Compiler forbids to match a generic object if type parameters are declared at the class level

[Piotr Kolaczkowski]

scala> trait Token[T]

defined trait Token

scala> trait TokenFactory[V, T <: Token[V]]

defined trait TokenFactory

scala> object TF1 extends TokenFactory[Long, Token[Long]]

defined module TF1

scala> def checkTF[V, T <: Token[V]](tf: TokenFactory[V, T]) = tf match { case TF1 => "TF1"; case _ => "other" }

checkTF: [V, T <: Token[V]](tf: TokenFactory[V,T])Any

scala> checkTF(TF1)

res1: String = TF1    /

// Now the only difference is we put it in the class:

trait Foo[V, T <: Token[V]] {  def checkTF(tf: TokenFactory[V, T]) = tf match { case TF1 => "TF1"; case _ => "other" } }

<console>:12: error: pattern type is incompatible with expected type;

 found   : TF1.type

 required: TokenFactory[V,T]

Note: if you intended to match against the class, try `case _: <none>`

       trait Foo[V, T <: Token[V]] {  def checkTF(tf: TokenFactory[V, T]) = tf match { case TF1 => println("TF1"); case _ => "other" } }

                                                                                            ^

Why? I don't understand why the place of declatring the type parameters (class / method) changes pattern matching validity.

Thanks,

Piotr

[Jason Zaugg]

On Sun, Jun 22, 2014 at 10:44 AM, Piotr Kolaczkowski <pkol...@datastax.com> wrote:

Why? I don't understand why the place of declatring the type parameters (class / method) changes pattern matching validity.

The difference comes down to the way that pattern type inference replaces method type parameters (by using freeTypesOfTerms) with fresh type variables when checking of the pattern type and scrutinee type are compatible. 

I don't know the rationale for this. I've cc-ed Martin who might be able to help out.

​

-jason

[martin odersky]

On Sun, Jun 22, 2014 at 10:44 AM, Piotr Kolaczkowski <pkol...@datastax.com> wrote:

In the method case, what you have here is a GADT: Patterns determine the type parameters of an corresponding methods in the scope of a pattern case. GADTs are not available for class parameters.

As far as I know, nobody has yet explored this combination, and it looks like it would be quite tricky to get this right.

 - Martin

Thanks,

Piotr

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Martin Odersky
EPFL