clone -
git clone https://github.com/twitter/scalding.git
Install homebrew -
brewsh -c "$(curl -fsSL https://raw.githubusercontent.com/Linuxbrew/install/master/install.sh)"
Or in Mac
/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
sbt assembly
#mv ~/.sbt/repositories ~
mv ~/repositories ~/.sbt/
vim tutorial/WONDERLAND.md
To build and launch the repl:
```
./sbt scalding-repl/assembly
./scripts/scald.rb --repl --local
```
chmod +x run-repl.sh
**What is SBT?**
When you write small programs that consist of only one, or just two or three source files, then it's easy enough to compile those source files by typing scalac MyProgram.scala in the command line. But when you start working on a bigger project with dozens or maybe even hundreds of source files, then it becomes too tedious to compile all those source files manually. You will then want to use a build tool to manage compiling all those source files.
sbt is such a tool. There are other tools too, some other well-known build tools that come from the Java world are Ant and Maven.
How it works is that you create a project file that describes what your project looks like; when you use sbt, this file will be called build.sbt. That file lists all the source files your project consists of, along with other information about your project. Sbt will read the file and then it knows what to do to compile the complete project.
Besides managing your project, some build tools, including sbt, can automatically manage dependencies for you. This means that if you need to use some libraries written by others, sbt can automatically download the right versions of those libraries and include them in your project for you.
Further Reading about SBT Directories:
- https://www.scala-sbt.org/1.x/docs/Directories.html
**Steps to run a job?**
Step 1: Compile code using sbt compile in the directory that contains the build.sbt file (first green arrow in image)
What does sbt compile actually do?
* compile Compiles the main sources (in the src/main/scala directory) into the "target" directory
*
- You can see the target dir in your project folders
- Step 2: Enter sbt command
- What does the sbt command actually do?
- Starts an sbt server
- Allows you to run sbt commands
- Examples
- Compile
- Test
- Verify
- runMain
- Step 3: Change sbt projects (3rd arrow)
- Read more about projects here: https://alvinalexander.com/scala/how-to-create-sbt-projects-with-subprojects
- This lets sbt know which set of "src" files to use
- Step 4: runMain (4th arrow)
- Execute the main function in a package
**http://twitter.github.io/scala_school/basics.html**
## About this class
The first few weeks will cover basic syntax and concepts, then we’ll start to open it up with more exercises.
Some examples will be given as if written in the interpreter and others as if written in a source file.
Having an interpreter available makes it easy to explore a problem space.
### Why Scala?
- Expressive
- First-class functions
- Closures
- Concise
- Type inference
- Literal syntax for function creation
- Java interoperability
- Can reuse java libraries
- Can reuse java tools
- No performance penalty
### How Scala?
- Compiles to java bytecode
- Works with any standard
JVM
- Or even some non-standard JVMs like Dalvik
- Scala compiler written by author of Java compiler
### Think Scala
Scala is not just a nicer Java. You should learn it with a fresh mind- you will get more out of these classes.
### Get Scala
Scala School’s examples work with [Scala 2.9.x](https://www.scala-lang.org/download/2.9.3.html) . If you use Scala 2.10.x or newer, *most* examples work OK, but not all.
### Start the Interpreter
Start the included `sbt console`.
```
$ sbt console
[...]
Welcome to Scala version 2.8.0.final (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_20).
Type in expressions to have them evaluated.
Type :help for more information.
scala>
```
## Expressions
```
scala> 1 + 1
res0: Int = 2
```
res0 is an automatically created value name given by the interpreter to the result of your expression. It has the type Int and contains the Integer 2.
(Almost) everything in Scala is an expression.
## Values
You can give the result of an expression a name.
```
scala> val two = 1 + 1
two: Int = 2
```
You cannot change the binding to a val.
### Variables
If you need to change the binding, you can use a `var` instead.
```
scala> var name = "steve"
name: java.lang.String = steve
scala> name = "marius"
name: java.lang.String = marius
```
## Functions
You can create functions with def.
```
scala> def addOne(m: Int): Int = m + 1
addOne: (m: Int)Int
```
In Scala, you need to specify the type signature for function parameters. The interpreter happily repeats the type signature back to you.
```
scala> val three = addOne(2)
three: Int = 3
```
You can leave off parens on functions with no arguments.
```
scala> def three() = 1 + 2
three: ()Int
scala> three()
res2: Int = 3
scala> three
res3: Int = 3
```
### Anonymous Functions
You can create anonymous functions.
