On Mon, Jan 8, 2018 at 4:29 PM, Ben Cohen via swift-evolution <email@example.com> wrote: > There exists in the standard library a type `DictionaryLiteral` that deserves > naming re-consideration before we declare ABI Stability, because it’s > confusingly misnamed, being neither a Dictionary (it doesn’t provide > key-based lookup of values) nor a Literal. > > Instead, it’s just an immutable collection of key-value pairs you can create > _from_ a literal.
I was around when we were naming this type, and I don't think it is misnamed. However, I just read the documentation and I don't believe it explains the intended use of the type, hence we have this confusion. The intended use case for DictionaryLiteral is to build DSLs. Swift provides types in the standard library that are efficient and lossless representations for all literals, but are also useful as your workhorse data types: string literal -- String, array literal -- Array, integer literal -- [U]Int64, kinda (there is some dissatisfaction with integer literals since there is no type to capture a >64 bit literal, which is useful for defining a bigint type). But there was no type that could capture a dictionary literal in a lossless form, maintaining the order of items, not imposing dictionary limitations like uniqueness or hashability of keys. Therefore, we added DictionaryLiteral. Yes, it did land together with Mirrors in commit 50c6e936d4b94b9736a8d060ddf052ef1ba9c74d, but the intent was that it is a generally useful library component unrelated to Mirrors, and therefore we did not add any Mirror-related words to the name (we couldn't nest it in Mirror because nested generics didn't work at the time). In fact you can see the comment in Mirrors.swift that has been preserved to this date: // This component could stand alone, but is used in Mirror's public interface. Mirrors just *use* DictionaryLiteral to build a small DSL in the initializer of Mirror, so that callers could call that initializer representing Mirror's children with a nice intuitive syntax using a dictionary literal. Therefore, based on this intended use, I think that trying to hint that this type is a general *collection* of any kind is a disservice for users -- DictionaryLiteral wasn't intended to be a collection, and it does not have performance characteristics anyone would reasonably expect from it. We could add more words to the name to paint it more an "advanced tool shade", for example, CapturedDictionaryLiteral, DictionaryLiteralSyntax or something along those lines, but I feel like the impact of that to fix the confusion would be minimal. Instead, I think the right way to fix the confusion is to adjust the documentation to not claim that it is first and foremost a collection, because currently documentation is only technically correct, but does not explain the intended use of the type at all. Dmitri _______________________________________________ swift-evolution mailing list firstname.lastname@example.org https://lists.swift.org/mailman/listinfo/swift-evolution