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More details about structural types
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‎compiler/src/dotty/tools/dotc/typer/Dynamic.scala‎

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@@ -117,7 +117,7 @@ trait Dynamic { self: Typer with Applications =>
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*
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* If `U` is a value type, map `x.a` to the equivalent of:
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*
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* (x: Selectable).selectDynamic(x, "a").asInstanceOf[U]
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* (x: Selectable).selectDynamic("a").asInstanceOf[U]
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*
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* If `U` is a method type (T1,...,Tn)R, map `x.a` to the equivalent of:
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*

‎docs/docs/reference/changed/structural-types.md‎

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title: "Programmatic Structural Types"
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---
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Previously, Scala supported structural types by means of
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reflection. This is problematic on other platforms, because Scala's
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reflection is JVM-based. Consequently, Scala.js and Scala.native don't
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support structural types fully. The reflection based implementation is
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also needlessly restrictive, since it rules out other implementation
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schemes. This makes structural types unsuitable for e.g. modelling
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rows in a database, for which they would otherwise seem to be an ideal
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match.
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Dotty allows to implement structural types programmatically, using
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"Selectables". `Selectable` is a trait defined as follows:
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trait Selectable extends Any {
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def selectDynamic(name: String): Any
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def selectDynamicMethod(name: String, paramClasses: ClassTag[_]*): Any =
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new UnsupportedOperationException("selectDynamicMethod")
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}
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The most important method of a `Selectable` is `selectDynamic`: It
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takes a field name and returns the value associated with that name in
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the selectable.
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Assume now `r` is a value with structural type `S`. In general `S` is
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of the form `C { Rs }`, i.e. it consists of a class reference `C` and
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refinement declarations `Rs`. We call a field selection `r.f`
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_structural_ if `f` is a name defined by a declaration in `Rs` whereas
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`C` defines no member of name `f`. Assuming the selection has type
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`T`, it is mapped to something equivalent to the following code:
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(r: Selectable).selectDynamic("f").asInstanceOf[T]
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That is, we make sure `r` conforms to type `Selectable`, potentially
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by adding an implicit conversion. We then invoke the `get` operation
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of that instance, passing the the name `"f"` as a parameter. We
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finally cast the resulting value back to the statically known type
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`T`.
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`Selectable` also defines another access method called
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`selectDynamicMethod`. This operation is used to select methods
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instead of fields. It gets passed the class tags of the selected
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method's formal parameter types as additional arguments. These can
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then be used to disambiguate one of several overloaded variants.
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Package `scala.reflect` contains an implicit conversion which can map
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any value to a selectable that emulates reflection-based selection, in
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a way similar to what was done until now:
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package scala.reflect
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object Selectable {
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implicit def reflectiveSelectable(receiver: Any): scala.Selectable =
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receiver match {
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case receiver: scala.Selectable => receiver
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case _ => new scala.reflect.Selectable(receiver)
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}
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}
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When imported, `reflectiveSelectable` provides a way to access fields
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of any structural type using Java reflection. This is similar to the
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current implementation of structural types. The main difference is
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that to get reflection-based structural access one now has to add an
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import:
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import scala.reflect.Selectable.reflectiveSelectable
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On the other hand, the previously required language feature import of
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`reflectiveCalls` is now redundant and is therefore dropped.
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As you can see from its implementation above, `reflectSelectable`
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checks first whether its argument is already a run-time instance of
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`Selectable`, in which case it is returned directly. This means that
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reflection-based accesses only take place as a last resort, if no
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other `Selectable` is defined.
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Other selectable instances can be defined in libraries. For instance,
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here is a simple class of records that support dynamic selection:
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case class Record(elems: (String, Any)*) extends Selectable {
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def selectDynamic(name: String): Any = elems.find(_._1 == name).get._2
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}
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`Record` consists of a list of pairs of element names and values. Its
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`selectDynamic` operation finds the pair with given name and returns
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its value.
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For illustration, let's define a record value and cast it to a
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structural type `Person`:
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type Person = Record { val name: String; val age: Int }
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val person = Record("name" -> "Emma", "age" -> 42).asInstanceOf[Person]
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Then `person.name` will have static type `String`, and will produce `"Emma"` as result.
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The safety of this scheme relies on the correctness of the cast. If
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the cast lies about the structure of the record, the corresponding
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`selectDynamic` operation would fail. In practice, the cast would
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likely be part if a database access layer which would ensure its
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correctness.
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### Notes:
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1. The scheme does not handle polymorphic methods in structural
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refinements. Such polymorphic methods are currently flagged as
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errors. It's not clear whether the use case is common enough to
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warrant the additional complexity of supporting it.
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2. There are clearly some connections with `scala.Dynamic` here, since
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## Syntax
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```
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SimpleType ::= ... | Refinement
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Refinement ::= ‘{’ RefineStatSeq ‘}’
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RefineStatSeq ::= RefineStat {semi RefineStat}
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RefineStat ::= ‘val’ VarDcl | ‘def’ DefDcl | ‘type’ {nl} TypeDcl
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```
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## Motivation
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Some usecases, such as modelling database access, are more awkward in
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statically typed languages than in dynamically typed languages: With
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dynamically typed languages, it's quite natural to model a row as a
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record or object, and to select entries with simple dot notation (e.g.
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`row.columnName`).
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Achieving the same experience in statically typed
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language requires defining a class for every possible row arising from
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database manipulation (including rows arising from joins and
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projections) and setting up a scheme to map between a row and the
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class representing it.
