Let's say I have the following action which does at first a read operation and then updates the fields.

action1: DBIOAction[Any, NoStream, Read with Write]

Let action2 do another read operation on the same table. To do the three operation sequentially I do the following:

val action3 = action1 andThen action2

So I presume the output should be of the format.

action3: DBIOAction[Any, NoStream, Read with Write with Read]

But when I see that the output is of the form again :

DBIOAction[Any, NoStream, Read with Write]

Indeed in the method's signature I wrote DBIOAction[Any, NoStream, Read with Write with Read] but the IntelliJ does not complain. This does not seem correct. Am I doing a mistake ?

CodePudding user response：

Read with Write with Read and Read with Write are mostly the same type:

implicitly[Read with Write with Read =:= Read with Write] // compiles
implicitly[Read with Write =:= Read with Write with Read] // compiles
implicitly[Write with Read =:= Read with Write] // compiles
implicitly[Read with Write =:= Write with Read] // compiles

although according to the spec they seem to be not equivalent (≡):

Two compound types are equivalent if the sequences of their component are pairwise equivalent, and occur in the same order, and their refinements are equivalent.

Looking into compiler:

https://github.com/scala/scala/pull/3981

RefinedType#normalize is responsible for flattening nested compound types to a flat representation.

Types are normalized during =:= in search of a successful result.

This means that ((A with B) with C) =:= (A with B with C).

https://github.com/retronym/scala/blob/a9182fbeaf018c1aa0f88f0aee7b921383b746f2/src/reflect/scala/reflect/internal/Types.scala#L1601-L1619

private def normalizeImpl = {
  // TODO see comments around def intersectionType and def merge
  // SI-8575 The dealias is needed here to keep subtyping transitive, example in run/t8575b.scala
  def flatten(tps: List[Type]): List[Type] = {
    def dealiasRefinement(tp: Type) = if (tp.dealias.isInstanceOf[RefinedType]) tp.dealias else tp
    tps map dealiasRefinement flatMap {
      case RefinedType(parents, ds) if ds.isEmpty => flatten(parents)
      case tp => List(tp)
    }
  }
  val flattened = flatten(parents).distinct
  if (decls.isEmpty && hasLength(flattened, 1)) {
    flattened.head
  } else if (flattened != parents) {
    refinedType(flattened, if (typeSymbol eq NoSymbol) NoSymbol else typeSymbol.owner, decls, NoPosition)
  } else if (isHigherKinded) {
    etaExpand
  } else super.normalize
}

Notice the usage of flatten(...) and .distinct.