Scala generic type constraints and specialized behavior

I have a class where I need an apply[T] method where T is only allowed to be Foo or Bar. Also, the method needs to behave differently depending on whether it is a Foo or a Bar. Becuase of type erasure, I cannot simple make apply[Foo] and apply[Bar] methods (that was my first attempt). To solve this, I tried doing something like this

apply[T]

T

Foo

Bar

Foo

Bar

apply[Foo]

apply[Bar]

def apply[T](ds: Dataset[T]): Dataset[T] =
ds match
case ds: Dataset[Foo] => ...
case ds: Dataset[Bar] => ...
case _ => ???




but this doesn't work because of type erasure of T. Also this doesn't even restrict this method to only be able to be called with T of type Foo or Bar, it just "does nothing" when it isn't of one of those two types. How can I make this apply method have these properties?

T

T

Foo

Bar

Thank you.

NotImplementedError

2 Answers
2

Here is a nice trick around type erasure:

def apply(ds: Dataset[Foo]): Dataset[Foo] = ???
def apply(ds: Dataset[Bar])(implicit dummy: DummyImplicit): Dataset[Bar] = ???


Lets you have multiple methods with the same run-time signature without having to use type parameters.

You could use an implicit typeclass:

trait Baz[T]
def apply(ds: DataSet[T]): DataSet[T]


implicit val fooImpl = new Baz[Foo]
def apply(ds: DataSet[Foo]): DataSet[Foo] = ???


implicit val barImpl = new Baz[Bar]
def apply(ds: DataSet[Bar]): DataSet[Bar] = ???


def apply[T](ds: Dataset[T])(implicit b: Baz[T]): Dataset[T] = b(ds)


T

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