Scala ClassManifest instead of Type[T]

Question

The following code

def httpPost[T: ClassManifest](data: AnyRef): T = {
  val webResource = client.resource("http://localhost..")
  val resp = webResource.post(classOf[ClientResponse], data)
  resp.getEntity(classManifest[T].erasure) //Need classOf[T] here 
}

results in this type mismatch compilation error

[INFO]  found   : _$1 where type _$1
[INFO]  required: T
[INFO]       resp.getEntity(classManifest[T].erasure)

Based on the answer to Scala classOf for type parameter it looks like it should work.

The erasure method returns java.lang.Class[_] and I presume that this is the problem so I have two questions:

Why does the class manifest return an existential type and not simply Class[T] - if it's the erasure of T, surely that will always be _ (underscore) because T is obviously unknown, which means its return value isn't as useful as I'd have expected.
What do I need to do to make the code work!

Update:

Thanks Kim and Jean-Phillipe for your answers.

I had previously tried a cast so the original last line was replaced with

val responseData = resp.getEntity(classManifest[T].erasure) //Runtime error
responseData.asInstanceOf[T]

and this compiles but there's now a runtime error because the getEntity method is passed the class of Object, which it can't process because it needs a more specific type (for which it has a handler). Although it's deferred until runtime, it again comes down to the erasure method not giving specific type information and that's why I thought that to solve the problem, the inline example must be solved.

score 2 · Accepted Answer · answered Jun 17 '11 at 01:42

There's something seriously wrong with this code. In particular:

def httpPost[T: ClassManifest](data: AnyRef): T = {
  val webResource = client.resource("http://localhost..")
  val resp = webResource.post(classOf[ClientResponse], data)
  resp.getEntity(classManifest[T].erasure) //Need classOf[T] here 
}

How is Scala supposed to know what the type of T is? Are you passing it explicitly when invoking httpPost? I suspect not, and that's the reason why erasure is returning Object for you.

As for why ClassManifest#erasure returns Class[_] instead of something else, I suspect the reason is that this is the type used by most Java methods, and since Class is invariant, if erasure returned Class[T], then you'd have to cast it to use it with those methods!

Thanks for for your comments. There's nothing wrong with the code as per my original update (about the cast). However, I wasn't using a type parameter at the invocation site - I thought that it would be inferred from the assignment target. It worked with that addition. — Dextaa, Jun 20 '11 at 09:23

score 0 · Answer 2 · answered Jun 16 '11 at 08:00

0

First question: No idea...

Second question: I think it is safe to cast here. You can use foo.asInstanceOf[Class[T]].

answered Jun 16 '11 at 08:00

Kim Stebel

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score 0 · Answer 3 · answered Jun 16 '11 at 08:25

I believe that an existential type is returned to make it clear that the cast that you may want to make is your responsibility. Class is a bit weird: for instance, a Class[List[String]] should actually be typed as a Class[List[_]] as it does not carry any information about the String parametrization of List. The cast suggested by Kim is always safe when T is not itself a parametrized type.

Scala ClassManifest instead of Type[T]

3 Answers3