Names

We use names to refer to things. For example, "Professeur Emile Perrot" refers to a very fragrant variety of rose, while "Cherry Parfait" is a highly disease resistant rose variety that barely smells at all. Much ink has been spilled, and many a chin stroked, on how exactly this relationship works in spoken language. Programming languages are much more constrained, which allows us to be much more precise: names refer to values. If we have named a value, wherever we would write out the expression that creates the value we can instead use the name. In other words, a name evaluates to the value it refers to. We will sometimes say names are bound to values, or a name introduces a binding. This naturally raises the question: how do we give names to values? There are several ways to do this in Scala. Let's see a few.

Object Literals

One way to name a value is to use an object literal. Here is an example.

object Example {
  Image.circle(100).fillColor(Color.paleGoldenrod).strokeColor(Color.indianRed).draw()
}

This is a literal expression, like other literals we've seen so far, but in this case it creates an object with the name Example. When we use the name Example in a program it evaluates to that object.

Example
// Example.type = Example$@76c39258

Try writing Example a few times in a worksheet. Do you notice any difference in uses of the name? You might have noticed that the first time you entered the name Example a picture was drawn, but on subsequent uses this didn't happen. The first time we use an object's name the body of the object is evaluated and the object is created. On subsequent uses of the name the object already exists and is not evaluated again. We can tell there is a difference in this case because the expression inside the object calls the draw method. If we replaced it with something like 1 + 1 (or just dropped the call to draw) we would not be able to tell the difference. We'll have plenty more to say about this in a later chapter.

Notice that the type of Example is Example.type, a unique type that no other value has.

val Declarations

Declaring an object literal mixes together object creation and defining a name. It would be useful if we could separate the two, so we could give a name to a pre-existing object. (Remember that all values are objects.) A val declaration allows us to do this.

We use val by writing

val <name> = <value>

replacing <name> and <value> with the name and the expression evaluating to the value respectively. For example

val one = 1
val anImage = Image.circle(100).fillColor(Color.red)

These two declarations define the names one and anImage. We can use these names to refer to the values in later code.

one
// res0: Int = 1
anImage
// res1: Image = FillColor(
//   image = Circle(d = 100.0),
//   color = RGBA(
//     r = UnsignedByte(value = 127),
//     g = UnsignedByte(value = -128),
//     b = UnsignedByte(value = -128),
//     a = Normalized(get = 1.0)
//   )
// )

Declarations

We've talked about declarations and definitions above. It's now time to be precise about what these terms mean.

We already know about expressions. They are a part of a program that evaluate to a value. A declaration or definition is a different part of a program. Declarations do not evaluate to a value. Instead they give a name to something (not always to a value as we can declare types in Scala.) Both object and val are declarations.

One consequence of declarations being separate from expressions is we can't write programs like

val one = ( val aNumber = 1 )

because val aNumber = 1 is not an expression and thus does not evaluate to a value.

We can however write

val aNumber = 1
// aNumber: Int = 1
val one = aNumber
// one: Int = 1

object name {
  body
}

val name = expression

object {}

object Example {
  val hi = "Hi!"
}

Example.hi
// res4: String = "Hi!"

hi
// error: 
// Not found: hi
// error: 
// Line is indented too far to the left, or a `}` is missing
// error: 
// Line is indented too far to the left, or a `}` is missing
// error: 
// Line is indented too far to the left, or a `}` is missing
// error: 
// Line is indented too far to the left, or a `}` is missing
// error: 
// A pure expression does nothing in statement position
// error: 
// A pure expression does nothing in statement position

Scope

If you did the last exercise (and you did, didn't you?) you'll have seen that a name declared inside an object can't be used outside the object without also referring to the object that contains the name. Concretely, if we declare

object Example {
  val hi = "Hi!"
}

we can't write

hi

We must tell Scala to look for hi inside Example.

Example.hi
// res8: String = "Hi!"

We say that a name is visible in the places where it can be used without qualification, and we call the places where a name is visible its scope. So using our fancy-pants new terminology, hi is not visible outside of Example, or alternatively hi is not in scope outside of Example.

How do we work out the scope of a name? The rule is fairly simple: a name is visible from the point it is declared to the end of the nearest enclosing braces (braces are { and }). In the example above hi is enclosed by the braces of Example and so is visible there. It's not visible elsewhere.

We can declare object literals inside object literals, which allows us to make finer distinctions about scope. For example in the code below

object Example1 {
  val hi = "Hi!"

  object Example2 {
    val hello = "Hello!"
  }
}

hi is in scope in Example2 (Example2 is defined within the braces that enclose hi). However the scope of hello is restricted to Example2, and so it has a smaller scope than hi.

What happens if we declare a name within a scope where it is already declared? This is known as shadowing. In the code below the definition of hi within Example2 shadows the definition of hi in Example1

object Example1 {
  val hi = "Hi!"

  object Example2 {
    val hi = "Hello!"
  }
}

Scala let's us do this, but it is generally a bad idea as it can make code very confusing.

We don't have to use object literals to create new scopes. Scala allows us to create a new scope just about anywhere by inserting braces. So we can write

object Example {
  val good = "Good"

  // Create a new scope
  {
    val morning = good ++ " morning"
    val toYou = morning ++ " to you"
  }

  val day = good ++ " day, sir!"
}

morning (and toYou) is declared within a new scope. We have no way to refer to this scope from the outside (it has no name) so we cannot refer to morning outside of the scope where it is declared. If we had some secrets that we didn't want the rest of the program to know about this is one way we could hide them.

Braces on their own are known as a block expression in Scala. Not only do they define a new scope, but, as their name suggests, they are an expression and therefore evaluate to a value. That value is the last expression in the block. So in the following the block evaluates to 3.

{
  val one = 1
  val two = 2
  1 + 2
}
// res11: Int = 3

We can also give a name to this value.

val three = {
  val one = 1
  val two = 2
  1 + 2
}
// three: Int = 3

The way nested scopes work in Scala is called lexical scoping. Not all languages have lexical scoping. For example, Ruby and Python do not, and Javascript has only recently acquired lexical scoping. It is the authors' opinion that creating a language without lexical scope is an idea on par with eating a bushel of Guatemalan insanity peppers and then going to the toilet without washing your hands.

val a = 1
val b = 2
val answer = a + b

object One {
  val a = 1

  object Two {
    val a = 3
    val b = 2
  }

  object Answer {
    val answer = a + Two.b
  }
}

object One {
  val a = 5
  val b = 2

  object Answer {
    val a = 1
    val answer = a + b
  }
}

object One {
  val a = 1
  val b = a + 1
  val answer = a + b
}

object One {
  val a = 1

  object Two {
    val b = 2
  }

  val answer = a + b
}

object One {
  val a = b - 1
  val b = a + 1

  val answer = a + b
}