j ) | i <− [ 0 . . x ] , j <− [ i . . x ] ,6 ( i+j ) ‘mod ‘ 2 == 0 ]7

8 > main = do9 > ( x :_) <− getArgs10 > putSt rLn $ show $ permute $ ( ( r ead x ) : : I n t )

11 o b j e c t Comprehend2 {12 de f permute ( n : I n t ) =13 f o r ( i <− 0 u n t i l n+1; j <− i u n t i l n+1 i f ( i+j )%2==0)14 y i e l d P a i r ( i , j )15

16 de f main ( a r g s : Ar ray [ S t r i n g ] ) = {17 v a l x = I n t e g e r p a r s e I n t ( a r g s ( 0 ) )18 p r i n t l n ( permute ( x ) )19 }20 }

GeorgSauthoff

Introduction

Features

Conclusion

List comprehensions


5 > permute x = [ ( i , j ) | i <− [ 0 . . x ] , j <− [ i . . x ] ,6 ( i+j ) ‘mod ‘ 2 == 0 ]7

8 > main = do9 > ( x :_) <− getArgs10 > putSt rLn $ show $ permute $ ( ( r ead x ) : : I n t )11 o b j e c t Comprehend2 {12 de f permute ( n : I n t ) =13 f o r ( i <− 0 u n t i l n+1; j <− i u n t i l n+1 i f ( i+j )%2==0)14 y i e l d P a i r ( i , j )15

16 de f main ( a r g s : Ar ray [ S t r i n g ] ) = {17 v a l x = I n t e g e r p a r s e I n t ( a r g s ( 0 ) )18 p r i n t l n ( permute ( x ) )19 }20 }


GeorgSauthoff

Introduction

Features

Conclusion

Lazy evaluation

1 o b j e c t Lazy {2

3 de f bottom : I n t = bottom4

5 de f f 1 ( x : I n t , y : ( ) => I n t ) : I n t = x ∗ 26

7 de f f 2 ( x : I n t , y : => I n t ) : I n t = x ∗ 28

9 de f g ( x : I n t , y : I n t ) : I n t = x ∗ 210

11 de f main ( a r g s : Ar ray [ S t r i n g ] ) = {12 v a l x = I n t e g e r p a r s e I n t ( a r g s ( 0 ) )13 p r i n t l n ( f 1 ( x , ( ) => bottom ) )14 p r i n t l n ( f 2 ( x , bottom ) )15 }16 }


GeorgSauthoff

Introduction

Features

Conclusion

Lazy evaluation

1 c l a s s Foo ( x : I n t ) {2 p r i v a t e de f e x p e n s i v e = x∗x∗x∗x∗x∗x3 p r i v a t e de f compute = {}4

5 l a z y v a l bar = { compute ; e x p e n s i v e }6 }7

8 o b j e c t Lazy2 {9

10 de f main ( a r g s : Ar ray [ S t r i n g ] ) = {11 v a l x = I n t e g e r p a r s e I n t ( a r g s ( 0 ) )12 v a l f = new Foo ( x )13 i f ( x >42)14 p r i n t l n ( f . bar )15 }16 }


GeorgSauthoff

Introduction

Features

Conclusion

Currying

different signature of curried functions (vs. e.g. Haskell)it is possible to convert between curry and non-curryfunctions

Example1 o b j e c t Cur ry {2 de f p l u s ( x : I n t , y : I n t ) = x + y3 de f p l u s 2 ( x : I n t ) ( y : I n t ) = x + y4 de f p l u s 3 ( x : I n t ) = ( y : I n t ) => x+y5

6 v a l p l u s 4 = Funct i on . c u r r i e d ( p l u s _)7

8 v a l f 2 = p l u s 2 (2 ) _9 v a l f 3 = p l u s 3 (2 )10 v a l f 4 = p l u s 4 (2 )11

12 de f main ( a r g s : Ar ray [ S t r i n g ] ) = {13 v a l x = I n t e g e r p a r s e I n t ( a r g s ( 0 ) )14 p r i n t l n ( f 2 ( x ) ) ; p r i n t l n ( f 3 ( x ) ) ; p r i n t l n ( f 4 ( x ) )15 }16 }


GeorgSauthoff

Introduction

Features

Conclusion

Uniform access

to avoid the getter/setter boilerplate hell

Example1 c l a s s FooOld { va r s t a t e : I n t = 0 }2 c l a s s Foo {3 p r i v a t e va r s : I n t = 04 de f s t a t e_ =(x : I n t ) = s = x∗25 de f s t a t e = s6 }7 c l a s s Bar { de f s t a t e : I n t = 23 }8 c l a s s Baz extends Bar { o v e r r i d e v a l s t a t e : I n t = 42 }9

10 o b j e c t Uniform {11 de f main ( a r g s : Ar ray [ S t r i n g ] ) = {12 v a l x = I n t e g e r p a r s e I n t ( a r g s ( 0 ) )13 va r f = new Foo ; f . s t a t e = x14 p r i n t l n ( f . s t a t e )15 v a l g = new Baz16 p r i n t l n ( g . s t a t e )17 }18 }


GeorgSauthoff

Introduction

Features

Conclusion

Actors

Message passing with fancy operatorsSyntax similar to Erlang message passing constructsnot part of the language, a lib with some operators


GeorgSauthoff

Introduction

Features

Conclusion

Conclusion

Scala fits well into the JVM world→ easy to mix Scala and Java modules

Notation not as elegant as Haskell’sType system limits type inferenceIn comparison to Java many sources oftedious boilerplate are eliminated


GeorgSauthoff

Introduction

Features

Conclusion

Discussion

Thanks! && Do you have questions?


GeorgSauthoff

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scala programming language - uni-bielefeld.degsauthof/docs/scala.pdf · scala programming language...

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