```
scala> (x: Int) => x + 1
res2: (Int) => Int = <function1>
```
This function adds 1 to an Int named x.
```
scala> res2(1)
res3: Int = 2
```
You can pass anonymous functions around or save them into vals.
```
scala> val addOne = (x: Int) => x + 1
addOne: (Int) => Int = <function1>
scala> addOne(1)
res4: Int = 2
```
If your function is made up of many expressions, you can use {} to give yourself some breathing room.
```
def timesTwo(i: Int): Int = {
println("hello world")
i * 2
}
```
This is also true of an anonymous function.
```
scala> { i: Int =>
println("hello world")
i * 2
}
res0: (Int) => Int = <function1>
```
You will see this syntax often used when passing an anonymous function as an argument.
### Partial application
You can partially apply a function with an underscore, which gives you another function. Scala uses the underscore to mean different things in different contexts, but you can usually think of it as an unnamed magical wildcard. In the context of `{ _ + 2 }` it means an unnamed parameter. You can use it like so:
```
scala> def adder(m: Int, n: Int) = m + n
adder: (m: Int,n: Int)Int
scala> val add2 = adder(2, _:Int)
add2: (Int) => Int = <function1>
scala> add2(3)
res50: Int = 5
```
You can partially apply any argument in the argument list, not just the last one.
### Curried functions
Sometimes it makes sense to let people apply some arguments to your function now and others later.
Here’s an example of a function that lets you build multipliers of two numbers together. At one call site, you’ll decide which is the multiplier and at a later call site, you’ll choose a multiplicand.
```
scala> def multiply(m: Int)(n: Int): Int = m * n
multiply: (m: Int)(n: Int)Int
```
You can call it directly with both arguments.
```
scala> multiply(2)(3)
res0: Int = 6
```
You can fill in the first parameter and partially apply the second.
```
scala> val timesTwo = multiply(2) _
timesTwo: (Int) => Int = <function1>
scala> timesTwo(3)
res1: Int = 6
```
You can take any function of multiple arguments and curry it. Let’s try with our earlier `adder`
```
scala> val curriedAdd = (adder _).curried
curriedAdd: Int => (Int => Int) = <function1>
scala> val addTwo = curriedAdd(2)
addTwo: Int => Int = <function1>
scala> addTwo(4)
res22: Int = 6
```
### Variable length arguments
There is a special syntax for methods that can take parameters of a repeated type. To apply String’s `capitalize` function to several strings, you might write:
```
def capitalizeAll(args: String*) = {
args.map { arg =>
arg.capitalize
}
}
scala> capitalizeAll("rarity", "applejack")
res2: Seq[String] = ArrayBuffer(Rarity, Applejack)
```
## Classes
```
scala> class Calculator {
| val brand: String = "HP"
| def add(m: Int, n: Int): Int = m + n
| }
defined class Calculator
scala> val calc = new Calculator
calc: Calculator = Calculator@e75a11
scala> calc.add(1, 2)
res1: Int = 3
scala> calc.brand
res2: String = "HP"
```
Contained are examples defining methods with def and fields with val. Methods are just functions that can access the state of the class.
### Constructor
Constructors aren’t special methods, they are the code outside of method definitions in your class. Let’s extend our Calculator example to take a constructor argument and use it to initialize internal state.
```
class Calculator(brand: String) {
/**
* A constructor.
*/
val color: String = if (brand == "TI") {
"blue"
} else if (brand == "HP") {
"black"
} else {
"white"
}
// An instance method.
def add(m: Int, n: Int): Int = m + n
}
```
Note the two different styles of comments.
You can use the constructor to construct an instance:
```
scala> val calc = new Calculator("HP")
calc: Calculator = Calculator@1e64cc4d
scala> calc.color
res0: String = black
```
### Expressions
Our Calculator example gave an example of how Scala is expression-oriented. The value color was bound based on an if/else expression. Scala is highly expression-oriented: most things are expressions rather than statements.
### Aside: Functions vs Methods
Functions and methods are largely interchangeable. Because functions and methods are so similar, you might not remember whether that *thing* you call is a function or a method. When you bump into a difference between methods and functions, it might confuse you.
```
scala> class C {
| var acc = 0
| def minc = { acc += 1 }
| val finc = { () => acc += 1 }
| }
defined class C
scala> val c = new C
c: C = C@1af1bd6
scala> c.minc // calls c.minc()
scala> c.finc // returns the function as a value:
res2: () => Unit = <function0>
```
When you can call one “function” without parentheses but not another, you might think *Whoops, I thought I knew how Scala functions worked, but I guess not. Maybe they sometimes need parentheses?* You might understand functions, but be using a method.
