- About Scala
- Documentation
- Code Examples
- Software
- Scala Developers
Inner types as abstract method arguments
Tue, 2011-08-16, 16:18
I'm getting a confusing compilation error message that I think should be fixed.
In some code requiring subclassing of inner traits/classes I'm having trouble successfully implementing an abstract method, apparently because my type signature doesn't match. For example in response to this code...
> trait Outer[This<:Outer[This]] {
> this: This =>
> trait Inner
> def score(i:This#Inner): Double
> }
> class Outer2 extends Outer[Outer2] {
> class Inner extends super.Inner
> def score(i:Outer2#Inner) = 0.0
> }
...the compiler says:
> error: class Outer2 needs to be abstract, since method score in trait Outer of type (i: this.Outer2#Inner)Double is not defined
> (Note that This#Inner does not match this.Outer2#Inner)
I don't understand why the "score" method type signature doesn't match.
When I follow the suggestion of the error message and re-write Outer2 with:
> def score(i:this.Outer2#Inner) = 0.0
I get a different error message:
> error: type Outer2 is not a member of this.Outer2
There are two issues:
(1) The error messages are confusing.
(2) How can I specify and then implement a method taking an inner class argument whose type will be a sub-class determined later?
Tue, 2011-08-16, 17:57
#2
Re: Inner types as abstract method arguments
On 8/16/11 8:18 AM, Andrew McCallum wrote:
> ...the compiler says:
>
>> error: class Outer2 needs to be abstract, since method score in trait Outer of type (i: this.Outer2#Inner)Double is not defined
>> (Note that This#Inner does not match this.Outer2#Inner)
This message is now:
abstract-concrete-methods.scala:7: error: class Outer2 needs to be abstract, since method score in trait Outer of type (i: Outer2#Inner)Double is not defined
(Note that This#Inner does not match Outer2#Inner: class Inner in class Outer2 is a subclass of trait Inner in trait Outer, but method parameter types must match exactly.)
class Outer2 extends Outer[Outer2] {
^
one error found
Thu, 2011-08-18, 18:27
#3
Re: Inner types as abstract method arguments
Hi David!
Thanks. The particular example I have in mind is more complicated than this, but I have essentially decided to go with a type member as you suggest.
I hope you are doing well.
I'm interested in the possibility of (re)-integrating scalala into FACTORIE. I removed the dependency out of concern about efficiency (of large sparse vectors, etc). Is now a good time to re-evaluate the situation?
Best,
Andrew
On Aug 16, 2011, at 12:13 PM, David Hall wrote:
> On Tue, Aug 16, 2011 at 8:18 AM, Andrew McCallum wrote:
>> I'm getting a confusing compilation error message that I think should be fixed.
>>
>> In some code requiring subclassing of inner traits/classes I'm having trouble successfully implementing an abstract method, apparently because my type signature doesn't match. For example in response to this code...
>>
>>> trait Outer[This<:Outer[This]] {
>>> this: This =>
>>> trait Inner
>>> def score(i:This#Inner): Double
>>> }
>>> class Outer2 extends Outer[Outer2] {
>>> class Inner extends super.Inner
>>> def score(i:Outer2#Inner) = 0.0
>>> }
>
> Try replacing "trait Inner" with "type Inner". You want Inner to be
> abstract in Outer, otherwise it's a concrete type and you can't
> implement score by only accepting a subtype of Outer#Inner.
>
> (As an example, take a look at:
> https://github.com/dlwh/scalanlp-core/blob/master/learn/src/main/scala/s...
> and
> https://github.com/dlwh/scalanlp-core/blob/master/learn/src/main/scala/s...
> )
>
On Tue, Aug 16, 2011 at 8:18 AM, Andrew McCallum wrote:
> I'm getting a confusing compilation error message that I think should be fixed.
>
> In some code requiring subclassing of inner traits/classes I'm having trouble successfully implementing an abstract method, apparently because my type signature doesn't match. For example in response to this code...
>
>> trait Outer[This<:Outer[This]] {
>> this: This =>
>> trait Inner
>> def score(i:This#Inner): Double
>> }
>> class Outer2 extends Outer[Outer2] {
>> class Inner extends super.Inner
>> def score(i:Outer2#Inner) = 0.0
>> }
Try replacing "trait Inner" with "type Inner". You want Inner to be
abstract in Outer, otherwise it's a concrete type and you can't
implement score by only accepting a subtype of Outer#Inner.
(As an example, take a look at:
https://github.com/dlwh/scalanlp-core/blob/master/learn/src/main/scala/s...
and
https://github.com/dlwh/scalanlp-core/blob/master/learn/src/main/scala/s...
)