The union type of any pair of singleton types is equivalent to their supertype

[propensive]

There are a couple of ways of demonstrating this. The following does not produce a type error when it should:

def foo: 1 | 2 = 1
def bar: 3 | 4 = foo

whereas this does (correctly):

def foo: 1 | 2 = 1
def bar: 1 = foo

Intersection types do, however, appear to work correctly. This produces the expected type error:

trait X {
  def foo: 1 & 2
  def bar: 2 & 3 = foo
}

[propensive]

Another useful test is to check whether this compiles:

implicitly[1 & (2 | 3) =:= (1 & 2) | (1 & 3)]

[smarter]

It does with #844 if you add a few parentheses to avoid precedence issues:

object Test {
  implicitly[(1 & (2 | 3)) =:= ((1 & 2) | (1 & 3))]
}

[propensive]

Great!

Do you have any thoughts on the precedence rules for type operators, in light of | and & becoming more widely used at the type-level, @odersky?

[smarter]

Yes, it's especially confusing for pattern matching:

x match {
  case x: 1 | 2 => // means case (x: 1) | 2, but is likely to be interpreted as case x: (1 | 2)
  case x @ 1 | 2 => // means case (x @ 1) | 2, but is likely to be interpreted as case x @ (1 | 2)
}

[odersky]

The idea to take precedence from Java is still valid I think. Tweaking here
else risks becoming bikeshedding.

• Martin

On Thu, Oct 22, 2015 at 9:45 PM, Guillaume Martres <notifications@github.com

wrote:

Yes, it's especially confusing for pattern matching:

x match {
case x: 1 | 2 => // means case (x: 1) | 2, but is likely to be interpreted as case x: (1 | 2)
case x @ 1 | 2 => // means case (x @ 1) | 2, but is likely to be interpreted as case x @ (1 | 2)
}

—
Reply to this email directly or view it on GitHub
#829 (comment).

Martin Odersky
EPFL

[smarter]

Fair enough, but note that this means that porting uses of intersection types from scalac to dotty is not trivial because with and & do not have the same precedence:

trait A
trait B
object Test {
  (new A with B) match { case x: A with B => x } // OK
  (new A with B) match { case x: A & B => x } // error
  (new A with B) match { case x: (A & B) => x } // OK
}
`

[odersky]

Superseded by #1551