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Calling Traversable#map from java: impossible? #4389

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scabug opened this Issue Mar 25, 2011 · 4 comments

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scabug commented Mar 25, 2011

Chosen as a sample of the impressive difficulty of calling into scala. I can't make this compile at all. Is there a way to make it work? If there is not, is it due to the signature? If nontrivial things aren't going to work anyway then there's no point in suffering so much for the cause.

import scala.*;
import scala.collection.Traversable;
import scala.collection.Traversable$$;
import scala.collection.generic.CanBuildFrom;

class A { };
class B { };

public class J {
  // The generic signature we are trying to implement:
  //
  // public abstract <B,That> That scala.collection.TraversableLike.map(scala.Function1<A, B>,scala.collection.generic.CanBuildFrom<Repr, B, That>)
  
  public void sum(Traversable<A> x) {
    // 1) This way, or any other deviation from 2), and we get "incompatible types"
    // at the declaration.
    CanBuildFrom<Traversable<A>, B, Traversable<B>> cbf1 = Traversable$$.MODULE$$.<B>canBuildFrom();
    // 2) This way, we get "cannot find symbol" at the call site.
    CanBuildFrom<Traversable<?>, B, Traversable<B>> cbf2 = Traversable$$.MODULE$$.<B>canBuildFrom();
    // A function.
    Function1<A, B> f = new scala.runtime.AbstractFunction1<A, B>() {
      public B apply(A a) {
        return null;
      }
    };
    x.map(f, cbf1);
    x.map(f, cbf2);
    
    // For our 3) and last trick, we try to take the working part from each of those, and
    // cast the cbf to the type it wants.  And it won't even let me cast.
    x.map(f, (CanBuildFrom<Traversable<A>, B, Traversable<B>>)cbf2);

    // Error 1:
    // J.java:12: incompatible types
    // found   : scala.collection.generic.CanBuildFrom<scala.collection.Traversable<?>,B,scala.collection.Traversable<B>>
    // required: scala.collection.generic.CanBuildFrom<scala.collection.Traversable<A>,B,scala.collection.Traversable<B>>
    //     CanBuildFrom<Traversable<A>, B, Traversable<B>> cbf1 = Traversable$$.MODULE$$.<B>canBuildFrom();
    //                                                                                                ^

    // Error 2:
    // J.java:23: cannot find symbol
    // symbol  : method map(scala.Function1<A,B>,scala.collection.generic.CanBuildFrom<scala.collection.Traversable<?>,B,scala.collection.Traversable<B>>)
    // location: interface scala.collection.Traversable<A>
    //     x.map(f, cbf2);
    //      ^
    
    // Error 3:
    // J.java:43: inconvertible types
    // found   : scala.collection.generic.CanBuildFrom<scala.collection.Traversable<?>,B,scala.collection.Traversable<B>>
    // required: scala.collection.generic.CanBuildFrom<scala.collection.Traversable<A>,B,scala.collection.Traversable<B>>
    //     x.map(f, (CanBuildFrom<Traversable<A>, B, Traversable<B>>)cbf2);
    //                                                               ^
    // 3 errors
    // 

    x.map(f, null); // "works"
  }
  
  // Nope, not these either
  //
  // CanBuildFrom<Traversable, B, Traversable<B>> cbf3 = Traversable$$.MODULE$$.<B>canBuildFrom();
  // CanBuildFrom<? super Traversable<A>, B, Traversable<B>> cbf4 = Traversable$$.MODULE$$.<B>canBuildFrom();
  // CanBuildFrom<Object, B, Traversable<B>> cbf5 = Traversable$$.MODULE$$.<B>canBuildFrom();
  // CanBuildFrom<Traversable<?>, B, Traversable> cbf6 = Traversable$$.MODULE$$.<B>canBuildFrom();
  // 
  // You can try explicit parameterization, it won't help.
  //
  // x.<B, Traversable<B>>map(f, cbf2);
}
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scabug commented Mar 25, 2011

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scabug commented Apr 11, 2011

@odersky said:
I played with it until it got too tedious. I think the signatures work fine. What does not work is that the variances of CanBuildFrom cannot be modelled in Java, so types do not match. And it seems Java does not even let me override with a cast. So short answer: You can't call these things from Java because instead of declaration side variance you have only a broken wildcard system.

@scabug scabug closed this May 18, 2011

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scabug commented Dec 11, 2015

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scabug commented Dec 14, 2015

@paulp said:
When I opened this ticket, the release version of scala was 2.8, with 2.9.0 a couple months out. Applicability of whatever is on offer at the stackoverflow question is unknown.

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