[swift-evolution] [Pitch] Raw mode string literals

[Xiaodi Wu]

On Thu, Nov 23, 2017 at 5:33 PM, Chris Lattner <clattner at nondot.org> wrote:

> On Nov 23, 2017, at 10:35 AM, Xiaodi Wu via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> This proposed addition addresses a known pain point, to be sure, but I
> think it has many implications for the future direction of the language and
> I'd like to explore them here.
>
>
> Thanks for writing this up Xiaodi,
>
> We should certainly move any discussion about regex literals into its own
> thread, but to make it clear that I'm not simply suggesting that we
> implement something in Swift 10 instead of addressing a known pain point
> now, here's a sketch of how Swift 5 could make meaningful progress:
>
> - Teach the lexer about basic /pattern/flag syntax.
> - Add an `ExpressibleByRegularExpressionLiteral`, where the initializer
> would be something like `init(regularExpressionLiteralPattern: String,
> flags: RegularExpressionFlags)` where RegularExpressionFlags would be an
> OptionSet type.
> - Add conformance to `ExpressibleByRegularExpressionLiteral` to
> `NSRegularExpression`.
> - Have no default `RegularExpressionLiteralType` for now so that, in the
> future, we can discuss and design a Swift standard library regular
> expression type, which is justifiable because we've baked in language
> support for the literal. This can be postponed.
>
>
> This approach could make sense, but it makes a couple of assumptions that
> I’m not certain are the right way to go (to be clear, I’m not certain that
> they’re wrong either!).
>
> Things I’d like to carefully consider:
>
> 1) We could make the compiler parse and validate regex literals at compile
> time:
>
> a) this allows the compiler to emit diagnostics (with fixits!) on
> malformed literals.
>
> b) When the compiler knows the grammar of the regex, it can precompile the
> regex into a DFA table or static executable code, rather than runtime
> compiling into a bytecode.
>
> c) however, the compiler can’t parse the literal unless it knows the
> dialect it corresponds to.  While we could parameterize this somehow (e.g.
> as a requirement in ExpressibleByRegularExpressionLiteral), if we weren’t
> bound by backwards compatibility, we would just keep things simple and say
> “there is one and only one grammar”.  I’d argue that having exactly one
> grammar supported by the // syntax is also *better* for users, rather than
> saying “it depends on what library you’re passing the regex into”.
>

I think we've circled back to a topic that we've discussed here before. I
do agree that having more of this validation at compile time would improve
the experience. However, I can see a few drawbacks to the _compiler_ doing
the validation:

- In the absence of a `constexpr`-like facility, supporting runtime
expressions would mean we'd be writing the same code twice, once in C++ for
compile-time validation of literal expressions and another time in Swift
for runtime expressions.

- As seen in these discussions about string literals where users want to
copy and paste text and have it "just work," supporting only one dialect in
regex literals will inevitably lead users to ask for other types of regex
literals for each individual flavor of regex they encounter.

Just like ExpressibleByDictionaryLiteral doesn't deduplicate keys, leaving
that to Dictionary, I think regex literals are better off not validating
literal expressions (or, maybe, doing only the barest sanity check),
leaving the rest to concrete regex types. As you point out with validation
of integer overflows during constant folding, we could get enough
compile-time validation even without teaching the compiler itself how to
validate the literal.

2) I’d like to explore the idea of making // syntax be *patterns* instead
> of simply literals.  As a pattern, it should be possible to bind submatches
> directly into variable declarations, eliminating the need to count parens
> in matches or other gross things.  Here is strawman syntax with a dumb
> example:
>
> if case /([a-zA-Z]+: let firstName) ([a-zA-Z]+: let lastName)/ =
> getSomeString() {
>    print(firstName, lastName)
> }
>
>
This is an interesting idea. But is it significantly more usable than the
same type having a collection of named matches using the usual Perl syntax?

  if case /(?<firstName>[a-zA-Z]+) (?<lastName>[a-zA-Z]+)/ =
getSomeString() {
    print(Regex.captured["firstName"], Regex.captured["lastName"])
  }

3) I see regex string matching as the dual to string interpolation.  We
> already provide the ability for types to specify a default way to print
> themselves, and it would be great to have default regex’s associated with
> many types, so you can just say “match an Int here” instead of having to
> match [0-9]+ and then do a failable conversion to Int outside the regex.
>
>
> 4) I’d like to consider some of the advances that Perl 6 added to its
> regex grammar.  Everyone knows that modern regex’s aren’t actually regular
> anyway, so it begs the question of how far to take it.  If nothing else, I
> appreciate the freeform structure supported (including inline comments)
> which make them more readable.
>

Sounds like we want multiline regex literals :)

We should also support a dynamic regex engine as well, because there are
> sometimes reasons to runtime construct regex’s.  This could be handled by
> having the Regex type support a conversion from String or something,
> orthogonal to the language support for regex literals/patterns.
>
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