For reference, The Book has explanatory material in 19.4, 19.5.
http://www.artima.com/pins1ed/type-parameterization.html


On Tue, Feb 21, 2012 at 12:36 PM, Adam Warski <adam@xxxxxxxxxx> wrote:

> Hello,
>
> I have a (sealed) series of commands, each of which has a different return
> type. The basic trait is a Command[R], where R is the return type.
> There is then a command executor, which has a def execute[R](cmd:
> Command[R]): R, and executes an action appropriate for a command using
> pattern matching.
> All works well until I add a variance annotation (+R) to the trait:
>
> https://gist.github.com/1878718
>
> Any ideas why? :)
>
> Thanks,
> Adam
>