Short intro to scala and the play framework

Short intro to Scala and the
Play! Framework
50 - 60 minutes
Last updated: Sept 2014 falmeida1988@gmail.com
Leveraging modern programming techniques to make
safer, faster and more predictable applications

Scala - First encounter
def hello() = {
println("Hello, world!")
}
var aMap = Map("Red" -> "Apple", "Yellow" -> "Peach")
aMap += ("Purple" -> "Grape")
def factorial(x: BigInt): BigInt =
if (x == 0) 1 else x * factorial(x - 1)
class MyClass(index: Int, name: String)
functions
collections
recursive functions
classes

Scala - Background
● Created by Martin Odersky at EPFL in 2003
● Aimed at overcoming Java’s weaknesses while allowing for easy migration
for former Java developers
● One of the few object-functional languages (also: F#)
● One of the JVM-compliant languages (also: groovy, clojure, jruby, jython)

Scala selling points
● Functional/Object Oriented hybrid - use one, the other, or both
● More elegant, less painful concurrency constructs
● Uncluttered, concise syntax
● Seamless interoperability and compatibility with Java
● Typing: expressive type system / static typing / type safety / type inference

Quick syntax walkthrough
● no type declaration prior to use (due to type-inference)
var msg = "Hello, world!"
● braces are only needed for multiline blocks
def max(x: Int, y: Int) = if (x > y) x else y
def max2(x: Int, y: Int) = {
if (x > y) x
else y
}
no braces
yes braces

● no semicolons at the end (unless you want multiple statements)
println(line); println(line)
println(line)
println(line)
equivalent code

● type comes after variable name (only when required or for usability)
def max(x: Int, y: Int): Int =
if (x > y) x else y
val m = new HashMap[Int, String] ()
required types for
val m1: Map[Int, String] = new HashMap()
arguments
optional return type
either is acceptable
not necessary to annotate both sides
val m2: HashMap[Int, String] = new HashMap[Int, String] ()

● val for immutable variables, var for mutable variables
val msg = "Hello, world!"
msg = "Another message!" // ERROR: reassignment to val
var msg2 = "Hello, world!"
msg2 = "Another message!" // no error

● statements also return a value (if, for, while, def) - which means they are
also expressions
var result1 = ""
if(marks >= 50)
result = "passed"
else
result = "failed"
val result2 = if(marks >= 50)
"passed"
else
"failed"
using if as a
statement
if as an expression

● return statements are allowed but generally not needed
def multiply(a: Int,b:Int):Int =
return a*b
def sum(x:Int,y:Int) =
x + y
def greet() = println( "Hello, world!" )
explicitly using return
When no return is provided,
last value computed by the
function is returned
Functions that return no
useful values have a result
type of Unit

● function literals or anonymous functions
(x: Int) => x * 2
val double = (x: Int) => x * 2
● example usage: as parameter to function map:
List(1,2,3,4,5).map{ (x: Int) => x * 2 }
//evaluates to List(2, 4, 6, 8, 10)
a function that takes an Int
and multiplies it by two
function value being
assigned to a variable
function map takes another
function as argument

Scala, Java Comparison
scala class MyClass(index: Int, name: String)
class MyClass {
private int index;
private String name;
public MyClass(int index, String name) {
this.index = index;
this.name = name;
}
}
java

Scala, Java Comparison
scala val nameHasUpperCase = name.exists(_.isUpper)
boolean nameHasUpperCase = false;
for (int i = 0; i < name.length(); ++i) {
if (Character.isUpperCase(name.charAt(i))) {
nameHasUpperCase = true;
break;
}
}
java

Scala, Java Interoperability
● You can use Java code in Scala as-is (i.e. no changes needed).
● Scala classes can subclass Java classes, you can instantiate Java classes
in Scala, you can access methods, fields (even if they are static), etc.
● You can also use Scala code in Java projects, as long as you don’t use
many advanced Scala concepts that are not possible in Java.
● Similar code generated by Scala and Java usually generates the exact
same bytecode, as can be verified using tools like javap

Environment
● Console
● sbt - dependency and package management
● JVM integration
● Typesafe products (Akka, Play, Activator, Spray, Slick, etc)
● All of your favourite Java libraries
● Testing suites (Unit, Integration, Acceptance, Property-based, etc)
● Small but high-level community

Weaknesses
● It’s a large language. Users are advised not to try to use many different
concepts at the same time, especially when starting out.
● Scala has a somewhat steep learning curve and its complex type system is
powerful but hard to grasp at times.
● Implicit conversions are useful but easily misused and may make code
harder to understand.
● Complex function signatures may put some off.

Play Framework
● MVC Web Framework; supports Java and Scala
● Created in 2007
● Used at Linkedin, Coursera, The Guardian, etc.
● Uses sbt for dependency management
● As of September 2014, it has had 5000 commits by over 350 contributors.
● The Simplest Possible Play App

Play Features
● Play has all default features one can expect from a modern framework:
○ MVC-based separation of concerns
○ Support for ORMs (Java) or FRMs (Scala)
○ Rich models with support for Forms, Validation, etc.
○ Database Migration (called evolutions)
○ Template engine (Scala-based)
○ Extensive routing
○ Support for REST-only Apps
○ Lots of community-provided plugins
○ Supported by Typesafe

Play - Application Layout
app/ -- Application sources
| assets/ -- LESS, Coffeescript sources
| controllers/ -- Controllers
| models/ -- Domain models
| views/ -- Templates
build.sbt -- Build configuration
conf/ -- Configuration files
| application.conf -- Main configuration file
| routes -- Routes definition
public/ -- Public folder (CSS, JS, etc)
project/ -- sbt configuration files
logs/ -- Logs
target/ -- Generated files - ignore
test/ -- Sources for tests

Play Framework - Examples
● Displaying a view from a controller action
package controllers
import play.api._
import play.api.mvc._
object Application extends Controller {
def index = Action {
Ok(views.html.index("Your new application is ready."))
}
}
import libraries
create a controller
define actions as methods

Play Framework - Examples
● Displaying a view with some data
package controllers
import models._
import play.api._
import play.api.mvc._
import play.api.Play.current
object Application extends Controller {
this method returns a List[Computer]
def index = Action { implicit request =>
val computers = Computer .list
Ok(views.html.web.index( "Hello! I'm the WEB!" , computers))
}
}
the Computer model was defined in
package models (not shown here)
instantiate a view file and feed it
a string and a list of computers

Activator
● Web-based IDE and project viewer, built by Typesafe
● A large number of sample applications (templates) to study and learn from

Resources
● Scala Programming Language (Wikipedia Article)
● Scala official site
● Typesafe
● Programming in Scala Book on Amazon
● A preset Virtual Machine for Scala/Play Development
● Functional Programming Principles in Scala on Coursera
● Principles of Reactive Programming on Coursera
● Play Framework Official Website
● ScalaJavaInterop Project
● Play Framework (Wikipedia Article)
● Using Scala on Heroku
● Typesafe Activator
● All Available Activator Templates

Short intro to scala and the play framework

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