Re:Why scientists are turning to Rust (2020)
by tialaramex on Thursday August 04, 2022 @04:20AM
(#32340514)
Rust's Edition system allows it to deprecate and eventually remove syntax that's now considered a mistake, yet that syntax still exists (forever!) for anybody who doesn't want to learn the modern way to write it.
Let's take two serious C++ proposals I've looked at over the past years and imagine if it had Editions (it doesn't, Vittorio's similar Epochs proposal was shot down and won't be coming back). First a try operator. C++ doesn't have one of these, it would be usual to spell it ? but that's prohibited in C++ by the existence of a ? character as part of the ternary operator ?:. If you had Editions you could abolish ?: in a new Edition or change the way it is parsed so that some weird utterances now don't work because ? is the try operator, without them the proposers instead reluctantly suggest ?? (which is already an operator in some languages and it isn't try...)
Second, pattern matching with _ as a wildcard in the pattern. C++ doesn't do that today. Unfortunately _ is a valid identifier in C++ and people use it. Not a lot of people, but enough for it to be a real pain if this stopped working. So the proposers reluctantly suggest you could write __ in this case. If you had Editions you could forbid _ in a new Edition and in that Edition have the nicer _ while __ is provided in older Editions if they want new pattern matching.
Editions only cover syntax (they're actually mutating the Abstract Syntax Tree at compile time) but because this affords some great opportunities for backward compatible changes, it encourages people to get creative and do even more. For example Rust's arrays learned to be IntoIterator. But that's not a syntax change, so how did that remain compatible? Well, there's a hack, if you have an edition prior to 2021 Edition, then one function from IntoIterator is hidden from resolution by the hack, so that array.into_iter() does what it did in your edition before arrays were IntoIterator. But every other feature works, so you can write "for thing in array", you can pass arrays to functions which need an iterator, and so on, and in newer editions the hack isn't used so array.into_iter() does what you'd expect given arrays are now IntoIterator.
Let's take two serious C++ proposals I've looked at over the past years and imagine if it had Editions (it doesn't, Vittorio's similar Epochs proposal was shot down and won't be coming back). First a try operator. C++ doesn't have one of these, it would be usual to spell it ? but that's prohibited in C++ by the existence of a ? character as part of the ternary operator ?:. If you had Editions you could abolish ?: in a new Edition or change the way it is parsed so that some weird utterances now don't work because ? is the try operator, without them the proposers instead reluctantly suggest ?? (which is already an operator in some languages and it isn't try...)
Second, pattern matching with _ as a wildcard in the pattern. C++ doesn't do that today. Unfortunately _ is a valid identifier in C++ and people use it. Not a lot of people, but enough for it to be a real pain if this stopped working. So the proposers reluctantly suggest you could write __ in this case. If you had Editions you could forbid _ in a new Edition and in that Edition have the nicer _ while __ is provided in older Editions if they want new pattern matching.
Editions only cover syntax (they're actually mutating the Abstract Syntax Tree at compile time) but because this affords some great opportunities for backward compatible changes, it encourages people to get creative and do even more. For example Rust's arrays learned to be IntoIterator. But that's not a syntax change, so how did that remain compatible? Well, there's a hack, if you have an edition prior to 2021 Edition, then one function from IntoIterator is hidden from resolution by the hack, so that array.into_iter() does what it did in your edition before arrays were IntoIterator. But every other feature works, so you can write "for thing in array", you can pass arrays to functions which need an iterator, and so on, and in newer editions the hack isn't used so array.into_iter() does what you'd expect given arrays are now IntoIterator.