@Unchecked warning suppression for higher existential type

Question

In Scala 2.10, given class Foo[F[_]] , I cannot write

 scala> x.isInstanceOf[Foo[_]] <console>:10: error: _$1 takes no type parameters, expected: one x.isInstanceOf[Foo[_]] ^ 

or

 scala> x.isInstanceOf[Foo[_[_]]] <console>:11: error: _$1 does not take type parameters x.isInstanceOf[Foo[_[_]]] ^ 

I can write x.isInstanceOf[Foo[F] forSome { type F[_]] } which gives an unchecked warning. I tried to post the @unchecked annotation in different places, but none of them work:

 scala> x.isInstanceOf[Foo[H] @unchecked forSome {type H[_]}] <console>:11: warning: abstract type H in type Foo[H] @unchecked forSome { type H[_] <: Any } is unchecked since it is eliminated by erasure x.isInstanceOf[Foo[H] @unchecked forSome {type H[_]}] ^ scala> x.isInstanceOf[Foo[H @unchecked] forSome {type H[_]}] <console>:11: warning: abstract type H in type Foo[H @unchecked] is unchecked since it is eliminated by erasure x.isInstanceOf[Foo[H @unchecked] forSome {type H[_]}] ^ <console>:11: error: kinds of the type arguments (? @unchecked) do not conform to the expected kinds of the type parameters (type F) in class Foo. ? @unchecked type parameters do not match type F expected parameters: <none> has no type parameters, but type F has one x.isInstanceOf[Foo[H @unchecked] forSome {type H[_]}] ^ scala> x.isInstanceOf[Foo[H] forSome {type H[_] @unchecked}] <console>:1: error: `=', `>:', or `<:' expected x.isInstanceOf[Foo[H] forSome {type H[_] @unchecked}] ^ 

Is there a way to write this existential type without warning?

Answer 1

Given the pattern matching, you can leave warnings:

 x match {case _: Foo[_] => ???} 

It is also a bit less verbose in my opinion. If you name the case variable (starting with a lowercase letter or escaping with quotes, i.e. Not _ , as in the previous example, before : , you already have asInstanceOf .

Answer 2

Type of guessing:

 $ scala210 -language:_ Welcome to Scala version 2.10.4 (OpenJDK 64-Bit Server VM, Java 1.7.0_65). Type in expressions to have them evaluated. Type :help for more information. scala> class Foo[F[_]] defined class Foo scala> (null: Any).isInstanceOf[(Foo[F] forSome { type F[_] }) @unchecked] res0: Boolean = false 

A popup just informed me that the code blocks are not very informative.

Oh and s/guessing/experimenting .