Yeah, this is really a consequence of Scala trying to do much shit for you automatically with the map function. See, it's not just a map function, but also an auto-conversion function from any generic sequence to any other generic sequence.
What the hell does that mean? Okay, so basically, take your standard map function. Run it on the structure you're mapping over. Great, now you've got your updated structure! Now, you don't just want to return the same type of structure you just had, you also want it to be a different structure, but with those modified elements you had for its contents. Now build that new structure from the thing you just modified (Scala may actually optimize the traversal and construction of the new collection into one pass, although I am not positive). And there's your return value.
It actually gets more complicated due to the Liskov principle and the way that Java handles type erasure, but thinking about that shit just makes me fuckin' tired.
You want to know what the map type signature should look like if the Scala guys weren't going overboard on trying to make things seem outwardly simple?
class Functor f where
fmap :: (a -> b) -> f a -> f b
What the hell does that mean? Okay, so basically, take your standard map function. Run it on the structure you're mapping over. Great, now you've got your updated structure! Now, you don't just want to return the same type of structure you just had, you also want it to be a different structure, but with those modified elements you had for its contents. Now build that new structure from the thing you just modified (Scala may actually optimize the traversal and construction of the new collection into one pass, although I am not positive). And there's your return value.
It actually gets more complicated due to the Liskov principle and the way that Java handles type erasure, but thinking about that shit just makes me fuckin' tired.
You want to know what the map type signature should look like if the Scala guys weren't going overboard on trying to make things seem outwardly simple?