Match type syntax does not allow writing type patterns for type members directly

[neko-kai]

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

[bblfish]

It looks very promising indeed.

To port banana-rdf to Scala 3 I need to remove all the type projections in banana-rdf, which fundamentally dependent on them.
This use of match types looks very promising. I am trying things out here src/test/scala/ideas/MatchTypes.scala

Previously I had tried dependent types in the same repo, but those were starting to look very bad.
I had been discussing this in dotty discussions thread 12527.

[OlivierBlanvillain]

I gave this a shot today, but it turns out to be trickier than it looks because of the way refinement types are handled in the compiler. I think having to define a type alias here isn't the end of the world. Scala already has many oddities with types patterns, match types naturally inherits some of them:

// With 2.13
scala> new Object match { case _: Any { type T = t } => null }                                  
                                                 ^                                              
       error: not found: type t

scala> type Foo[T] = Any { type X = T }
defined type alias Foo

scala> new Object match { case _: Foo[t] => null }                                                            
res1: Null = null

scala> new Object match { case _: t => null }
                                  ^
       error: not found: type t

scala> type Id[X] = X
defined type alias Id

scala> new Object match { case _: Id[t] => null }                                               
res3: Null = null

[bblfish]

There was a long discussion on the contributors forum Algebras of opaque types and pattern matching.

I have succeeded in getting this to work In quite a big project, as you can see in the scala-3 branch of banana-rdf.

The only worry I have is that I get the following types of exceptions, even though I have exceptions such as the following even though there are unapply and TypeTests available.

[warn] -- Unchecked Warning: /Volumes/Dev/Programming/Scala3/banana-rdf/scratch/shared/src/main/scala/org/w3/banana/test3/Scastie.scala:137:5 
[warn] 137 |		case t : banana.RDF.Literal[R] => "success matching "+t
[warn]     |		     ^
[warn]     |the type test for org.w3.banana.test3.banana.RDF.Literal[R] cannot be checked at runtime
[warn] -- Unchecked Warning: /Volumes/Dev/Programming/Scala3/banana-rdf/rdf-test-suite/shared/src/main/scala/org/w3/banana/TripleTest.scala:41:5 
[warn] 41 |		case t : RDF.Literal[R] =>
[warn]    |		     ^
[warn]    |the type test for org.w3.banana.RDF.Literal[R] cannot be checked at runtime
[warn] -- Unchecked Warning: /Volumes/Dev/Programming/Scala3/banana-rdf/rdf-test-suite/shared/src/main/scala/org/w3/banana/TripleTest.scala:149:18 
[warn] 149 |			case Triple(s, p, l: Literal[R]) =>
[warn]     |			                  ^
[warn]     |the type test for org.w3.banana.RDF.Literal[R] cannot be checked at runtime

[neko-kai]

@OlivierBlanvillain

I think having to define a type alias here isn't the end of the world.

As I see it, the bigger issue is that the lack of syntax makes it seem as if it's impossible to do this in the first place.

Scala already has many oddities with types patterns, match types naturally inherits some of them:

In case of value matches, extracting a type member into a variable is redundant, since you could instead refer to it with a selection

new {} match { case t: { type T } => summon[t.T] }

The equivalent ability for types has been removed in Scala 3 ;)

[bblfish]

I am using this pattern quite heavily.

In banana-rdf for Scala3 I start by defining a set of types and specify an inheritance hierarchy for them.
eg

trait RDF: 
  ...
  type Node <: rNode
  type BNode <: Node
  type URI <: Node & rURI

Then I put together an object RDF (below line 115) that is able to find those types given a subtype of RDF. So for example:

 type URI[R <: RDF] <: Matchable = R match
      case GetURI[u] => u

 private type GetURI[U <: Matchable]              = RDF { type URI = U }

But I'd like to capture that `URI[R <: RDF] is a subtype of Node[R] as in the initial RDF trait. I can't see how to do that....

Without that it seems like I need to write a lot of implicit conversion objects for every pair of inherited types. This is still doable here but perhaps there is a simpler solution?

[neko-kai]

@bblfish
Maybe constraining GetURI parameter could help?

type URI[R <: RDF] <: Node = R match case GetURI[u] => u

private type GetURI[U <: Node]

[bblfish]

The node type will depend on the RDF subclass. So the best I could do I think is:

    type Node[R <: RDF] = //URI[R] | BNode[R] | Literal[R]
       R match
          case GetNode[n] => n

   type URI[R <: RDF] <: Node[R] = R match
          case GetURI[u] => u

   private type GetURI[U]              = RDF { type URI = U }
   private type GetNode[N]             = RDF { type Node = N }

I get the error

[error] -- [E007] Type Mismatch Error: /Volumes/Dev/hjs/Programming/Scala3/CoSy/banana-rdf/scala3/rdf/shared/src/main/scala/org/w3/banana/RDF.scala:130:24
[error] 130 |      case GetURI[u] => u
[error]     |                        ^
[error]     |               Found:    u
[error]     |               Required: org.w3.banana.RDF.Node[R]
[error]     |
[error]     |               Note: a match type could not be fully reduced:
[error]     |
[error]     |                 trying to reduce  org.w3.banana.RDF.Node[R]
[error]     |                 failed since selector  R
[error]     |                 does not match  case org.w3.banana.RDF{Node = n} => n
[error]     |                 and cannot be shown to be disjoint from it either.
[error]     |
[error]     | longer explanation available when compiling with `-explain`
[error] one error found
[error] (rdfJVM / Compile / compileIncremental) Compilation failed
[error] Total time: 0 s, completed Oct 18, 2022, 12:19:54 PM

I tried to see if I could give more information about URI in GetURI

   type URI[R <: RDF] <: Node[R] = R match
      case GetURI[R, u] => u

   private type GetURI[R <: RDF, U <: Node[R]]              = RDF { type URI = U }

Which gives me the error

[error] -- [E007] Type Mismatch Error: /Volumes/Dev/hjs/Programming/Scala3/CoSy/banana-rdf/scala3/rdf/shared/src/main/scala/org/w3/banana/RDF.scala:130:27
[error] 130 |      case GetURI[R, u] => u
[error]     |                           ^
[error]     |Found:    u
[error]     |Required: org.w3.banana.RDF.Node[R]
[error]     |
[error]     |where:    u is a type in type URI with bounds <: org.w3.banana.RDF.Node[R]
[error]     |
[error]     |
[error]     |Note: a match type could not be fully reduced:
[error]     |
[error]     |  trying to reduce  org.w3.banana.RDF.Node[R]
[error]     |  failed since selector  R
[error]     |  does not match  case org.w3.banana.RDF{Node = n} => n
[error]     |  and cannot be shown to be disjoint from it either.
[error]     |
[error]     | longer explanation available when compiling with `-explain`
[error] one error found

Perhaps I should try to work out how this -explain flag works from sbt... Ah found it explained here and it works...

Btw. this pattern I think is extremely useful, and could be widely used across the scala ecosystem to bring many libraries written by different groups in different languages under a common API. I have used this to help me integrate Http4s and Akka for example here https://github.com/bblfish/httpSig/blob/trackIO/ietfSig/shared/src/main/scala/run/cosy/http/Http.scala