| name | scala-generics-variance |
| description | Guide for translating Python TypeVars and generics to Scala covariant and contravariant types. |
Generics and Variance in Scala
Python's typing module provides TypeVar to represent generics, including covariant and contravariant parameters. Scala has built-in support for declaration-site variance.
Type Parameters
- Invariant:
class MyClass[A] (Must be exact type, like Python TypeVar("T"))
- Covariant:
class MyClass[+A] (Can be subtype, like Python TypeVar("T_co", covariant=True))
- Contravariant:
class MyClass[-A] (Can be supertype, like Python TypeVar("T_contra", contravariant=True))
Example: Covariant Container
Python:
T_co = TypeVar("T_co", covariant=True)
class Container(Generic[T_co]):
def __init__(self, items: Sequence[T_co]):
self._items = tuple(items)
def get_all(self) -> tuple[T_co, ...]:
return self._items
Scala:
class Container[+A](items: Seq[A]) {
private val _items: Vector[A] = items.toVector
def getAll: Vector[A] = _items
}
Protocols vs Traits
Python Protocol allows duck-typing, but Scala requires nominal typing via trait.
Python:
class Tokenizable(Protocol):
def to_token(self) -> str: ...
Scala:
trait Tokenizable {
def toToken: String
}
If an object needs to implement this, it must explicitly extend Tokenizable.
