On Wed, Apr 1, 2009 at 8:06 PM, Kris Nuttycombe
<[email protected]>wrote:
>
> Something that occurred to me recently along these lines - perhaps
> someone can disabuse me of this notion. In Java, such recursive types
> are necessary because you don't have abstract types. To refer to the
> implementation type in the declaring class you have to use the
> self-type.
>
> But in Scala, what application would not be satisfied by:
>
> trait Mapper {
> type T <: Mapper
> }
>
> class User extends Mapper {
> type T = User
> }
>
> Is it just that the restriction on T is not sufficiently narrow?
No... becayse you can say:
class User extends Mapper {
type T = Address
}
I think this captures things, but I'm not 100% sure (and self-types weren't
around when I did Mapper):
trait Mapper[T <: Mapper] {
self: T =>
}
>
>
> Kris
>
> On Fri, Mar 27, 2009 at 5:26 PM, Alex Boisvert <[email protected]>
> wrote:
> > Or said another way,
> >
> > MappedTextarea[ T <: Mapper[T] ]
> >
> > declares a type parameter T and fixes the upper bound of the
> MappedTextarea
> > type parameter to Mapper[T], which means that the type passed to
> > MappedTextArea must be a subtype of Mapper.
> >
> > I, too, found this notation confusing at first and wished I could write
> > MappedTextarea[ <: Mapper[T] ] directly but declaring the T before its
> use
> > is necessary to disambiguate it from existing class names.
> >
> > alex
> >
> >
> >
> > On Fri, Mar 27, 2009 at 1:52 PM, Stefan Scott <
> [email protected]>
> > wrote:
> >>
> >> Hi -
> >>
> >> Sorry to be asking a Scala syntax question here in the Lift group, but
> >> I figured somebody here would know, since this Scala syntax occurs
> >> quite a bit in the Lift source code.
> >>
> >> When reading some of the Lift source I come across a particular Scala
> >> "idiom" involving parameterized types with bounds, whose semantics I'm
> >> unsure of.
> >>
> >> For example:
> >>
> >> class MappedTextarea[ T <: Mapper[ T ] ] ( owner : T, maxLen: Int )
> >> extends
> >> MappedString[ T ]( owner, maxLen ) { ... }
> >>
> >> What I'm unsure about here is the part where it says:
> >>
> >> T <: Mapper[ T ]
> >>
> >> At first, this made no sense to me - how could a type T be a subtype
> >> of type Mapper[ T ] ?
> >>
> >> Then I guessed that maybe the two occurrences of T are unrelated to
> >> each other - ie, class MappedTextarea is parameterized over a type T,
> >> which must be a subtype of a type Mapper[ T ] -- where the second T is
> >> actually in a separate scope so that it has nothing to do with the
> >> first T.
> >>
> >> Is that what this really means?
> >>
> >> And, if that's really the case, then I guess the other occurrences of
> >> T later in the text:
> >>
> >> owner : T
> >> MappedString[ T ]
> >>
> >> are also referring to the first occurrence of T -- the type T which is
> >> upper-bounded by type Mapper[ T ].
> >>
> >> Finally, does this mean that the above code could also have been
> >> written equivalently as follows:
> >>
> >> class MappedTextarea[ T <: Mapper[ U ] ] ( owner : T, maxLen: Int )
> >> extends
> >> MappedString[ T ]( owner, maxLen ) { ... }
> >>
> >> using U instead of T for the type parameter that's in a separate
> >> scope?
> >>
> >> Thanks for any help.
> >>
> >> - Stefan Scott
> >>
> >>
> >>
> >>
> >>
> >>
> >
> >
> > >
> >
>
> >
>
--
Lift, the simply functional web framework http://liftweb.net
Beginning Scala http://www.apress.com/book/view/1430219890
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