Simple generic function which doesn't compile

Question

Please take a look at this function in Scala and tell me why it doesn't compile.

def alternative_identity[A <: Any, B <: Any](obj: A): B = { obj }

I know that the function itself doesn't make sense, however I am trying to solve more complex problem and this example illustrates the fundamental problem which I have. Thank you in advance!

How is the compiler supposed to know what B is? The function is supposed to return B, but it's returning A. Are they supposed to have the same type? What is the function supposed to do? — Michael Zajac
– Michael Zajac, Commented Jan 21, 2015 at 18:30
But compiler knows that obj is a subtype of Any, because A is subtype of Any. Then compiler can infer that obj satisfies requirements for B (being subtype of Any). What is wrong in this reasoning? — Piotr Niedzwiedz
– Piotr Niedzwiedz, Commented Jan 21, 2015 at 18:34
What if I call alternative_identity[Int, Process](1) ? How on earth will an Int magically become a Process? — Michael Zajac
– Michael Zajac, Commented Jan 21, 2015 at 18:45
@PiotrNiedzwiedz - Monkeys are animals and Rhinos are animals, but Monkeys are not Rhinos. — Lee
– Lee, Commented Jan 21, 2015 at 18:47

mohit · Accepted Answer · 2015-01-21 18:45:04Z

It does not compile because obj is object of type A and you are returning it from a function where the return type is B

Even though A and B is subtype ofAny there is no way to know whether B is the base class of A.

For example, String and Int are subtypes of Any but they can't be used interchangeably.

Collectives™ on Stack Overflow

Simple generic function which doesn't compile

1 Answer 1

Comments

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Related