[scala] polymorphic collection construction in 2.8

[daniel mahler]

I am trying to write a generic function to collect elements from a Responder
into an actual collection, but to be generic about the type of collection.
I get to something like:

def collect[A, C <: Traversable[A]] (r: Responder[A]) : C = {
    val b: Builder[A,C] = ... // new Builder[A, C] ??
    for (val a <- r) b += a
    return b.result

but what should ... be?
it can't be new Builder[A, C], since Builder is just a trait
and you need a colection specific subclass
Every companion object of a collection class has newBuilder method,
but how do you get at it in a generic way?
Colection classes do have a companion method,
but that requires a collection instance to begin with.

I have thought about something like Traversable.empty[A].genericBuilder[A, CC]
but that only covers collection classes that correspondto unary
functors, ie no Maps.
I also looked at the implicit newBuilderFactory mehods
but I did not get anywhere with those either.

thanks
Daniel

[daniel mahler]

On Sat, Oct 17, 2009 at 11:01 PM, Daniel Mahler <dmahler@...> wrote:

> I am trying to write a generic function to collect elements from a Responder
> into an actual collection, but to be generic about the type of collection.
> I get to something like:
>
> def collect[A, C <: Traversable[A]] (r: Responder[A]) : C = {
>    val b: Builder[A,C] = ... // new Builder[A, C] ??
>    for (val a <- r) b += a
>    return b.result
>
> but what should ... be?
> it can't be new Builder[A, C], since Builder is just a trait
> and you need a colection specific subclass
> Every companion object of a collection class has newBuilder method,
> but how do you get at it in a generic way?
> Colection classes do have a companion method,
> but that requires a collection instance to begin with.
>
> I have thought about something like Traversable.empty[A].genericBuilder[A, CC]

And that is not even how genericBuilder works :)

> but that only covers collection classes that correspondto unary
> functors, ie no Maps.
> I also looked at the implicit newBuilderFactory mehods
> but I did not get anywhere with those either.
>
> thanks
> Daniel
>

[Jorge Ortiz-3]

The best I can come up with is:

  def collect[A, CC[X] <: Traversable[X]](r: Responder[A])(to: GenericCompanion[CC]) = {

    val b = to.newBuilder[A]

    for (a <- r) b += a

    b.result

  }

Unfortunately you have to specify explicitly what kind of collection you want:

  collect(responder)(List)

or

  collect(responder)(Set)

--j

On Sat, Oct 17, 2009 at 2:33 PM, Daniel Mahler <dmahler@...> wrote:

On Sat, Oct 17, 2009 at 11:01 PM, Daniel Mahler <dmahler@...> wrote:
> I am trying to write a generic function to collect elements from a Responder
> into an actual collection, but to be generic about the type of collection.
> I get to something like:
>
> def collect[A, C <: Traversable[A]] (r: Responder[A]) : C = {
>    val b: Builder[A,C] = ... // new Builder[A, C] ??
>    for (val a <- r) b += a
>    return b.result
>
> but what should ... be?
> it can't be new Builder[A, C], since Builder is just a trait
> and you need a colection specific subclass
> Every companion object of a collection class has newBuilder method,
> but how do you get at it in a generic way?
> Colection classes do have a companion method,
> but that requires a collection instance to begin with.
>
> I have thought about something like Traversable.empty[A].genericBuilder[A, CC]

And that is not even how genericBuilder works :)

> but that only covers collection classes that correspondto unary
> functors, ie no Maps.
> I also looked at the implicit newBuilderFactory mehods
> but I did not get anywhere with those either.
>
> thanks
> Daniel
>