Re: [RFC] Nullable intersection types

On Friday, July 23, 2021 at 4:58 AM Nicolas Grekas <nicolas.grekas@gmail.com> wrote: > Hi everyone, > > as proposed by Nikita and Joe, I'm submitting this late RFC for your > consideration for inclusion in PHP 8.1. Intersection types as currently > accepted are not nullable. This RFC proposes to make them so. > > I wrote everything down about the reasons why here: > https://wiki.php.net/rfc/nullable_intersection_types > > Please have a look and let me know what you think. Hi Nicolas, Thanks for your work on this. The RFC states: > the preference of the author of this RFC is to define “?” as having > a lower precedence than any other type-operator, and thus to use > the “?X&Y” syntax. I also strongly believe this would be a mistake. When union types were introduced, it was explicitly decided not to use the ?(T1|T2) syntax, for the following reason [1]: > this notation is both rather awkward syntactically, and differs from > the well-established T1|T2|null syntax used by phpdoc comments. > The > discussion feedback was overwhelmingly in favor of supporting the > T1|T2|null notation. ?T is a shorthand alias for T|null, and the alias cannot be used to add null to a union type. Although PHP doesn't yet support generalized union and intersection type combinations, this is almost certainly something we'll want to enable in the future (in fact, this very RFC is a step towards this). But if union types don't support the shorthand nullability syntax, and intersection types require it, how will the syntaxes be combined in the future? E.g. will we write null | A & B | C or will it be required to use something like ?A & B | C? The latter is not only syntactically awkward but also very confusing. Since |null is the only supported syntax to add null to a union type, for consistency the same syntax should be used to add null to an intersection type. This will avoid confusion and inconsistency down the road if a future RFC enables more general union and intersection type combinations. It also avoids the need to define a precedence for the ? type operator. As you note in the RFC, PHP already defines | as having a lower precedence than &, so X & Y | null can only be interpreted as (X & Y) | null. This is consistent with other languages such as TypeScript, where A & B | C & D is parsed as (A & B) | (C & D). [2] Since precedence is already defined for this syntax consistently with other languages, I don't think it's necessary to require parentheses if we use the A & B | null syntax. Best regards, Theodore [1]: https://wiki.php.net/rfc/union_types_v2#nullable_union_types [2]: https://github.com/Microsoft/TypeScript/pull/3622