Allow given instances to alias structural instances

[mpilquist]

Consider a structural given:

object Instances:
  given listInstance: Monad[List] with Traverse[List] with ...

Now imagine we want to define an alias for this instance in another module:

object Functor:
  given Monad[List] with Traverse[List] = Instances.listInstance

This is not currently supported. Structural instances cannot be aliased via =. Note the following works fine though:

object Functor:
  export Instances.listInstance

[LPTK]

What is a structural given? I don't think I've seen such terminology before.

Can't you just write the following?:

  given Monad[List] & Traverse[List] = Instances.listInstance

[bishabosha]

What is a structural given? I don't think I've seen such terminology before.

It is the terminology for a given where there is no "rhs"
e.g. given FromString[Int] with {def fromString(s: String) = s.toInt} or given Ctx() (simplified from given Ctx with {})

[mpilquist]

Thanks @bishabosha. I grabbed the term from the grammar:

TmplDef             ::=  ...
                     |   ‘given’ GivenDef
GivenDef            ::=  [GivenSig] StructuralInstance
                     |   [GivenSig] AnnotType ‘=’ Expr
                     |   [GivenSig] AnnotType
GivenSig            ::=  [id] [DefTypeParamClause] {UsingParamClause} ‘:’
StructuralInstance  ::=  ConstrApp {‘with’ ConstrApp} ‘with’ TemplateBody

https://docs.scala-lang.org/scala3/reference/contextual/givens.html#syntax

[neko-kai]

@LPTK

What is a structural given? I don't think I've seen such terminology before.

Can't you just write the following?:

  given Monad[List] & Traverse[List] = Instances.listInstance

But you can't! Although you can with parentheses:

trait Monad[F[_]]
trait Traverse[F[_]]

object Instances:
  given listInstance: (Monad[List] & Traverse[List]) = new Monad[List] with Traverse[List] {}

object Functor:
  given (Monad[List] & Traverse[List]) = Instances.listInstance

@main def main = { import Functor.given ; println(summon[Monad[List]] -> summon[Traverse[List]]) }

https://scastie.scala-lang.org/Y006CKiITOO1hz8Hivy2fA

Essentially everything in the feature request works, but requires parentheses in a way that's inconsistent with the rest of the grammar – you don't need parentheses for an equivalent def or val definition.
Parentheses could be justified because with in given starts a template definition. But & does not start a definition, so there's no particular reason the above code should require parentheses around the type, I think.

[mpilquist]

@neko-kai Nice! I had tried the intersection but hadn't thought to use parens!