What is the eta expansion in Scala?

by

The definition, and some examples, are given in http://scala-lang.org/files/archive/spec/2.11/06-expressions.html#method-values.

someMethod _ will roughly be translated to a new function object like this:

Not quite: it’s actually

new Function1[Int, Int] {
  def apply(x: Int): Int = someMethod(x)
}

The difference matters e.g. if someMethod is overridden somewhere.

Is it all that Scala does?

You also need to take into account what happens if the method takes multiple parameter lists (you get a function which returns a function) or by-name parameters.

What are the scenarios that eta expansion are needed?

  1. When you specifically ask for it (e.g. someMethod _).

  2. When you use a method (with parameters) where a value of a function type (or a SAM type in Scala 2.12) is expected. E.g.

    def foo(f: Int => Int) = ???

foo(someMethod)

  3. That’s it.

Note that using eta-expansion and an anonymous function with placeholders (someMethod(_)) can behave differently due to type inference, implicits, etc.

More Related Contents:

  1. What is a “context bound” in Scala?
  2. What is a Manifest in Scala and when do you need it?
  3. Why “avoid method overloading”?
  4. How can I change column types in Spark SQL’s DataFrame?
  5. Need clarification on Scala literal identifiers (backticks)
  6. how to make saveAsTextFile NOT split output into multiple file?
  7. Scala how can I count the number of occurrences in a list
  8. What is the difference between def foo = {} and def foo() = {} in Scala?
  9. Case class to map in Scala
  10. Difference between using App trait and main method in scala
  11. Apache Spark how to append new column from list/array to Spark dataframe
  12. Functional programming, Scala map and fold left [closed]
  13. What does => and () => mean in Scala [duplicate]
  14. Load Scala file into interpreter to use functions?
  15. Is there a way to use “type” word as a variable name in Scala?
  16. Any reason why scala does not explicitly support dependent types?
  17. Reduce, fold or scan (Left/Right)?
  18. Apache Spark, add an “CASE WHEN … ELSE …” calculated column to an existing DataFrame
  19. How to suppress info and success messages in sbt?
  20. Replace null values in Spark DataFrame
  21. Scala: write string to file in one statement
  22. Scala: pass Seq to var-args functions
  23. How does the “scala.sys.process” from Scala 2.9 work?
  24. Calculate Cosine Similarity Spark Dataframe
  25. Why does Scala support shadow variables? [closed]
  26. How to wait for several Futures?
  27. forall in Scala
  28. Scala asInstanceOf with parameterized types
  29. How do I run an sbt main class from the shell as normal command-line program?
  30. Spark ML VectorAssembler returns strange output

Leave a Comment