PHP Internals News: Episode 103: Disjunctive Normal Form (DNF) Types

Friday, June 24th 2022, 09:07 BST

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

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Transcript

Derick Rethans 0:15: Hi, I'm Derick. Welcome to PHP internals news, a podcast dedicated to explaining the latest developments in the PHP language. This is episode 103. Today I'm talking with George Peter Banyard again, this time about a disjunctive normal form types RFC, or DNF, for short, which he's proposing together with Larry Garfield. George Peter, would you please introduce yourself?
George Peter Banyard 0:39: Hello, my name is George Peter Banyard, I work on PHP paid part time, by the PHP foundation.
Derick Rethans 0:44: Just like last time, we are still got colleagues.
George Peter Banyard 0:46: Yes, we are indeed still call it.
Derick Rethans 0:48: What is this RFC about? What is it trying to solve?
George Peter Banyard 0:52: The problems of this RFC is to be able to mix intersection and union types together. Last year, when intersection types were added to PHP, they were explicitly disallowed to be used with Union types. Because: a) mental framework, b) implementation complexity, because intersection types were already complicated on their own, to try to get them to work with Union types was kind of a big step. So it was done in chunks. And this is the second part of the chunk, being able to use it with Union types in a specific way.
Derick Rethans 1:25: What is the specific way?
George Peter Banyard 1:27: The specific way is where the disjoint normal form thing comes into play. So the joint normal form just means it's a normalized form of the type, where it's unions of intersections. The reason for that it helps the engine be able to like handle all of the various parts it needs to do, because at one point, it would need to normalize the type anyway. And we currently is just forced on to the developer because it makes the implementation easier. And probably also the source code, it's easier to read.
Derick Rethans 1:54: When you say, forcing it up on a developer to check out you basically mean that PHP won't try to normalize any types, but instead throws a compilation error?
George Peter Banyard 2:05: Exactly. It's, it's the job of the developer to do the normalization step. The normalization step is pretty easy, because I don't expect people to do too many stuff as intersection types. But as can always be done as a future scope of like adding a normalization step, then you get into the issues of like, maybe not having deterministic code, because normalization steps can take very, very long, and you can't necessarily prove that it will terminate, which is not a great situation to be in. Imagine just having PHP not running at all, because it's stuck in an infinite loop trying to normalize the format. It's just like, oh, I can't compile
Derick Rethans 2:39: Would a potential type alias kind of syntax help with that?
George Peter Banyard 2:44: Maybe, I'm not really sure. Actually reading like research about it from computer scientists, in functional programming languages, which is everything is compiled on my head. And they have the whole thing was like, well, they need to type type normalize, and especially with type aliases, they haven't really figured out a way yet. So I'm not sure how we are going to figure out a way if experts and PhD students and researchers haven't really figured out a way.
Derick Rethans 3:08: And is the reason for that mostly, because PHP, resolves types while it is running code sometimes because it has to overload classes, and then it might find out it is an inherited class, for example?
George Peter Banyard 3:19: Yes, I think it's like this weird thing where might maybe PHP has like kind of an advantage, because it doesn't need to, like resolve all of the types at once. And if you have a type alias, it's just oh, if it's used, and you just need to resolve it, and then try to figure it out. There's also the added complexity of like, variance checks, because most functional programming languages, they have variance to some degree, but they don't have the whole inheritance of like typical OOP languages have. It's kind of a very strange field, the fact that yeah, PHP is just like, well, we kind of do stuff at runtime, and you don't necessarily need everything. And it just works is like, well, we'll do. That's mainly the reason why the dev needs to do the normalization step, the form is done. It's also I think, the most easiest to understand, it's just like, Oh, you have this and this, or this group, or stuff, or this group of stuff, or this thing, simple type. The other form would be another normalized form would be conjunctive normal form, which is a list of ANDs of ORs to just have this thing, or X, like (A or B or C) and X and (Y or Z), which I think is harder to understand.
Derick Rethans 4:26: What is the exact syntax then?
George Peter Banyard 4:28: So the exact syntax is, if you want to have an intersection type was in a union type, you need to like bracket it by parentheses. And then you have like the normal pipe union operator and you can mix it with single types, you can mix it with true, you can mix it with false, which are literal types, which now exist, or just normal, bool types.
Derick Rethans 4:48: The parenthesis is actually required. You don't rely on operator precedence to make things work?
George Peter Banyard 4:53: Yes. Relying on operator precedence is terrible.
Derick Rethans 4:57: Yep, I agree.
George Peter Banyard 4:58: I'd say Oh, yeah, but I think I've heard this argument on the list like a couple of times, it's just, oh, yeah, but maths, like, has like, and as priority over like, or, I mean, I did three years of a maths degree and not gonna lie. Maths notation is terrible for most of us. People don't even agree on terminology. I'm just gonna say, let's, let's just do better.
Derick Rethans 5:19: I agree. I mean, most coding standards for any sort of variable for like conditions, will already require parenthesis around multiple complex clauses anyway, right? I mean, it's a sensible thing to do, just for readability, in my opinion. So the RFC also talks about a few syntax that you aren't allowed to do, and that you have to normalize or deconstruct yourself, what kinds of things are these?
George Peter Banyard 5:41: if you would want to have a type which has an intersection of a class A with at least one other class, so let's say X or Y, but you can always convert it into DNF form, how this type would be, it would be (A and X) or (A and Y). This seems to be the more unusual case, I would imagine. One of the motivating cases of DNF types is to do something like Array or (Traversable and Countable). I don't really see mixing and match...