[swift-evolution] String update

[George Leontiev]

Thanks, Michael. This is very interesting!

I wonder if it is worth considering (for lack of a better word) *verbose* regular expression for Swift.

For instance, your example:
```
let usPhoneNumber = /
  (let area: Int? <- \d{3}?) -
  (let routing: Int <- \d{3}) -
  (let local: Int <- \d{4}) /
```
would become something like (strawman syntax):
```
let usPhoneNumber = /let area: Int? <- .numberFromDigits(.exactly(3))/ + "-" +
                    /let routing: Int <- .numberFromDigits(.exactly(3))/ + "-"
                    /let local: Int <- .numberFromDigits(.exactly(4))/
```
With this format, I also noticed that your code wouldn't match "555-5555", only "-555-5555", so maybe it would end up being something like:
```
let usPhoneNumber = .optional(/let area: Int <- .numberFromDigits(.exactly(3))/ + "-") +
                    /let routing: Int <- .numberFromDigits(.exactly(3))/ + "-"
                    /let local: Int <- .numberFromDigits(.exactly(4))/
```
Notice that `area` is initially a non-optional `Int`, but becomes optional when transformed by the `optional` directive.
Other directives may be:
```
let decimal = /let beforeDecimalPoint: Int <-- .numberFromDigits(.oneOrMore)/ +
              .optional("." + /let afterDecimalPoint: Int <-- .numberFromDigits(.oneOrMore)/
```

In this world, the `/<--/` format will only be used for explicit binding, and the rest will be inferred from generic `+` operators.


I also think it would be helpful if `Regex` was generic over all sequence types.
Going back to the phone example, this would looks something like:
```
let usPhoneNumber = .optional(/let area: Int <- .numberFromDigits(.exactly(3))/ + "-") +
                    /let routing: Int <- .numberFromDigits(.exactly(3))/ + "-"
                    /let local: Int <- .numberFromDigits(.exactly(4))/
print(type(of: usPhoneNumber)) // => Regex<UnicodeScalar, (area: Int?, routing: Int, local: Int)>
```
Note the addition of `UnicodeScalar` to the signature of `Regex`. Other interesting signatures are `Regex<JSONToken, JSONEnumeration>` or `Regex<HTTPRequestHeaderToken, HTTPRequestHeader>`. Building parsers becomes fun!

- George

> On Jan 10, 2018, at 11:58 AM, 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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