[swift-evolution] String update

[John Holdsworth]

Hi Michael,

Thanks for sending this through. It’s an interesting read. One section gave me pause however. I feel Swift should resist the siren call of combining Swift Syntax with Regex syntax as it falls on the wrong side of Occam's razor. ISO Regex syntax is plenty complex enough without trying to incorporate named capture, desirable as it may be. Also, if I was to go down that route, I’d move away from / as the delimiter which is a carry over from Perl to something like e”I am a regex” to give the lexer more to go on which could represent say, a cached instance of NSRegularExpression.

And now for something completely different...

Common usage patterns for a regex fall into 4 categories: deconstruction, replacement, iteration and switch/case. Ideally the representation of a regex match would the same for all four of these categories and I’d like to argue a set of expressive regex primitives can be created without building them into the language.

I’ve talked before about a regex match being coded as a string/regex subscripting into a string and I’ve been able to move this forward since last year. While this seems like an arbitrary operator to use it has some semantic sense in that you are addressing a sub-part of the string with pattern as you might use an index or a key. Subscripts also have some very interesting properties in Swift compared to other operators or functions: You don’t have to worry about precedence, they can be assigned to, used as an interator, and I've learned since my last email on this topic that the Swift type checker will disambiguate multiple subscript overloads on the basis of the type of the variable is being assigned to.

An extension to String can now realise the common use cases by judicious use of types:

var input = "Now is the time for all good men to come to the aid of the party"

if input["\\w+"] {
    print("match")
}

// receiving type controls data you get
if let firstMatch: Substring = input["\\w+"] {
    print("match: \(firstMatch)")
}

if let groupsOfFirstMatch: [Substring?] = input["(all) (\\w+)"] {
    print("groups: \(groupsOfFirstMatch)")
}

// "splat" out up to N groups of first match
if let (group1, group2): (String, String) = input["(all) (\\w+)"] {
    print("group1: \(group1), group2: \(group2)")
}

if let allGroupsOfAllMatches: [[Substring?]] = input["(\\w)(\\w*)"] {
    print("allGroups: \(allGroupsOfAllMatches)")
}

// regex replace by assignment
input["men"] = "folk"
print(input)

// parsing a properties file using regex as iterator
let props = """
    name1 = value1
    name2 = value2
    """

var params = [String: String]()
for groups in props["(\\w+)\\s*=\\s*(.*)"] {
    params[String(groups[1]!)] = String(groups[2]!)
}
print(params)

The case for switches is slightly more opaque in order to avoid executing the match twice but viable.

let match = RegexMatch()
switch input {
case RegexPattern("(\\w)(\\w*)", capture: match):
    let (first, rest) = input[match]
    print("\(first) \(rest)")
default:
    break
}

This is explored in the attached playground (repo: https://github.com/johnno1962/SwiftRegex4 <https://github.com/johnno1962/SwiftRegex4>)


I’m not sure I really expect this to take off as an idea but I’d like to make sure it's out there as an option and it certainly qualifies as “out there”.

John

> On 10 Jan 2018, at 19:58, Michael Ilseman via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Hello, I just sent an email to swift-dev titled "State of String: ABI, Performance, Ergonomics, and You!” at https://lists.swift.org/pipermail/swift-dev/Week-of-Mon-20180108/006407.html, whose gist can be found at https://gist.github.com/milseman/bb39ef7f170641ae52c13600a512782f. I posted to swift-dev as much of the content is from an implementation perspective, but it also addresses many areas of potential evolution. Please refer to that email for details; here’s the recap from it:
> 
> ### Recap: Potential Additions for Swift 5
> 
> * Some form of unmanaged or unsafe Strings, and corresponding APIs
> * Exposing performance flags, and some way to request a scan to populate them
> * API gaps
> * Character and UnicodeScalar properties, such as isNewline
> * Generalizing, and optimizing, String interpolation
> * Regex literals, Regex type, and generalized pattern match destructuring
> * Substitution APIs, in conjunction with Regexes.
> 
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution

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