CODEWITHSUNDEEP
scalasaddle
SkyWalker
SkyWalker

Reputation: 14309

What's the meaning of new and curly brace (without type to create)?

I found the following definition in Scala Saddle and just want to be sure I understood it correctly. There is an object defining an implicit function that exposes some HDF5 I/O functionality to the Frame type so that the writeHdfFile function becomes available to any Frame: 

object H5Implicits {
  /**
   * Provides enrichment on Frame object for writing to an HDF5 file.
   */
  implicit def frame2H5Writer[RX: ST: ORD, CX: ST: ORD, T: ST](frame: Frame[RX, CX, T]) = new {

    /**
     * Write a frame in HDF5 format to a file at the path provided
     *
     * @param path File to write
     * @param id Name of the HDF group in which to store frame data
     */
    def writeHdfFile(path: String, id: String) {
      H5Store.writeFrame(path, id, frame)
    }

  } // end new
}

However, I have never seen the = new { syntax before. Does it mean it is creating and returning a new function each time? why would that make more sense as opposed to simply doing = {

Upvotes: 0

Views: 96

Answers (2)

pedrorijo91
pedrorijo91

Reputation: 7845

it is creating an anonymous class. You can learn more about anonymous classes in scala here

Upvotes: 2

Stephen
Stephen

Reputation: 4296

Its a new anonymous class with 1 function.

In this case its used to provide syntax to the frame: Frame[RX, CX, T].

With this helper class in scope you can write.

frame.writeHdfFile(...)

Without this technique you would need to write.

writeHdfFile(frame, ...)

Normally this is done with a implicit class rather than a implicit def like that.

One benefit of this technique is that you can add helper methods to a class without changing them directly. Note how writeHdfFile is not defined on Frame

This is done pretty similar to how type classes are implemented in scala.

Upvotes: 5

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