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This requires a large amount of boilerplate, which leads developers to
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trade the advantages of static typing for simpler schemes where colum
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names are represented as strings and passed to other operators (e.g.
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`row.select("columnName")`). This approach forgoes the advantages of
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static typing, and is still not as natural as the dynamically typed
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version.
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Structural types help in situations where we would like to support
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simple dot notation in dynamic contexts without losing the advantages
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of static typing. They allow developers to use dot notation and
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configure how fields and methods should be resolved.
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An [example](#example) is available at the end of this document.
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## Implementation of structural types
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The standard library defines a trait `Selectable` in the package
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`scala`, defined as follows:
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```scala
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trait Selectable extends Any {
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def selectDynamic(name: String): Any
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def selectDynamicMethod(name: String, paramClasses: ClassTag[_]*): Any =
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new UnsupportedOperationException("selectDynamicMethod")
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}
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```
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An implementation of `Selectable` that relies on Java reflection is
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available in the standard library: `scala.reflect.Selectable`. Other
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implementations can be envisioned for platforms where Java reflection
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is not available.
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`selectDynamic` takes a field name and returns the value associated
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with that name in the `Selectable`. Similarly, `selectDynamicMethod`
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takes a method name, `ClassTag`s representing its parameters types and
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will return the function that matches this
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name and parameter types.
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Given a value `v` of type `C { Rs }`, where `C` is a class reference
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and `Rs` are refinement declarations, and given `v.a` of type `U`, we
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consider three distinct cases:
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- If `U` is a value type, we map `v.a` to the equivalent of:
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```scala
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v.a
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--->
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(v: Selectable).selectDynamic("a").asInstanceOf[U]
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```
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- If `U` is a method type `(T1, ..., Tn) => R` with at most 7
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parameters and it is not a dependent method type, we map `v.a` to
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the equivalent of:
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```scala
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v.a
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--->
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(v: Selectable).selectDynamic("a", CT1, ..., CTn).asInstanceOf[(T1, ..., Tn) => R]
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```
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- If `U` is neither a value nor a method type, or a dependent method
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type, or has more than 7 parameters, an error is emitted.
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We make sure that `r` conforms to type `Selectable`, potentially by
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introducing an implicit conversion, and then call either
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`selectDynamic` or `selectMethodDynamic`, passing the name of the
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member to access and the class tags of the formal parameters, in the
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case of a method call. These parameters could be used to disambiguate
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one of several overload variants in the future, but overloads are not
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supported in structural types at the moment.
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## Extensibility
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New instances of `Selectable` can be defined to support means of
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access other than Java reflection, which would enable usages such as
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the database access example given in the "Motivation" section.
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## Limitations of structural types
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- Methods with more than 7 formal parameters cannot be called via
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structural call.
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- Dependent methods cannot be called via structural call.
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- Overloaded methods cannot be called via structural call.
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- Refinement do not handle polymorphic methods.
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## Differences with Scala 2 structural types
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- Scala 2 supports structural types by means of Java reflection. Unlike
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Scala 3, structural calls do not rely on a mechanism such as
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`Selectable`, and reflection cannot be avoided.
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- In Scala 2, structural calls to overloaded methods are possible.
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- In Scala 2, mutable `var`s are allowed in refinements. In Scala 3,
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they are no longer allowed.
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## Migration
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Receivers of structural calls need to be instances of `Selectable`. A
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conversion from `Any` to `Selectable` is available in the standard
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library, in `scala.reflect.Selectable.reflectiveSelectable`. This is
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similar to the implementation of structural types in Scala 2.
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## Relation with `scala.Dynamic`
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There are clearly some connections with `scala.Dynamic` here, since
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both select members programmatically. But there are also some
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differences.
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- Fully dynamic selection is not typesafe, but structural selection
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is, as long as the correspondence of the structural type with the
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underlying value is as stated.
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- Fully dynamic selection is not typesafe, but structural selection
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is, as long as the correspondence of the structural type with the
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underlying value is as stated.
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- `Dynamic` is just a marker trait, which gives more leeway where and
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how to define reflective access operations. By contrast
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`Selectable` is a trait which declares the access operations.
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- `Dynamic` is just a marker trait, which gives more leeway where and
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how to define reflective access operations. By contrast
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`Selectable` is a trait which declares the access operations.
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- One access operation, `selectDynamic` is shared between both
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approaches, but the other access operations are
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different. `Selectable` defines a `selectDynamicMethod`, which
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takes class tags indicating the method's formal parameter types as
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additional argument. `Dynamic` comes with `applyDynamic` and
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`updateDynamic` methods, which take actual argument values.
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- One access operation, `selectDynamic` is shared between both
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approaches, but the other access operations are
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different. `Selectable` defines a `selectDynamicMethod`, which
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takes class tags indicating the method's formal parameter types as
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additional argument. `Dynamic` comes with `applyDynamic` and
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`updateDynamic` methods, which take actual argument values.
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### Reference
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## Example
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For more info, see [Issue #1886](https://github.com/lampepfl/dotty/issues/1886).
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<script src="https://scastie.scala-lang.org/Duhemm/HOZFKyKLTs294XOSYPU5Fw.js"></script>
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## Reference
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For more info, see [Rethink Structural
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Types](https://github.com/lampepfl/dotty/issues/1886).

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