Scala - Difference between parametric method and forSome

Question

In Scala:

def rules[T]: Array[Rule[T]]= { ... }

vs.

def rules: Array[Rule[T]] forSome {type T} = { ... }

They are not the same because in some cases only the latter one is working.

Answer 1

forSome marks an existential type. Existential types may be used for client side type variance, which might be required for your use case. You can read more under this question.

What is the forSome keyword in Scala for?

or in the linked article

http://scalada.blogspot.de/2008/01/existential-types.html