GCD issue in both old and new patmat?

[√iktor Ҡlang]

scala> (new IllegalStateException).asInstanceOf[Throwable] match { case e @ (_: IllegalStateException | _: ClassCastException) => val sanity: Exception = e }

<console>:8: error: type mismatch;

 found   : Throwable

 required: Exception

              (new IllegalStateException).asInstanceOf[Throwable] match { case e @ (_: IllegalStateException | _: ClassCastException) => val sanity: Exception = e }

--
Viktor Klang

Akka Tech Lead

Typesafe - The software stack for applications that scale

Twitter: @viktorklang

[Eugene Burmako]

I think it's because typeTest in ' static' mode emits the test that ensures that the static type of scrutinee conforms to a case. I was also puzzled by that, when tried to implement tag-aware matching for abstract types.

[√iktor Ҡlang]

Is it a bug, and if so, is there a ticket for it already?

[Adriaan Moors]

I agree this would be nice, but this is as spec'ed, I believe:

A pattern binder x@p consists of a pattern variable x and a pattern p. The type of the variable x is the static type T of the pattern p.
All alternative patterns are type checked with the expected type of the pattern.  

in your example, the expected type of the alternative pattern is Throwable, and thus, so is the type of e

(btw, to see the types using the compiler/repl:  -Xprint-types -Xprint:typer:

case (e @ ((_{IllegalStateException}: IllegalStateException){IllegalStateException}| (_{ClassCastException}: ClassCastException){ClassCastException}){Throwable}){Throwable} =>

)

[Eugene Burmako]

No idea, we did not have time to discuss this with Adriaan

On May 16, 2012 4:05 PM, "√iktor Ҡlang" <viktor...@gmail.com> wrote:
Is it a bug, and if so, is there a ticket for it already?

On Wed, May 16, 2012 at 3:01 PM, Eugene Burmako <eugene....@epfl.ch> wrote:
>

> I think it's b...

[Eugene Burmako]

Does this mean that I cannot match something of static type Tree against, say, Apply?

[Adriaan Moors]

I think we're talking about different things here.

recall Viktor's example:

(new IllegalStateException).asInstanceOf[Throwable] match { case e @ (_: IllegalStateException | _: ClassCastException) => val sanity: Exception = e }

`e` has type `Throwable` since that's how alternatives are type checked, thus you can't assign it to sanity

it's trivial to fix this in the type checker for this example, but we'd also have to update the spec

don't know if now's a good time for that (well, actually, I would say it's not)

        case Alternative(alts) =>

          val alts1 = alts mapConserve (alt => typed(alt, mode | ALTmode, pt))

          treeCopy.Alternative(tree, alts1) setType lub(alts1 map (_.tpe)) //pt

[Miles Sabin]

On Wed, May 16, 2012 at 2:20 PM, Adriaan Moors <adriaa...@epfl.ch> wrote:
> I think we're talking about different things here.
>
> recall Viktor's example:
> (new IllegalStateException).asInstanceOf[Throwable] match { case e @ (_:
> IllegalStateException | _: ClassCastException) => val sanity: Exception = e
> }
>
> `e` has type `Throwable` since that's how alternatives are type checked,
> thus you can't assign it to sanity

But surely the LUB here should be Exception not Throwable?

I'm not sure I see how Viktor's example is different from,

scala> class A ; class B extends A ; class C extends A
defined class A
defined class B
defined class C

scala> ((new B) : A) match { case a @ (_ : B | _ : C) => val sanity : A = a }

scala>

Which is just fine.

Cheers,


Miles

--
Miles Sabin
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[martin odersky]

On Wed, May 16, 2012 at 3:27 PM, Miles Sabin <mi...@milessabin.com> wrote:

On Wed, May 16, 2012 at 2:20 PM, Adriaan Moors <adriaa...@epfl.ch> wrote:
> I think we're talking about different things here.
>
> recall Viktor's example:
> (new IllegalStateException).asInstanceOf[Throwable] match { case e @ (_:
> IllegalStateException | _: ClassCastException) => val sanity: Exception = e
> }
>
> `e` has type `Throwable` since that's how alternatives are type checked,
> thus you can't assign it to sanity

But surely the LUB here should be Exception not Throwable?

No, there is no LUB computed. We can't because types in patterns flow from the root (selector type) to the leaves (variables in patterns),

 

I'm not sure I see how Viktor's example is different from,

scala> class A ; class B extends A ; class C extends A
defined class A
defined class B
defined class C

scala> ((new B) : A) match { case a @ (_ : B | _ : C) => val sanity : A = a }

Selector type is A, so a has type A.

Cheers

 - Martin

[Adriaan Moors]

Miles, your example is different from Viktor's:

contrast

  val sanity: Exception = (e: Throwable)

to

  val sanity : A = (a: A)

On Wed, May 16, 2012 at 3:27 PM, Miles Sabin <mi...@milessabin.com> wrote:

[Miles Sabin]

On Wed, May 16, 2012 at 2:30 PM, martin odersky <martin....@epfl.ch> wrote:
> No, there is no LUB computed. We can't because types in patterns flow from
> the root (selector type) to the leaves (variables in patterns),

Fair enough, but it seems a little unfortunate that,

(new IllegalStateException).asInstanceOf[Throwable] match {
case e : IllegalStateException => val sanity: Exception = e
case e : ClassCastException => val sanity : Exception = e
}

which you might naively think is equivalent to Viktor's eg. is
perfectly OK, especially since one of the main uses of pattern
alternatives is to eliminate duplicate case clause RHS's in precisely
this kind of situation.

[√iktor Ҡlang]

On Wed, May 16, 2012 at 3:43 PM, Miles Sabin <mi...@milessabin.com> wrote:

On Wed, May 16, 2012 at 2:30 PM, martin odersky <martin....@epfl.ch> wrote:
> No, there is no LUB computed. We can't because types in patterns flow from
> the root (selector type) to the leaves (variables in patterns),

Fair enough, but it seems a little unfortunate that,

(new IllegalStateException).asInstanceOf[Throwable] match {
 case e : IllegalStateException => val sanity: Exception = e
 case e : ClassCastException => val sanity : Exception = e
}

which you might naively think is equivalent to Viktor's eg. is
perfectly OK, especially since one of the main uses of pattern
alternatives is to eliminate duplicate case clause RHS's in precisely
this kind of situation.

Heck, I'd even settle for:

case (e: Exception) @ (_: ClassCastException | _: CancelledKeyException | _: IOException | _: ActorInitializationException) ⇒

But I think it's intuitive to assume that the type of the e will be the LUB of the match.

Cheers,

√

 

Cheers,


Miles

--
Miles Sabin
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