Identifiers, Names and Scopes

Names in Scala identify types, values, methods, and classes which are collectively called entities. Names are introduced by local definitions and declarations, inheritance, import clauses, or package clauses which are collectively called bindings.

Bindings of different kinds have a precedence defined on them:

1. Definitions and declarations that are local, inherited, or made available by a package clause in the same compilation unit where the definition occurs have highest precedence.
2. Explicit imports have next highest precedence.
3. Wildcard imports have next highest precedence.
4. Definitions made available by a package clause not in the compilation unit where the definition occurs have lowest precedence.

There are two different name spaces, one for types and one for terms. The same name may designate a type and a term, depending on the context where the name is used.

A binding has a scope in which the entity defined by a single name can be accessed using a simple name. Scopes are nested. A binding in some inner scope shadows bindings of lower precedence in the same scope as well as bindings of the same or lower precedence in outer scopes.

A reference to an unqualified (type- or term-) identifier $x$ is bound by the unique binding, which

• defines an entity with name $x$ in the same namespace as the identifier, and
• shadows all other bindings that define entities with name $x$ in that namespace.

It is an error if no such binding exists. If $x$ is bound by an import clause, then the simple name $x$ is taken to be equivalent to the qualified name to which $x$ is mapped by the import clause. If $x$ is bound by a definition or declaration, then $x$ refers to the entity introduced by that binding. In that case, the type of $x$ is the type of the referenced entity.

A reference to a qualified (type- or term-) identifier $e.x$ refers to the member of the type $T$ of $e$ which has the name $x$ in the same namespace as the identifier. It is an error if $T$ is not a value type. The type of $e.x$ is the member type of the referenced entity in $T$.

Example

Assume the following two definitions of a objects named X in packages P and Q.

package P {
  object X { val x = 1; val y = 2 }
}

package Q {
  object X { val x = true; val y = "" }
}

The following program illustrates different kinds of bindings and precedences between them.

package P {                  // `X' bound by package clause
import Console._             // `println' bound by wildcard import
object A {
  println("L4: "+X)          // `X' refers to `P.X' here
  object B {
    import Q._               // `X' bound by wildcard import
    println("L7: "+X)        // `X' refers to `Q.X' here
    import X._               // `x' and `y' bound by wildcard import
    println("L8: "+x)        // `x' refers to `Q.X.x' here
    object C {
      val x = 3              // `x' bound by local definition
      println("L12: "+x)     // `x' refers to constant `3' here
      { import Q.X._         // `x' and `y' bound by wildcard import
//      println("L14: "+x)   // reference to `x' is ambiguous here
        import X.y           // `y' bound by explicit import
        println("L16: "+y)   // `y' refers to `Q.X.y' here
        { val x = "abc"      // `x' bound by local definition
          import P.X._       // `x' and `y' bound by wildcard import
//        println("L19: "+y) // reference to `y' is ambiguous here
          println("L20: "+x) // `x' refers to string "abc" here
}}}}}}