Java higher-order methods inconsistently require type annotations

[joshlemer]

This works:

scala> import java.util.stream.Stream
import java.util.stream.Stream

scala> Stream.of(1,2,3).map(_ + 1)
res20: java.util.stream.Stream[?0] = java.util.stream.ReferencePipeline$3@27014faf

But then, you'd think that this should work:

scala> Stream.of(1,2,3).map(_.toString)
                              ^
       error: type mismatch;
        found   : Int => String
        required: java.util.function.Function[_ >: Int, _]

It can be made to work only by including the map type param:

scala> Stream.of(1,2,3).map[String](_.toString)
res22: java.util.stream.Stream[String] = java.util.stream.ReferencePipeline$3@7e7daeac

This would hopefully work because it does actually work in java:

Stream.of(1,2,3).map(i -> i.toString());

[SethTisue]

(quick, off-the-cuff reaction: reminiscent of scala/scala-dev#184 , is it the same?)

[dwijnand]

I guess the inconsistency is that + exists on Int, but .toString is on Object and Function isn't contravariant.

[joshlemer]

@dwijnand no that's not the issue, it happens whenever the return-type of the function differs from the input type.

@SethTisue this does seem to be related or the same issue, yes.

[adriaanm]

I've been looking at this. Fertile ground for bug fixin'! Already found an issue introduced by scala/scala#6492, but no full fix for this issue yet. (WIP: https://github.com/adriaanm/scala/tree/t11558)

[som-snytt]

Seen on SO by someone "stuck" on 2.12, and not in the Valentine's Day sense of "stuck on you."