<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Here’s another fun one: the following type checks despite the literal closure; we’ve constrained the return type of filter() to [Int], and not LazyFilterCollection<Int>, so again the type checker picks the overload that takes the non-escaping closure:<div class=""><br class=""></div><div class="">func myFilter(array: [Int], predicate: (Int) -> Bool) -> [Int] {<br class=""> return array.lazy.filter { predicate($0) }<br class="">}</div><div class=""><br class=""></div><div class="">Slava</div><div class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Mar 26, 2017, at 1:14 AM, Slava Pestov via swift-users <<a href="mailto:swift-users@swift.org" class="">swift-users@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Hi Ray,<div class=""><br class=""></div><div class="">There are two overloads of filter() available on ‘array.lazy’; the version that takes an escaping closure and returns a LazyFilterCollection, and the version that takes a non-escaping closure and returns [Int].</div><div class=""><br class=""></div><div class="">In the first example, we pick the LazyFilterCollection-returning overload, because the literal closure { predicate($0) } can be coerced to both an escaping or a non-escaping closure type, and in the absence of additional constraints we go with the overload from a concrete type over an overload in a protocol extension. After the overload has been picked we validate the body of the closure, and notice that it is invalid because whole the closure is already known to be @escaping, it references the non-@escaping ‘predicate’.</div><div class=""><br class=""></div><div class="">In the second example, ‘predicate’ is known to be non-@escaping, which rules out the first overload completely, so we go with the second overload and perform a non-lazy filter.</div><div class=""><br class=""></div><div class="">I would argue this is somewhat confusing, but it might be difficult to change the overload resolution rules in a way where the first overload is always chosen.</div><div class=""><br class=""></div><div class="">Slava</div><div class=""><br class=""><div class=""><blockquote type="cite" class=""><div class="">On Mar 26, 2017, at 12:13 AM, Ray Fix via swift-users <<a href="mailto:swift-users@swift.org" class="">swift-users@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class=""><br class=""></div><div class="">Hi,</div><div class=""><br class=""></div><div class="">One of the motivating examples for withoutActuallyEscaping looks like the following. This predictably doesn’t work because "use of non-escaping parameter 'predicate' may allow it to escape"</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);" class="">func</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> myFilter(array: [</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(112, 61, 170);" class="">Int</span><span style="font-variant-ligatures: no-common-ligatures;" class="">], predicate: (</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(112, 61, 170);" class="">Int</span><span style="font-variant-ligatures: no-common-ligatures;" class="">) -> </span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(112, 61, 170);" class="">Bool</span><span style="font-variant-ligatures: no-common-ligatures;" class="">) -> [</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(112, 61, 170);" class="">Int</span><span style="font-variant-ligatures: no-common-ligatures;" class="">] {</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);" class="">return</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(112, 61, 170);" class="">Array</span><span style="font-variant-ligatures: no-common-ligatures;" class="">(array.</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(112, 61, 170);" class="">lazy</span><span style="font-variant-ligatures: no-common-ligatures;" class="">.</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(62, 30, 129);" class="">filter</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> { predicate($0) })</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures;" class="">}</span></div></div><div class=""><br class=""></div><div class="">The solution is to use withoutActuallyEscaping. This works and produces the expected results.</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);" class="">func</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> myFilter(array: [</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(112, 61, 170);" class="">Int</span><span style="font-variant-ligatures: no-common-ligatures;" class="">], predicate: (</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(112, 61, 170);" class="">Int</span><span style="font-variant-ligatures: no-common-ligatures;" class="">) -> </span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(112, 61, 170);" class="">Bool</span><span style="font-variant-ligatures: no-common-ligatures;" class="">) -> [</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(112, 61, 170);" class="">Int</span><span style="font-variant-ligatures: no-common-ligatures;" class="">] {</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);" class="">return</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(62, 30, 129);" class="">withoutActuallyEscaping</span><span style="font-variant-ligatures: no-common-ligatures;" class="">(predicate) { predicate </span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);" class="">in</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(112, 61, 170);" class="">Array</span><span style="font-variant-ligatures: no-common-ligatures;" class="">(array.</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(112, 61, 170);" class="">lazy</span><span style="font-variant-ligatures: no-common-ligatures;" class="">.</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(62, 30, 129);" class="">filter</span><span style="font-variant-ligatures: no-common-ligatures;" class="">({predicate($0)}))</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""> }</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures;" class="">}</span></div></div><div class=""><br class=""></div><div class="">What I find puzzling is the below example compiles and runs correctly. It seems like it should be the same compiler error as in the first example.</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);" class="">func</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> myFilter(array: [</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(112, 61, 170);" class="">Int</span><span style="font-variant-ligatures: no-common-ligatures;" class="">], predicate: (</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(112, 61, 170);" class="">Int</span><span style="font-variant-ligatures: no-common-ligatures;" class="">) -> </span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(112, 61, 170);" class="">Bool</span><span style="font-variant-ligatures: no-common-ligatures;" class="">) -> [</span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(112, 61, 170);" class="">Int</span><span style="font-variant-ligatures: no-common-ligatures;" class="">] {</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><div style="margin: 0px; line-height: normal;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">return</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Array</span><span style="font-variant-ligatures: no-common-ligatures" class="">(array.</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">lazy</span><span style="font-variant-ligatures: no-common-ligatures" class="">.</span><span style="font-variant-ligatures: no-common-ligatures; color: #3e1e81" class="">filter</span><span style="font-variant-ligatures: no-common-ligatures" class="">(predicate))</span></div><div style="margin: 0px; line-height: normal;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">}</span></div></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div></div><div style="margin: 0px; line-height: normal; min-height: 13px;" class="">If you understand why this is so, it would be very helpful.</div><div style="margin: 0px; line-height: normal; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; line-height: normal; min-height: 13px;" class="">Thank you and best wishes,</div><div style="margin: 0px; line-height: normal; min-height: 13px;" class="">Ray</div></div>_______________________________________________<br class="">swift-users mailing list<br class=""><a href="mailto:swift-users@swift.org" class="">swift-users@swift.org</a><br class=""><a href="https://lists.swift.org/mailman/listinfo/swift-users" class="">https://lists.swift.org/mailman/listinfo/swift-users</a><br class=""></div></blockquote></div><br class=""></div></div>_______________________________________________<br class="">swift-users mailing list<br class=""><a href="mailto:swift-users@swift.org" class="">swift-users@swift.org</a><br class="">https://lists.swift.org/mailman/listinfo/swift-users<br class=""></div></blockquote></div><br class=""></div></body></html>