It is possible to define Scala case objects programmatically?

Question

In my current project, I need to define a lot of case objects subclassing a single sealed trait. The names of these objects have a pattern, e.g., Case1, Case2, Case3, ..., CaseN. I was wondering if there is way in scala to define all these case objects in a programmatical way, for example with a loop, instead of writing N lines of definitions.

If this is feasible, it would also be great to do this at compile time so that the case objects defined in this way are known to type checking. This sounds impossible in most languages (you can have either dynamically defined types like in python or have static type safety like in C/C++, but not both), but maybe it is doable in Scala for its powerful compile-time reflection?

But why won't you create class, and then make objects at runtime? — Kamil Banaszczyk, Oct 21 '16 at 16:14
Because you cannot do pattern matching on instances of a class, especially exhaustive pattern matching. — uraj, Oct 21 '16 at 16:18
What about pattern matching by some property of class? Create objects with pattern on property, and then match them by that property — Kamil Banaszczyk, Oct 21 '16 at 16:20
@KamilBanaszczyk Then that property has to be a bunch of case object, otherwise the matching won't be exhaustive — uraj, Oct 21 '16 at 16:28
@talex Could you point out some materials that can help me actually implement it? Thanks. — uraj, Oct 21 '16 at 16:30
@uraj sorry, but do not have any specific link, but google offer lots of them if you ask for "sacala macros define class" — talex, Oct 21 '16 at 16:32
I've added response with what i was thinking, i'm new at scala, so if you can tell me what can be wrong with this i will appreciate :) — Kamil Banaszczyk, Oct 21 '16 at 16:38
possible duplicate of http://stackoverflow.com/questions/22850340/dynamically-creating-case-classes-with-macros — jacks, Oct 21 '16 at 16:40
another possible duplicate http://stackoverflow.com/questions/10583283/dynamically-generate-case-class-in-scala?noredirect=1&lq=1 — dkolmakov, Oct 22 '16 at 08:33

score 1 · Answer 1 · answered Oct 21 '16 at 16:37

1

I'm thinking about this solution:

trait customTrait{
  val property: String
}

case class toMatch(property: String) extends customTrait{
}

val cases: Seq[toMatch] = for{
  x <- 0 until 10
} yield toMatch("property" + x)

def matchCase(caseToMatch: customTrait): String = caseToMatch match{

  case toMatch("property1") => "1"
  case toMatch("property2") => "2"
  case toMatch("property3") => "3"
  case toMatch("property4") => "4"
  case _ => "non"

}

for {
  x <- cases
} yield matchCase(x)

answered Oct 21 '16 at 16:37

Kamil Banaszczyk

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This piece of code works, but like I mentioned it lacks static checking on the cases you wrote in the match block. If someone forgets to add a case for, say, "property3", the compiler won't issue a warning. On the other hand, if you leave out the wild card case, the compiler will always issue a warning. – uraj Oct 21 '16 at 16:54

It is possible to define Scala case objects programmatically?

1 Answers1