[Discussion] is_string(), string type and objects implementing __toString()

Hi all, I submitted a GitHub PR* to allow objects implementing __toString() to *optionally* pass is_string() validation. More verbose wording of my motivation can be seen in the PR description, but here are the main points: - Simpler way to do checks like: is_string($var) || method_exists($var, '__toString') - Can be used for stricter string parameter validation in strict_types=0 mode (otherwise any scalar type is accepted) - Can be used for looser string parameter validation in strict_types=1 mode (__toString() objects aren't accepted there) - Regardless of the last 2 points, it is intentionally not limited to parameter types * https://github.com/php/php-src/pull/2408 --- I didn't have time to write this email right after submitting the patch, and in the meantime got some feedback from Fleshgrinder on GitHub, which I'll quote and address here: > Thanks for your effort and initiative. > > However, I strongly believe that this is the wrong approach. Adding a flag to a function > directly results in the fact that the function violates the single responsibility principle. What we > actually need to make this work is a "stringable" pseudo-type like the iterable type that > was introduced in PHP 7.1. This "stringable" pseudo-type is the union of the scalar > primitive string and any class that implements the __toString method. > > This has the advantage that we are actually able to use it together with strict_types, plus we > have separate dedicated functions like "is_stringable" that adhere to the single > responsibility principle. I actually wanted to create an RFC for that along with an implementation > since iterable was accepted, but did not find the time yet. > > Closing note: these pseudo-types are necessary in PHP because it has no coherent type system, > and there is nothing we can do about this in short term. Hence, adding such pseudo-types is the only > short term solution that we actually have. I ultimately wouldn't care if it's a separate function and did in fact think of an is_stringable() function, but wasn't happy with the naming - who's to say that e.g. integers aren't stringable? Bar horribly-verbose names like "string_or_objects_implementing__toString", I don't think there's a way to avoid that ambiguity. :/ If we want a "stringable" type though, I guess we'll have to live with that.. I feel that debating the actual type system is way broader than I intended this to be, so I'll refrain from going further on that for now, as I've got some more radical ideas about it. --- Thoughts? Cheers, Andrey.