In practice, you can do great things in Scala while remaining hazy on the difference between methods and functions. If you’re new to Scala and read [explanations of the differences](https://www.google.com/search?q=difference+scala+function+method), you might have trouble following them. That doesn’t mean you’re going to have trouble using Scala. It just means that the difference between functions and methods is subtle enough such that explanations tend to dig into deep parts of the language.
## Inheritance
```
class ScientificCalculator(brand: String) extends Calculator(brand) {
def log(m: Double, base: Double) = math.log(m) / math.log(base)
}
```
**See Also** Effective Scala points out that a [Type alias](https://twitter.github.com/effectivescala/#Types and Generics-Type aliases) is better than `extends` if the subclass isn’t actually different from the superclass. A Tour of Scala describes [Subclassing](https://www.scala-lang.org/node/125).
### Overloading methods
```
class EvenMoreScientificCalculator(brand: String) extends ScientificCalculator(brand) {
def log(m: Int): Double = log(m, math.exp(1))
}
```
### Abstract Classes
You can define an *abstract class*, a class that defines some methods but does not implement them. Instead, subclasses that extend the abstract class define these methods. You can’t create an instance of an abstract class.
```
scala> abstract class Shape {
| def getArea():Int // subclass should define this
| }
defined class Shape
scala> class Circle(r: Int) extends Shape {
| def getArea():Int = { r * r * 3 }
| }
defined class Circle
scala> val s = new Shape
<console>:8: error: class Shape is abstract; cannot be instantiated
val s = new Shape
^
scala> val c = new Circle(2)
c: Circle = Circle@65c0035b
```
## Traits
`traits` are collections of fields and behaviors that you can extend or mixin to your classes.
```
trait Car {
val brand: String
}
trait Shiny {
val shineRefraction: Int
}
class BMW extends Car {
val brand = "BMW"
}
```
One class can extend several traits using the `with` keyword:
```
class BMW extends Car with Shiny {
val brand = "BMW"
val shineRefraction = 12
}
```
**See Also** Effective Scala has opinions about [trait](https://twitter.github.com/effectivescala/#Object oriented programming-Traits).
**When do you want a Trait instead of an Abstract Class?** If you want to define an interface-like type, you might find it difficult to choose between a trait or an abstract class. Either one lets you define a type with some behavior, asking extenders to define some other behavior. Some rules of thumb:
- Favor using traits. It’s handy that a class can extend several traits; a class can extend only one class.
- If you need a constructor parameter, use an abstract class. Abstract class constructors can take parameters; trait constructors can’t. For example, you can’t say `trait t(i: Int) {}`; the `i` parameter is illegal.
You are not the first person to ask this question. See fuller answers at [stackoverflow:Scala traits vs abstract classes](https://stackoverflow.com/questions/1991042/scala-traits-vs-abstract-classes), [Difference between Abstract Class and Trait](https://stackoverflow.com/questions/2005681/difference-between-abstract-class-and-trait), and [Programming in Scala: To trait, or not to trait?](https://www.artima.com/pins1ed/traits.html#12.7)
## Types
Earlier, you saw that we defined a function that took an `Int` which is a type of Number. Functions can also be generic and work on any type. When that occurs, you’ll see a type parameter introduced with the square bracket syntax. Here’s an example of a Cache of generic Keys and Values.
```
trait Cache[K, V] {
def get(key: K): V
def put(key: K, value: V)
def delete(key: K)
}
```
Methods can also have type parameters introduced.
```
def remove[K](key: K)
```
Built at [@twitter](https://twitter.com/twitter) by [@stevej](https://twitter.com/stevej), [@marius](https://twitter.com/marius), and [@lahosken](https://twitter.com/lahosken) with much help from [@evanm](https://twitter.com/evanm), [@sprsquish](https://twitter.com/sprsquish), [@kevino](https://twitter.com/kevino), [@zuercher](https://twitter.com/zuercher), [@timtrueman](https://twitter.com/timtrueman), [@wickman](https://twitter.com/wickman), [@mccv](https://twitter.com/mccv) and [@garciparedes](https://github.com/garciparedes); Russian translation by [appigram](https://github.com/appigram); Chinese simple translation by [jasonqu](https://github.com/jasonqu); Korean translation by [enshahar](https://github.com/enshahar);
