Difference in behavior between `first` and `first(where: { _ in true })` used on lazy sequence

KoCMoHaBTa · March 17, 2023, 1:00pm

Hi,
I would like to ask, why there is a difference in the behavior of using first vs first(where: { _ in true }) on lazy sequence. I was expecting both to be the same.
Is this a bug or expected. If expected - why?
Here are playground screenshots showing the output of both cases:

xwu · March 17, 2023, 2:13pm

github.com/apple/swift

[SR-12928] `.first` is less efficient than `.first { _ in true }` on LazyMapSequence

opened 09:41PM - 02 Jun 20 UTC

swift-ci

bug standard library

| | | |------------------|-----------------|… |Previous ID | SR-12928 | |Radar | None | |Original Reporter | bubski (JIRA User) | |Type | Bug | Attachment: [Download](https://user-images.githubusercontent.com/2727770/164963820-5004506d-d955-40ca-8404-f89e22b8367d.gz) <details> <summary>Environment</summary> Apple Swift version 5.2.2 (swiftlang-1103.0.32.6 clang-1103.0.32.51) Target: x86_64-apple-darwin19.4.0 macOS 10.15.4 (19E287) Xcode 11.4.1 </details> <details> <summary>Additional Detail from JIRA</summary> | | | |------------------|-----------------| |Votes | 0 | |Component/s | Standard Library | |Labels | Bug | |Assignee | None | |Priority | Medium | md5: 38b32ecf66be259b2052152057060ea8 </details> **Issue Description:** ``` swift _ = (1...1) .lazy .compactMap { value -> String? in print("lazy compactMap a \(value)") return "\(value)" }.first _ = (1...1) .lazy .compactMap { value -> String? in print("lazy compactMap b \(value)") return "\(value)" }.first { _ in true } _ = (1...1) .compactMap { value -> String? in print("eager compactMap \(value)") return "\(value)" }.first // output: // lazy compactMap a 1 // lazy compactMap a 1 // lazy compactMap b 1 // eager compactMap 1 ``` Shouldn't **.first** be synonymous to **.first { \_ in true }** in terms of not only result, but also effort? Also, is lazy ``` swift _ = (1...1) .lazy .compactMap { value -> String? in print("lazy compactMap a \(value)") return "\(value)" }.first ``` being less efficient than eager ``` swift _ = (1...1) .compactMap { value -> String? in print("eager compactMap \(value)") return "\(value)" }.first ``` expected?