Suppressing @unchecked warning for a higher-kinded existential type

问题

In Scala 2.10, given class Foo[F[_]], I can't write

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

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've tried placing @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's type parameters do not match type F's 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 any way to write this existential type without a warning?

回答1:

With pattern matching you can keep the warnings away:

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

It is also a bit less verbose in my opinion. In case you name the case variable (starting with lower case letter or escaped with back quotes, ie. not _ as in the above example before :), you already have an asInstanceOf.

回答2:

Kind 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

The pop-up just told me code blocks are not very informative.

Oh, and s/guessing/experimenting.

来源：https://stackoverflow.com/questions/28176807/suppressing-unchecked-warning-for-a-higher-kinded-existential-type

标签

scala

scala-2.10

higher-kinded-types