Scala for dummies

MADRID · MAR · 2015

Scala Programming @ Madrid

Scala

David Vallejo & Javier Santos



About us

Javier Santos @jpaniego

«Hay dos formas de programar sin errores; solo la tercera funciona»

Alan J Perlis

David Vallejo @dvnavarro

«Una expresión regular es una expresión que no es del todo buena, pero tampoco del todo mala.»

@borjamonserrano



What’s Scala?

According Wikipedia:

Scala (/ˈskɑːlə/ SKAH-lə)[6] is an object-functional[7] programming language for general software

applications. Scala has full support for functional programming and a very strong static type system. This

allows programs written in Scala to be very concise and thus smaller in size than other general-purpose

programming languages. Many of Scala's design decisions were inspired by criticism over the shortcomings

of Java.[5]

http://en.wikipedia.org/wiki/Help:IPA_for_English

http://en.wikipedia.org/wiki/Help:IPA_for_English#Key



http://en.wikipedia.org/wiki/Wikipedia:Pronunciation_respelling_key



http://en.wikipedia.org/wiki/Scala_%28programming_language%29#cite_note-6

http://en.wikipedia.org/wiki/Scala_%28programming_language%29#cite_note-6

http://en.wikipedia.org/wiki/Multi-paradigm_programming_language

http://en.wikipedia.org/wiki/Scala_%28programming_language%29#cite_note-StepsInScala-7

http://en.wikipedia.org/wiki/Scala_%28programming_language%29#cite_note-StepsInScala-7

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

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

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



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

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

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

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

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




http://en.wikipedia.org/wiki/General-purpose_programming_language




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




http://en.wikipedia.org/wiki/Scala_%28programming_language%29#cite_note-overview-5




What’s Scala?

According to the official website:



What’s Scala?

According to newbies:



What’s Scala?

● General purpose language that runs on the JVM.

● Martin Odersky

● Multiple Inheritance

● Multi-paradigm: functional & object-oriented

● Typesafe (2011)

● TIOBE (41st position)

http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html

http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html



Why so fashionable these days?

● Useful tools to build concurrent systems○ More cores, more concurrency○ Avoid side effects using functional programming and immutable values○ Actor paradigm with the toolkit called Akka

● Awesome frameworks and tool-kits: Play Framework, Akka, Spark, Spray ...

● Others○ Powerful mechanisms for embedding DSL like, for example, implicits○ The best of functional world and the best of OO world○ Elegant & concise



Who uses it?



Developer environment

● REPL (Read-eval-print loop)http://scala-lang.org/download/

● SBThttp://www.scala-sbt.org/

● IDES: Eclipse http://scala-ide.org/download/sdk.htmlIntellij https://www.jetbrains.com/idea/features/scala.htmlNetBeans http://wiki.netbeans.org/Scala

http://scala-lang.org/download/

http://scala-lang.org/download/

http://www.scala-sbt.org/

http://www.scala-sbt.org/

http://scala-ide.org/download/sdk.html

https://www.jetbrains.com/idea/features/scala.html

http://wiki.netbeans.org/Scala



Features



val vs var● Type inference● var: variables (mutable)

● val: values (immutable)

● lazy vals: Not evaluated until necessary



val vs var● vals are always immutable?

Stateful objects (mutable collections for example) (watch out with side effects)



Methods/functions

● Type inference● Only braces means Unit

● No return necessary

● Variable args number (*)



What about class?● A class may contain

○ attributes (var/val)○ methods (def)○ expressions that

will be evaluated at instantiation time.



What about class?

● A simple class with no parameters



What about class?

● A simple class with parameters



What about class?

● A simple class with parameter accessors



What about abstract class?

● It cannot be instantiated, unless ...



What about case class?

● A case class provides:○ parameter accessors○ companion object with apply and unapply○ extends from Serializable○ copy method

● ...but it cannot be extended (We can handle that)



What about case class?





Rich interfaces: trait

● Methods can be implemented.● Multiple inheritance.



Inheritance: mixing in stuff...

● Linearization (Watch out the order)



What about nulls in Scala?

● Use option instead● Two possible values: Some(t) and None



Options in Scala

● Besides it’s iterable!● Nice functional methods like fold.



Generics

● For classes and traits



Generics

● For methods



Generics

● Variance: C[T] != C[+T] != C[-T]



Conditional statements: if



Conditional statements: match

● Similar to a Java switch...




● ...except that it’s not only a switch :-)● Pattern matching.




● So it’s great, but be carefull with



Loop statements: while

● Not very commonly used (vars and so on…)



Loop statements: for

● for-comprehensions: syntactic sugar for



Loop statements: for

● Typical example:

Is this purely functional?



Loops in Scala



Your new friend: fold

● Two signatures: foldLeft and foldRight (iterative vs. recursive)

● Same example:



Other common friends: map

● Convert each element to another (or same) type.

● Example:



Other common friends: flatMap

● Map each element to some monad-like type and flatten them

● Example:



More cool stuff: Type alias

● Useful for specifying anonymous tuple types



More cool stuff: Higher order functions

● Javascript ninjas gonna like…









More cool stuff: Infix notation, semicolon inference...

● Allows writing readable code● Specially useful for DSLs.



More cool stuff: Infix notation, semicolon inference...



More cool stuff: Implicits



More cool stuff: Implicits

● Defined by scope (be careful with ambiguity)● Specially difficult to trace: use only in very located places



More cool stuff: Self types

● Used for disambiguating with duplicated val/function names



Conclusions

● Steep learning curve

● It's hard to know which is the best way to do something

● Language expanding. More and more Scala enthusiasts.

● Mix the functional world and the OO world :)





Scala

David Vallejo & Javier Santos



scalerablog.wordpress.com

@scalerablog

http://scalerablog.wordpress.com

http://scalerablog.wordpress.com