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

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---
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layout: doc-page
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title: "Programmatic Structural Types - More Details"
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---
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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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## 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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## 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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- Refinements 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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## Reference
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For more info, see [Rethink Structural
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Types](https://github.com/lampepfl/dotty/issues/1886).

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

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title: "Programmatic Structural Types"
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---
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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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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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## Example
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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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<script src="https://scastie.scala-lang.org/Duhemm/HOZFKyKLTs294XOSYPU5Fw.js"></script>
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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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the database access example given at the beginning of this document.
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## Relation with `scala.Dynamic`
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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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## Example
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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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[More details](structural-types-spec.html)

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