[swift-evolution] Strings in Swift 4
matthew at anandabits.com
Thu Jan 26 13:27:38 CST 2017
> On Jan 26, 2017, at 1:15 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
> on Wed Jan 25 2017, Chris Lattner <sabre-AT-nondot.org> wrote:
>> On Jan 25, 2017, at 7:32 PM, Dave Abrahams <dabrahams at apple.com> wrote:
>>>> There are two important use cases for regex's: the literal case
>>>> (e.g. /aa+b*/) and the dynamically computed case. The former is
>>>> really what we’re talking about here, the latter should obviously be
>>>> handled with some sort of Regex type which can be formed from string
>>>> values or whatever.
>>> Ideally these patterns interoperate so that you can combine them.
>> Yes, as I mentioned, the regex literal should form something of the
>> Regex type. Any API that takes a Regex would work with them.
> But I think we want distinct types for some of these patterns so they
> can capture compile-time knowledge. That's why
> has a Pattern protocol. If you mean “type” in a looser sense that
> admits protocols, then we are aligned.
>>>> You should instead be able to directly bind subexpressions into local
>>>> variables. For example if you were trying to match something like
>>>> “42: Chris”, you should be able to use straw man syntax like this:
>>>> case /(let id: \d+): (let name: \w+)/: print(id); print(name)
>>> This is a good start, but inadequate for handling the kind of recursive
>>> grammars to which you want to generalize regexes, because you have to
>>> bind the same variable multiple times—often re-entrantly—during the same
>>> match. Actually the Kleene star (*) already has this basic problem,
>>> without the re-entrancy, but if you want to build real parsers, you need
>>> to do more than simply capture the last substring matched by each group.
>> Please specify some more details about what the problem is, because
>> I’m not seeing it. Lots of existing regex implementations work with
>> "(…)*” patterns by binding to the last value. From my perspective,
>> this is pragmatic, useful, and proven. What is your specific concern?
> My specific concern is that merely capturing the last match is
> inadequate to many real parsing jobs.
>> When you say “real” parsers, you’re implicitly insulting the “unreal"
> No offense intended, truly. As a PL guy I assumed you'd know what I
> meant. As you know, regexes aren't sufficiently powerful to handle
> parsing languages like Swift, and even if they were, retaining only the
> last match of a capture would be insufficient to go from recognizing
> valid input (parsing) to semantic analysis.
>> without explaining what the “real” ones are, or why they matter.
>> Please provide specific use cases that would be harmed by this
> I'm talking about the kinds of parsers made possible by Perl 6 grammars,
> which can be recursive. Some examples:
>>>> Unless we were willing to dramatically expand how patterns work, this
>>>> requires baking support into the language.
>>> I don't understand the "Unless" part of that sentence. It seems obvious
>>> that no expansion of how patterns work could make the above work without
>>> language changes.
>> I’m not a believer in this approach, but someone could argue that we
>> should allow arbitrary user-defined syntactic expansion of the pattern
>> grammar, similar to how we allow syntactic expansion of the expression
>> grammar through operator definitions. This is what I meant by
>> “dramatically expanding” how patterns work.
> Oh, I see what you mean. You need to bake *something* into the
> language. That thing could either be regex support or it could be
> something more general, like a macro system that allowed beautiful regex
> support to be built in a library. Well, I'd love to have the latter,
> but wouldn't be willing to sacrifice much quality-of-user-experience
> with in order to get it. It would have to be roughly indistinguishable
> from the end-user's point-of-view.
+1. I really like the directions the team is thinking about!
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