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

Question

Since Scala 2.7.2 there is something called Manifest which is a workaround for Java's type erasure. But how does Manifest work exactly and why / when do you need to use it?

The blog post Manifests: Reified Types by Jorge Ortiz explains some of it, but it doesn't explain how to use it together with context bounds.

Also, what is ClassManifest, what's the difference with Manifest?

I have some code (part of a larger program, can't easily include it here) that has some warnings with regard to type erasure; I suspect I can solve these by using manifests, but I'm not sure exactly how.

Answer 1

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")
  }

Answer 2

A Manifest was intended to reify generic types that get type-erased to run on the JVM (which does not support generics). However, they had some serious issues: they were too simplistic, and were unable to fully support Scala's type system. They were thus deprecated in Scala 2.10, and are replaced with TypeTags (which are essentially what the Scala compiler itself uses to represent types, and therefore fully support Scala types). For more details on the difference, see:

• Scala: What is a TypeTag and how do I use it?
• How do the new Scala TypeTags improve the (deprecated) Manifests?

In other words

when do you need it?

Before 2013-01-04, when Scala 2.10 was released.

Answer 3

Not a complete answer, but regarding the difference between Manifest and ClassManifest, you can find an example in the Scala 2.8 Array paper:

The only remaining question is how to implement generic array creation. Unlike Java, Scala allows an instance creation new Array[T] where T is a type parameter. How can this be implemented, given the fact that there does not exist a uniform array representation in Java?

The only way to do this is to require additional runtime information which describes the type T. Scala 2.8 has a new mechanism for this, which is called a Manifest. An object of type Manifest[T] provides complete information about the type T.
Manifest values are typically passed in implicit parameters; and the compiler knows how to construct them for statically known types T.

There exists also a weaker form named ClassManifest which can be constructed from knowing just the top-level class of a type, without necessarily knowing all its argument types.
It is this type of runtime information that’s required for array creation.

Example:

One needs to provide this information by passing a ClassManifest[T] into the method as an implicit parameter:

def  tabulate[T](len:Int,  f:Int=>T)(implicit m:ClassManifest[T]) =  { 
  val  xs  =  new  Array[T](len) 
  for   (i  <- 0  until   len)  xs(i)   = f(i) 
  xs 
} 

As a shorthand form, a context bound1 can be used on the type parameter T instead,

(See this SO question for illustration)

, giving:

def  tabulate[T:    ClassManifest](len:Int,  f:Int=>T)  =  { 
  val  xs  =  new  Array[T](len) 
  for   (i  <- 0  until   len)  xs(i)   = f(i) 
  xs 
} 

When calling tabulate on a type such as Int, or String, or List[T], the Scala compiler can create a class manifest to pass as implicit argument to tabulate.

Answer 4

Let's also chck out manifest in scala sources (Manifest.scala), we see:

Manifest.scala:
def manifest[T](implicit m: Manifest[T])           = m

So with regards to following example code:

def foo[A](somelist: List[A])(implicit m: Manifest[A]): String = {
  if (m <:< manifest[String]) {
    "its a string"
  } else {
    "its not a string"
  }
}

we can see that the manifest function searches for an implicit m: Manifest[T] which satisfies the type parameter you provide in our example code it was manifest[String]. So when you call something like:

if (m <:< manifest[String]) {

you are checking if the current implicit m which you defined in your function is of type manifest[String] and as the manifest is a function of type manifest[T] it would search for a specific manifest[String] and it would find if there is such an implicit.