On Thu, Jun 18, 2009 at 11:18 PM, Eric Bowman <[email protected]> wrote:
>
> The basic trick where a superclass has its subclass as a type parameter,
> e.g.
>
> class User extends MegaProtoUser[User]
>
> I've run into this before, I remember struggling to "get it", then
> getting it, but I can't recall the epiphany. But obviously this is a
> relatively common technique, so something to google is much appreciated.
>
I agree with Kris, this circular/recursive type pattern has two edges and
I'm not sure one is worth the other.
The main benefit I see with this pattern is that it enforces class User to
pass the User type parameter to Mapper, so you don't end up with a mixin
that doesn't make sense... although I doubt this error would actually happen
in reality because you'd quickly realize at design-time that calling any
method on User/MetaUser would return you something different than what you
expect. (
The drawback is that type signatures are more verbose, complex and viral (if
you care about extensibility). Just try explaining the type signature to
Scala newbie and they'll run away scared! :)
An alternative design would be something of the form,
trait Mapper {
type MapperType
}
class User extends Mapper {
type MapperType <: User
}
I think this design would be simpler to read and understand, without any
practical loss in type checking.
(As a side note, if you don't specify the correct type to Mapper with the
current design, you get an error message that's hardly helpful to newbies,
so it doesn't help much either... but I disgress.)
While I'm at it, there are also places in the code that could use
"this.type" instead of MapperType (also called "A" as parameter) to give out
an even more specific type without any tradeoffs. A prime example of this
would be Mapper.saveMe().
Just to be clear, I'm not proposing anything be changed in Lift's current
mapper since it would break compatibility.... I'm just suggesting it for
consideration in future design work... perhaps in Lift's new record module?
alex
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