To expand on this, Set([1,2,3,4,5]).hasPrefix([1,2,3]) currently returns true. But let’s say a year from now, we change Set to return an ordering based on hash values (which is entirely reasonable). Suddenly the same code may return true or false.
No guarantees will be broken by doing that, but the result has still changed because we are building on top of undefined behavior. Collection says nothing about the ordering over different builds of a program. Thanks, Jon > On Oct 16, 2017, at 4:11 PM, Jonathan Hull via swift-evolution > <swift-evolution@swift.org> wrote: > >> >> On Oct 16, 2017, at 1:05 PM, Xiaodi Wu <xiaodi...@gmail.com >> <mailto:xiaodi...@gmail.com>> wrote: >> >> >> On Mon, Oct 16, 2017 at 10:49 Jonathan Hull <jh...@gbis.com >> <mailto:jh...@gbis.com>> wrote: >> >>> On Oct 16, 2017, at 7:20 AM, Xiaodi Wu <xiaodi...@gmail.com >>> <mailto:xiaodi...@gmail.com>> wrote: >>> >>> To start with, the one you gave as an example at the beginning of this >>> discussion: Two sets with identical elements which have different internal >>> storage and thus give different orderings as sequences. You yourself have >>> argued that the confusion around this is enough of a problem that we need >>> to make a source-breaking change (renaming it) to warn people that the >>> results of the ‘elementsEqual’ algorithm are undefined for sets and >>> dictionaries. >>> >>> No, I am arguing that the confusion about ‘elementsEqual’ is foremost a >>> problem with its name; the result of this operation is not at all undefined >>> for two sets but actually clearly defined: it returns true if two sets have >>> the same elements in the same iteration order, which is a publicly >>> observable behavior of sets (likewise dictionaries). >> >> But that iteration order is undefined and could easily change due to changes >> in the private/internal structure of sets/dictionaries. Algorithms that >> rely on that “publicly observable behavior” (i.e. leaking of internals) will >> suddenly break. >> >> And an algorithm in which such “sudden breakage” would occur is…? > > Here are a few off the top of my head: > > func hasPrefix(Sequence)->Bool > func hasSuffix(Sequence)->Bool > func containsSubsequence(Sequence)->Bool > > What do these methods mean with regards to Set’s “publicly observable > behavior”? > >> >> You keep claiming that this bug is a feature because it is the current >> behavior… but that is tautological reasoning. >> >> Thanks, >> Jon > > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org <mailto:swift-evolution@swift.org> > https://lists.swift.org/mailman/listinfo/swift-evolution > <https://lists.swift.org/mailman/listinfo/swift-evolution>
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