PHP Internals News: Episode 88: Pure Intersection Types

Thursday, June 10th 2021, 09:16 BST

In this episode of "PHP Internals News" I talk with George Peter Banyard (Website, Twitter, GitHub, GitLab) about the "Pure Intersection Types" RFC that he has proposed.

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Transcript

Derick Rethans 0:14: Welcome to PHP internals news, a podcast dedicated to explaining the latest developments in the PHP language. This is Episode 88. Today I'm talking with George Peter Banyard about pure intersection types. George, could you please introduce yourself?
George Peter Banyard 0:30: Hello, my name is George Peter Banyard. I work on PHP code development in my free time. And on the PHP Docs.
Derick Rethans 0:36: This RFC is about intersection types. What are intersection types?
George Peter Banyard 0:40: I think the easiest way to explain intersection types is to use something which we already have, which are union types. So union types tells you I want X or Y, whereas intersection types tell you that I want X and Y to be true at the same time. The easiest example I can come up with is a traversable that you want to be countable as well. So traversable and countable. Currently, you can do intersection types in very hacky ways. So you can either create a new interface which extends both traversable and countable, but then all the classes that you want to be using this fashion, you need to make them implement the interface, which might not be possible if you using a library or other things like that. The other very hacky way of doing it is using reference and typed properties. You assign two typed properties by reference, one being traversable, one being countable, and then your actual property, you type alias reference it, with both of these properties. And then my PHP will check: does the property respect type A those reference? If yes, move to the next one. It doesn't respect type B, which basically gives you intersection types.
Derick Rethans 1:44: Yeah, I saw that in the RFC. And I was wondering like, well, people actually do that?
George Peter Banyard 1:49: The only reason I know that is because of Nikita's slide.
Derick Rethans 1:51: The thing is, if it is possible, people wil