Motivation

Some usecases, such as modelling database access, are more awkward in statically typed languages than in dynamically typed languages: With dynamically typed languages, it's quite natural to model a row as a record or object, and to select entries with simple dot notation (e.g. row.columnName).

Achieving the same experience in statically typed language requires defining a class for every possible row arising from database manipulation (including rows arising from joins and projections) and setting up a scheme to map between a row and the class representing it.

This requires a large amount of boilerplate, which leads developers to trade the advantages of static typing for simpler schemes where colum names are represented as strings and passed to other operators (e.g. row.select("columnName")). This approach forgoes the advantages of static typing, and is still not as natural as the dynamically typed version.

Structural types help in situations where we would like to support simple dot notation in dynamic contexts without losing the advantages of static typing. They allow developers to use dot notation and configure how fields and methods should be resolved.

Example

Here's an example of a structural type Person:

type Person = Record { val name: String; val age: Int }

The type Person adds a refinement to its parent type Record that defines the two fields name and age. We say the refinement is structural since name and age are not defined in the parent type. But they exist nevertheless as members of type Person.

This allows us to check at compiletime if accesses are valid:

val person: Person = ???
println(s"${person.name} is ${person.age} years old.") // works
println(person.email) // error: value email is not a member of Person

How is Record defined, and how does person.name resolve ?

Record is a class that extends the marker trait scala.Selectable and defines a method selectDynamic, which maps a field name to its value. Selecting a member of a structural type is syntactic sugar for a call to this method. The selections person.name and person.age are translated by the Scala compiler to:

person.selectDynamic("name").asInstanceOf[String]
person.selectDynamic("age").asInstanceOf[Int]

For example, Record could be defined as follows:

class Record(elems: (String, Any)*) extends Selectable:
  private val fields = elems.toMap
  def selectDynamic(name: String): Any = fields(name)

Which allows us to create instances of Person like so:

val person = Record("name" -> "Emma", "age" -> 42).asInstanceOf[Person]

The parent type Record in this example is a generic class that can represent arbitrary records in its elems argument. This argument is a sequence of pairs of labels of type String and values of type Any. When we create a Person as a Record we have to assert with a typecast that the record defines the right fields of the right types. Record itself is too weakly typed so the compiler cannot know this without help from the user. In practice, the connection between a structural type and its underlying generic representation would most likely be done by a database layer, and therefore would not be a concern of the end user.

Besides selectDynamic, a Selectable class sometimes also defines a method applyDynamic. This can then be used to translate function calls of structural members. So, if a is an instance of Selectable, a structural call like a.f(b, c) would translate to

a.applyDynamic("f")(b, c)

Using Java Reflection

Using Selectable and Java reflection, we can select a member from unrelated classes.

Before resorting to structural calls with Java reflection one should consider alternatives. For instance, sometimes a more a modular and efficient architecture can be obtained using type classes.

For example, we would like to provide behavior for both FileInputStream and Channel classes by calling their close method, however, these classes are unrelated, i.e. have no common supertype with a close method. Therefore, below we define a structural type Closeable that defines a close method.

type Closeable = { def close(): Unit }

class FileInputStream:
  def close(): Unit

class Channel:
  def close(): Unit

Ideally we would add a common interface to both these classes to define the close method, however they are defined in libraries outside of our control. As a compromise we can use the structural type to define a single implementation for an autoClose method:

import scala.reflect.Selectable.reflectiveSelectable

def autoClose(f: Closeable)(op: Closeable => Unit): Unit =
  try op(f) finally f.close()

The call f.close() requires Closeable to extend Selectable to identify and call the close method in the receiver f. A universal implicit conversion to Selectable is enabled by an import of reflectiveSelectable shown above, based on Java reflection. What happens "under the hood" is then the following:

• The implicit conversion wraps f in an instance of scala.reflect.Selectable (which is a subtype of Selectable).

• The compiler then transforms the close call on the wrapped f to an applyDynamic call. The end result is:

  reflectiveSelectable(f).applyDynamic("close")()
  

• The implementation of applyDynamic in reflectiveSelectable's result uses Java reflection to find and call a method close with zero parameters in the value referenced by f at runtime.

Structural calls like this tend to be much slower than normal method calls. The mandatory import of reflectiveSelectable serves as a signpost that something inefficient is going on.

Note: In Scala 2, Java reflection is the only mechanism available for structural types and it is automatically enabled without needing the reflectiveSelectable conversion. However, to warn against inefficient dispatch, Scala 2 requires a language import import scala.language.reflectiveCalls.

Extensibility

New instances of Selectable can be defined to support means of access other than Java reflection, which would enable usages such as the database access example given at the beginning of this document.

Local Selectable Instances

Local and anonymous classes that extend Selectable get more refined types than other classes. Here is an example:

trait Vehicle extends reflect.Selectable:
  val wheels: Int

val i3 = new Vehicle: // i3: Vehicle { val range: Int }
  val wheels = 4
  val range = 240

i3.range

The type of i3 in this example is Vehicle { val range: Int }. Hence, i3.range is well-formed. Since the base class Vehicle does not define a range field or method, we need structural dispatch to access the range field of the anonymous class that initializes id3. Structural dispatch is implemented by the base trait reflect.Selectable of Vehicle, which defines the necessary selectDynamic member.

Vehicle could also extend some other subclass of scala.Selectable that implements selectDynamic and applyDynamic differently. But if it does not extend a Selectable at all, the code would no longer typecheck:

trait Vehicle:
  val wheels: Int

val i3 = new Vehicle: // i3: Vehicle
  val wheels = 4
  val range = 240

i3.range // error: range is not a member of `Vehicle`

The difference is that the type of an anonymous class that does not extend Selectable is just formed from the parent type(s) of the class, without adding any refinements. Hence, i3 now has just type Vehicle and the selection i3.range gives a "member not found" error.

Note that in Scala 2 all local and anonymous classes could produce values with refined types. But members defined by such refinements could be selected only with the language import reflectiveCalls.

Relation with scala.Dynamic

There are clearly some connections with scala.Dynamic here, since both select members programmatically. But there are also some differences.

• Fully dynamic selection is not typesafe, but structural selection is, as long as the correspondence of the structural type with the underlying value is as stated.

• Two access operations, selectDynamic and applyDynamic are shared between both approaches. In Selectable, applyDynamic also may also take java.lang.Class arguments indicating the method's formal parameter types.

• updateDynamic is unique to Dynamic but as mentionned before, this fact is subject to change, and shouldn't be used as an assumption.

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