How to get around of invariance of MongoRecord in Scala

On Sat, Oct 15, 2011 at 05:23:10PM -0700, Lucas Wang wrote:
> Say I have a class defined as follows:
>
> class A extends MongoRecord[A]{
>
> }
> Now I need to create a new class B, that is a subclass of A:
>
> class B extends A{
>
> }
> Object B extends B with MongoMetaRecord[B]

Hi Lucas,

In my opinion the easiest way to fix this would be to create a trait
with A's implementation (call it whatever you want, say Impl) which
does not extend MongoRecord[A]. Then your definitions can be:

class A extends MongoRecord[A] with Impl { ... }

class B extends MongoRecord[B] with Impl { ... }

When writing Scala I find myself making most of my (non-abstract)
classes final, and using traits to share implementation. In these cases
it's easier to dodge inheritance issues like this one.

On Sun, Oct 16, 2011 at 11:23 AM, Lucas Wang wrote:
> class A extends MongoRecord[A]{
>
> }
> Now I need to create a new class B, that is a subclass of A:
>
> class B extends A{
>
> }
> Object B extends B with MongoMetaRecord[B]
>
> The compiler would give an error like: type arguments [B] do not
> conform to trait MongoMetaRecord's type parameter bounds [BaseRecord
> <: net.liftweb.mongodb.record.MongoRecord[BaseRecord]]
> [error] object B extends B with MongoMetaRecord[B]
>
> It seems that the class B inherits MongoRecord[A], but because of the
> invariance of MongoRecord, MongoRecord[B] cannot substitute
> MongoRecord[A]. And therefore class B does not conform to the type
> constraint. Any idea how to get around this problem? Thanks a lot.
>
Hi Lucas,

I dont know how MongoRecord works, but if its mutable it has to be
invariant to be typesafe. See Array Variance here for an example
[http://www.artima.com/pins1ed/type-parameterization.html#19.3]

I like to break up invariant interfaces into separate Covariant and
Contravariant parts, but I realize this isnt an option for you if its
Lift's code.

Basically, MongoRecord isnt designed to support passing a MR[B] as an
MR[A], where B <: A I think you'll need to change your design.

-Ben

On Sun, Oct 16, 2011 at 11:23 AM, Lucas Wang <wangjiangzhe2007@gmail.com> wrote:
> class A extends MongoRecord[A]{
>
> }
> Now I need to create a new class B, that is a subclass of A:
>
> class B extends A{
>
> }
> Object B extends B with MongoMetaRecord[B]
>
> The compiler would give an error like: type arguments [B] do not
> conform to trait MongoMetaRecord's type parameter bounds [BaseRecord
> <: net.liftweb.mongodb.record.MongoRecord[BaseRecord]]
> [error] object B extends B with MongoMetaRecord[B]
>
> It seems that the class B inherits MongoRecord[A], but because of the
> invariance of MongoRecord, MongoRecord[B] cannot substitute
> MongoRecord[A]. And therefore class B does not conform to the type
> constraint. Any idea how to get around this problem? Thanks a lot.
>
Hi Lucas,

I dont know how MongoRecord works, but if its mutable it has to be
invariant to be typesafe. See Array Variance here for an example
[http://www.artima.com/pins1ed/type-parameterization.html#19.3]

I like to break up invariant interfaces into separate Covariant and
Contravariant parts, but I realize this isnt an option for you if its
Lift's code.

Basically, MongoRecord isnt designed to support passing a MR[B] as an
MR[A], where B <: A I think you'll need to change your design.

-Ben