A fully defined trait cannot be instantiated

a trait cannot be instantiated, only mixed in. you need a class for instantiation, new T {} creates such an anonymous class.

best, -sciss-

On 24 Apr 2011, at 19:55, Julien Richard-Foy wrote:

> In the below code:
>
> scala> trait T { def foo = "hello" }
> defined trait T
>
> scala> new T
> :7: error: trait T is abstract; cannot be instantiated
> new T
> ^
>
> scala> new T {}
> res1: java.lang.Object with T = $anon$1@5e159d10
>
> Is this behavior expected? Why do I need to write braces to be able
> to
> instanciate my trait? Why does the error message say my trait is
> abstract?
>
> Regards,
> Julien

short answer: everything is working as it should
long answer:
a trait is like an enhances interface in java, it cannot be created
directly. you need to subclass it, which is what you do by adding {}.

Am 24.04.2011 20:55, schrieb Julien Richard-Foy:
> In the below code:
>
> scala> trait T { def foo = "hello" }
> defined trait T
>
> scala> new T
> :7: error: trait T is abstract; cannot be instantiated
> new T
> ^
>
> scala> new T {}
> res1: java.lang.Object with T = $anon$1@5e159d10
>
> Is this behavior expected? Why do I need to write braces to be able
> to
> instanciate my trait? Why does the error message say my trait is
> abstract?
>
> Regards,
> Julien
>

On Sunday April 24 2011, HamsterofDeath wrote:
> short answer: everything is working as it should
> long answer:
> a trait is like an enhances interface in java, it cannot be created
> directly. you need to subclass it, which is what you do by adding {}.

In particular, traits have no constructors
and thus cannot be constructed.

Randall Schulz

hmm....

scala> trait T { println( "gaga" )}
defined trait T

scala> new T {}
gaga
res0: java.lang.Object with T = $anon$1@17553743

it seems they have a constructor body (although no constructor arguments). and they do have type parameters!

best, -sciss-

On 24 Apr 2011, at 20:21, Randall R Schulz wrote:

> On Sunday April 24 2011, HamsterofDeath wrote:
>> short answer: everything is working as it should
>> long answer:
>> a trait is like an enhances interface in java, it cannot be created
>> directly. you need to subclass it, which is what you do by adding {}.
>
> In particular, traits have no constructors
> and thus cannot be constructed.
>
>
> Randall Schulz

On Sunday April 24 2011, Sciss wrote:
> hmm....
>
> scala> trait T { println( "gaga" )}
> defined trait T
>
> scala> new T {}
> gaga
> res0: java.lang.Object with T = $anon$1@17553743

You wrote the constructor body (an empty one).
You instantiated a class derived from the trait T, not T itself.

> it seems they have a constructor body (although no constructor
> arguments). and they do have type parameters!
>
> best, -sciss-

Randall Schulz