PHP Internals News: Episode 102: Add True Type

Thursday, June 2nd 2022, 09:06 BST

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

The RSS feed for this podcast is https://derickrethans.nl/feed-phpinternalsnews.xml, you can download this episode's MP3 file, and it's available on Spotify and iTunes. There is a dedicated website: https://phpinternals.news

Transcript

Derick Rethans 0:00: Hi I'm Derick. Welcome to PHP internals news, the podcast dedicated to explaining the latest developments in the PHP language. This is episode 102. Today I'm talking with George Peter Banyard about the Add True Type RFC that he's proposing. Hello George Peter, would you please introduce yourself?
George Peter Banyard 0:33: Hello, my name is George Peter Banyard, I work part time for the PHP Foundation. And I work on the documentation.
Derick Rethans 0:40: Very well. We're co workers really aren't we?
George Peter Banyard 0:43: Yes, indeed, we all co workers.
Derick Rethans 0:45: Excellent. We spoke in the past about related RFCs. I remember, which one was that again?
George Peter Banyard 0:51: Making null and false stand alone types
Derick Rethans 0:53: That's the one I was thinking of him. But what is this RFC about?
George Peter Banyard 0:56: So this RFC is about adding true as a single type. So we have false, which is one part of the Boolean type, but we don't have true. Now the reasons for that are a bit like historical in some sense, although it's only from PHP 8.0. So talking about something historical. When it's only a year ago, it's a bit weird. The main reason was that like PHP has many internal functions, which return false on failure. So that was a reason to include it in the Union types RFC, so that we could probably document these types because I know it would be like, string and Boolean when it could only return false and never true. So which is a bit pointless and misleading, so that was the point of adding false. And this statement didn't apply to true for the most part. With PHP 8, we did a lot of warning to value error promotions, or type error promotions, and a lot of cases where a lot of functions which used to return false, stopped returning false, and they would throw an exception instead. These functions now always return true, but we can't type them as true because we don't have it, and have so they are typed as bool, which is kind of also misleading in the same sense, with the union type is like, well, it only returns false. So no point using the boolean, but these functions always return true. But if you look at the type signature, you can see like, well, I need to cater to the case where the returns true and when returns false.
Derick Rethans 2:19: Do they return true or throw an exception?
George Peter Banyard 2:22: Yeah, so they either return true, or they either throw an exception. If you would design these functions from scratch, you would make them void, but legacy... and we did, I know it was like PHP 8.0, we did change a couple of functions from true to void. But then you get into these weird shenanigans where like, if you use the return value of the function in a in an if statement, null gets because in PHP, any function does return a value, even a void function, which returns null. Null gets coerced to false. So you now get like, basically a BC break, which you can't really? Yeah, we did a bit and then probably we sort of, it's probably a bad idea. That's also the point of like, making choices, things that are static analysers can be like, more informants being like, Okay, your if statement is kind of pointless here.
Derick Rethans 3:06: Yeah, you don't want to end up breaking BC. Now, we already had false and bool, you're adding true to this. How does that work with Union types? Can you make a union type of true or false?
George Peter Banyard 3:18: No. So there are two reasons mainly. A. true and false is the same as like boolean, which is like just use Boolean in this case. But you can say, well, it's more specific, so just allow it. So that's would be reasonable. But the problem is, false has different semantics than boolean. False does not coerce values. So it only accepts false as a literal value. Whereas boolean, if you're not in strict type, which is a lot of code, it will cause values like zero to false one, or any other integers to true. It will coerce every other integer to true, like the true type follows the behaviour of false of being a value type. So it only accepts true, you would get into this weird distinction of does true or false, mean exactly true or false? Or do you get the same behaviour as using the boolean type?
Derick Rethans 4:07: So I would say that true or false would than be more restrictive than bool.
George Peter Banyard 4:12: Exactly, which is a bit of a problem, because PHP internally has true and false and separate types, which also makes the implementation of this RFC extremely easy, because PHP already makes the distinction of them. But at the same time, the boolean type is just a union of the bitmask of true and false. You can't really distinguish between the types, true or false, or the boolean type within the type system. Currently just does it by checking if it only has one then it can do like two checks. Specifically, you would need to add like an extra flag. I mean, it's doable, but it's just like, Well, who knows which semantics we want? Therefore, just leave it for future discussion because I'm not very keen on it to be fair.
Derick Rethans 4:55: True or false are really only useful for return values and not so much for arguments types, because if you have an argument that that always must be true, then it's kind of pointless to have of course.
George Peter Banyard 5:05: Same as like it was with the null type RFC. Although there might be one case where PHP internal functions might change the value to true for an argument, I can maybe two types, would be like with the define function, this thing being like case insensitive or case sensitive, I don't remember what the parameter actually; could actually either be false or true, because at the moment, I think emits a notice, things do like the this thing is not supported, therefore the values what was ignored. But we could conceivably see that in PHP 9, we would actually implement this as a proper like: Okay, this only accepts true, yes, this argument is pointless, but it's in the middle of the function signature, so you can't really move it. The spl_register_overload function has like as its second argument, the throw on error or not, which since PHP 8 only accepts true, but it's in the middle of the function. The last argument is still very useful. It's prepend, instead of append the autoloader, I think, or might be the other way around, check the docs. Since PHP 8, this only accepts true. So if you pass in false, it will emit a notice and saying you'd like this argument has just been ignored. So whatever. But we can't really remove the argument. Because well, it's, if you use the third argument, as with positional arguments, then you would change like the signature and you would break it. Now, we don't have a way to enforce in PHP to use named arguments, because that would be a solution. It's just like, well, if you want to set this argument, you need to use named arguments, but we can't do that. Otherwise, then creating a new function, which has an alias, which is also kind of terrible. That would be one of the maybe only cases where you would actually get like true as a as an argument