Scala: type hint for lambda

Question

I'm refreshing scala. This looks very simple to me but I can't get it to run:

import java.nio.file.{FileSystems, Files}

object ScalaHello extends App {
    val dir = FileSystems.getDefault.getPath("/home/intelli/workspace")
    Files.walk(dir).map(_.toFile).forEach(println)
}

It throws error at the mapping lambda:

argument expression's type is not compatible with formal parameter type;
 found   : java.util.function.Function[java.nio.file.Path,java.io.File]
 required: java.util.function.Function[_ >: java.nio.file.Path, _ <: ?R]

I suspect it has something to do with providing type hints for the lambda but I can't find anything surfing Google. Much appreciated

And also please show your build file (build.sbt?). Especially scalacOptions. — Dmytro Mitin
– Dmytro Mitin, Commented Feb 5, 2023 at 13:51
Does anything change for you if you specify .map[File]((_: Path).toFile)? — Dmytro Mitin
– Dmytro Mitin, Commented Feb 5, 2023 at 14:16

Kolmar · Accepted Answer · 2023-02-05 14:16:56Z

4

Note that Files.walk returns a Java Stream, so map and forEach come from Java.

Assuming you are using Scala 2.12, your code will work if you either:

Update Scala version to 2.13 (no need to make any other changes in this case)
Specify return type of map:

Files.walk(dir).map[File](_.toFile).forEach(println)

Convert to Scala collections before calling map:

import scala.collection.JavaConverters._
Files.walk(dir).iterator().asScala.map(_.toFile).foreach(println)

answered Feb 5, 2023 at 14:16

Kolmar

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3 Comments

Dmytro Mitin Over a year ago

Cool. So it seems to be 2.12

diegoruizbarbero Over a year ago

Kolmar, thanks for the explanation: I somehow assumed that the stages of Scala streams would apply directly ...but as you explain, I was applying the transformations from Java streams!

Dmytro Mitin Over a year ago

@Kolmar "2.13 (no need to make any other changes in this case)" This seems to depend on scalacOptions sometimes. With -Ydelambdafy:inline .forEach(println) doesn't compile in 2.13 (class type required but java.util.function.Consumer[_ >: java.io.File] found) while .forEach(println(_)) does. With -Ydelambdafy:method (default) .forEach(println) compiles.

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