{"id":20746,"date":"2018-02-28T14:55:29","date_gmt":"2018-02-28T19:55:29","guid":{"rendered":"https:\/\/mjtsai.com\/blog\/?p=20746"},"modified":"2021-06-01T16:27:15","modified_gmt":"2021-06-01T20:27:15","slug":"swift-protocols-wishlist","status":"publish","type":"post","link":"https:\/\/mjtsai.com\/blog\/2018\/02\/28\/swift-protocols-wishlist\/","title":{"rendered":"Swift Protocols Wish List"},"content":{"rendered":"

Dave DeLong<\/a>:<\/p>\n

\n

If you’re adopting a protocol (especially one from the standard library), the tools don’t provide any help whatsoever in knowing what you actually have to implement.<\/p>\n

[…]<\/p>\n

Basically any time you find yourself using a PAT, you know that you’re either going to have to create a type eraser, or you’re going to have to make things generic that have no business being generic.<\/p>\n

[…]<\/p>\n

I wish the compiler generated type erasers for me.<\/p>\n

[…]<\/p>\n

I wish I could provide default implementations of protocol methods that assign to self<\/em>.<\/p>\n

[…]<\/p>\n

I wish I could extend a protocol to conform to another protocol.<\/p>\n<\/blockquote>\n\n

Previously: Swift Protocols With Associated Types<\/a>, Swift Type Erasure<\/a>, Patterns for Working With Associated Types<\/a>.<\/p>\n\n

Update (2018-03-05): See also: Swift Unwrapped<\/a>.<\/p>\n\n

Update (2018-07-27): Dave DeLong<\/a> (tweet<\/a>):<\/p>\n

What I really<\/em> want to do is this:<\/p>\n

protocol Predicate<Element> {\n    func contains(_ other: Element) -> Bool\n    func union(_ other: Predicate<Element>) -> Predicate<Element>\n}<\/pre>\n
Of course, the compiler doesn't let me do this.<\/p>\n
I want to express the idea that union()<\/code> can take any Predicate<\/code>-conforming value with the same element type, and you're going to get back some sort<\/em> of Predicate<\/code>-conforming value that contains elements of that type.<\/p>\n
Then in specific situations where I can provide a much more efficient implementation (like the IndexSet<\/code> + IndexSet<\/code> = IndexSet<\/code> scenario), I want to provide those explicit overrides for that method, and otherwise default back to the implementations I provide in my protocol extension (which happen<\/em> to return an OrPredicate<Int><\/code>)<\/p>\n<\/blockquote>\n
Brent Royal-Gordon<\/a>:<\/p>\n
What I think you actually want here is<\/em> the generalized existential feature. “Generalized existential” is a ten-dollar term for a ten-cent idea: “Swift should have built-in type-erasing wrappers for PATs, much like it has them for ordinary protocols.” Their syntax might be slightly different—they’d probably use a where<\/code> clause instead of a generic parameter list—but they would do the thing you want to do here.<\/p><\/blockquote>","protected":false},"excerpt":{"rendered":"
Dave DeLong: If you’re adopting a protocol (especially one from the standard library), the tools don’t provide any help whatsoever in knowing what you actually have to implement. […] Basically any time you find yourself using a PAT, you know that you’re either going to have to create a type eraser, or you’re going to […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"apple_news_api_created_at":"2018-07-27T18:38:04Z","apple_news_api_id":"089038d3-c958-45f5-937c-38252e0b4549","apple_news_api_modified_at":"2021-06-01T20:27:19Z","apple_news_api_revision":"AAAAAAAAAAAAAAAAAAAABA==","apple_news_api_share_url":"https:\/\/apple.news\/ACJA408lYRfWTfDglLgtFSQ","apple_news_coverimage":0,"apple_news_coverimage_caption":"","apple_news_is_hidden":false,"apple_news_is_paid":false,"apple_news_is_preview":false,"apple_news_is_sponsored":false,"apple_news_maturity_rating":"","apple_news_metadata":"\"\"","apple_news_pullquote":"","apple_news_pullquote_position":"","apple_news_slug":"","apple_news_sections":"\"\"","apple_news_suppress_video_url":false,"apple_news_use_image_component":false,"footnotes":""},"categories":[4],"tags":[46,71,901],"class_list":["post-20746","post","type-post","status-publish","format-standard","hentry","category-programming-category","tag-languagedesign","tag-programming","tag-swift-programming-language"],"apple_news_notices":[],"_links":{"self":[{"href":"https:\/\/mjtsai.com\/blog\/wp-json\/wp\/v2\/posts\/20746","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/mjtsai.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/mjtsai.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/mjtsai.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/mjtsai.com\/blog\/wp-json\/wp\/v2\/comments?post=20746"}],"version-history":[{"count":8,"href":"https:\/\/mjtsai.com\/blog\/wp-json\/wp\/v2\/posts\/20746\/revisions"}],"predecessor-version":[{"id":32663,"href":"https:\/\/mjtsai.com\/blog\/wp-json\/wp\/v2\/posts\/20746\/revisions\/32663"}],"wp:attachment":[{"href":"https:\/\/mjtsai.com\/blog\/wp-json\/wp\/v2\/media?parent=20746"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/mjtsai.com\/blog\/wp-json\/wp\/v2\/categories?post=20746"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/mjtsai.com\/blog\/wp-json\/wp\/v2\/tags?post=20746"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}