What is a Manifest in Scala and when do you need it?

by

The compiler knows more information about types than the JVM runtime can easily represent. A Manifest is a way for the compiler to send an inter-dimensional message to the code at runtime about the type information that was lost.

It isn’t clear if a Manifest would benefit the errors you are seeing without knowing more detail.

One common use of Manifests is to have your code behave differently based on the static type of a collection. For example, what if you wanted to treat a List[String] differently from other types of a List:

 def foo[T](x: List[T])(implicit m: Manifest[T]) = {
    if (m <:< manifest[String])
      println("Hey, this list is full of strings")
    else
      println("Non-stringy list")
  }
  
  foo(List("one", "two")) // Hey, this list is full of strings
  foo(List(1, 2)) // Non-stringy list
  foo(List("one", 2)) // Non-stringy list

A reflection-based solution to this would probably involve inspecting each element of the list.

A context bound seems most suited to using type-classes in scala, and is well explained here by Debasish Ghosh:
http://debasishg.blogspot.com/2010/06/scala-implicits-type-classes-here-i.html

Context bounds can also just make the method signatures more readable. For example, the above function could be re-written using context bounds like so:

  def foo[T: Manifest](x: List[T]) = {
    if (manifest[T] <:< manifest[String])
      println("Hey, this list is full of strings")
    else
      println("Non-stringy list")
  }

More Related Contents:

  1. error in if return in Scala
  2. How to convert scala List in to Map
  3. How do I get around type erasure on Scala? Or, why can’t I get the type parameter of my collections?
  4. Call by name vs call by value in Scala, clarification needed
  5. Best way to merge two maps and sum the values of same key?
  6. Return in Scala
  7. What does the `#` operator mean in Scala?
  8. How do I skip a header from CSV files in Spark?
  9. Where does the “flatmap that s***” idiomatic expression in Scala come from?
  10. Caused by: java.lang.NullPointerException at org.apache.spark.sql.Dataset
  11. What is the Scala identifier “implicitly”?
  12. How to save DataFrame directly to Hive?
  13. Scala underscore – ERROR: missing parameter type for expanded function
  14. Setting up sbt to use Java 7 for compilation?
  15. About how to add a new column to an existing DataFrame with random values in Scala
  16. How to use Column.isin with list?
  17. Is there a generic way to memoize in Scala?
  18. How to compare two dataframe and print columns that are different in scala
  19. How to pattern match into an uppercase variable?
  20. Scala double definition (2 methods have the same type erasure)
  21. What is going wrong with `unionAll` of Spark `DataFrame`?
  22. Scala macros: What is the difference between typed (aka typechecked) and untyped Trees
  23. What are all the instances of syntactic sugar in Scala?
  24. Scala: short form of pattern matching that returns Boolean
  25. How can I convert immutable.Map to mutable.Map in Scala?
  26. Install sbt on ubuntu [closed]
  27. Should I use the final modifier when declaring case classes?
  28. Scala equivalent of Java’s Number
  29. Is there a simple example of how to generate verilog from Chisel3 module?
  30. Difference between two rows in Spark dataframe

Leave a Comment