On Tue, May 17, 2016 at 2:38 AM, Greg Ewing <greg.ewing@canterbury.ac.nz> wrote:
Guido van Rossum wrote:
Hmm... In Java there is a form of syntactic sugar that automatically deals with such boxes called auto-(un)boxing, IIUC. So I still think it can be called syntactic sugar.
That's not the same thing either. Boxing in Java is a hack to make up for the fact that some types are not objects, and the auto boxing and unboxing is there so that you can forget about the boxes and pretend that e.g. int and Integer are the same type (at least for some purposes).
But with algebraic types, the boxes are entities in their own right whose presence conveys information. You can have more than one kind of box with the same contents:
data Box = Matchbox Int | Shoebox Int
Not only is a Matchbox distinguishable from a Shoebox at run time, but Box is a distinct type from Int -- you can't pass an Int directly to something expecting a Box or vice versa.
A realisation of algebraic types in Python (or any other language, for that matter) would require carrying information at run time about which kind of box is present. This is in contrast to a union type, which is purely a compile-time concept and has no run-time implications.
I'm sorry, I wasn't trying to claim that Java's auto-(un)boxing was anything like ADTs; I know better. I was just quipping that just because there's a difference between a box containing a piece of fruit and the piece of fruit itself, that doesn't mean handling the box can't be considered syntactic sugar -- your original remark claimed something wasn't syntactic sugar because of the difference between the box and its contents, and that's what I disagree with. -- --Guido van Rossum (python.org/~guido)