scala/scala3
Voir sur GitHubMatch type syntax does not allow writing type patterns for type members directly
Open
#13 416 ouverte le 29 août 2021
area:match-typeshelp wanteditype:enhancement
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Description
Minimized code
trait RDF {
type Triple
}
object RDF {
type Triple[R <: RDF] = R match {
case (RDF { type Triple = t }) => t
}
type GetTriple[T] = RDF { type Triple = T }
}
Output
Not found: type t
Expectation
Expected to be able to extract a variable type from inside a type refinement.
This syntax limit can be worked around by introducing a "pattern type":
trait RDF {
type Triple
}
object RDF {
type Triple[R <: RDF] = R match {
case GetTriple[t] => t
}
type GetTriple[T] = RDF { type Triple = T }
}
Why write something like that? Apparently this pattern of extracting a field using a match type can substitute for some usages of generalized type projections and is enough to port some of the libraries using type projections, e.g. https://github.com/bblfish/banana-play