[swift-evolution] URL Literals

Tony Allevato allevato at google.com
Mon Dec 19 13:18:52 CST 2016

+1 to the sentiment in your last paragraph.

In general, I'm not a very big fan of the #foo(...) syntax for literals and
I think using that as the starting point for discussion biases us towards
those when more general and powerful alternatives could exist. That syntax
exists for Playground support and I'd hate to see it proliferate into
non-playground sources. Furthermore, if the user could write a
compile-time-validating initializer for `URL("http://foo.com/bar")` (or
`let url: URL = "http://foo.com/bar"`), then that expression becomes just
as much a "literal" as something like `#url("http://foo.com/bar")`. There's
no functional difference between the two: both are sequences of tokens that
the compiler can evaluate statically. The benefit is that the end user
doesn't have to worry about the distinction; the compiler picks the
appropriate evaluation automatically depending on whether the initializer
argument is statically known or not.

Taking this a step further, if we can get constexpr-like validation for
initializers like this, then I wonder if the #foo literal syntax needs to
exist at all, and I'd propose that we abandon it and just let the
playground UI and other IDEs transform things like `UIColor(red: 0.5,
green: 0.5, blue: 0.5)` into a swatch when it sees them if they can be
evaluated at compile-time. What you lose is, as Erica mentioned earlier,
the fact that `#colorLiteral(...)` is untyped and can be turned into
multiple color types, but the value already has to be assigned to a
concrete type eventually anyway.

Am I missing any use cases for things like #colorLiteral that doing this
would make difficult or impossible?

On Mon, Dec 19, 2016 at 10:53 AM David Sweeris via swift-evolution <
swift-evolution at swift.org> wrote:

> On Dec 19, 2016, at 1:26 AM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> URLs are unlikely to be something that can be validated by regex. See, for
> instance, this discussion: <
> https://webkit.org/blog/7086/url-parsing-in-webkit/>. The full spec is
> here: <https://url.spec.whatwg.org>. If Swift were to implement parsing
> of URLs at the level of the compiler or core library, I'd expect it to be
> the full spec, as we do with Unicode.
> On Mon, Dec 19, 2016 at 2:26 AM, Benjamin Spratling via swift-evolution <
> swift-evolution at swift.org> wrote:
> Howdy,
> Yes, I was also intrigued by the “Regex” validation mentioned in another
> post.  It could offer a convenient way to get some literals support in
> without the headaches associated with the constexpr C++ approach.
> I’m curious, though, how many types can we image in can be validated by
> this method?  If it really is just URL’s, then I’d actually lean towards
> making this a compiler magic feature.
> Someone else mentioned fetching the URL’s for a preview.  Given that we
> might be coding “deletes” in URL’s (yes, I recently met a backend developer
> who coded a delete as a GET), I really highly suggest we not ping people’s
> API’s artificially.  At least we shouldn’t for non-file-scheme URLs.  IMHO,
> verifying that a service is active isn’t really the Swift compiler’s job.
> It might happen as part of coordinated run-time tests, which sometimes have
> to be balanced to keep test data correct, something the IDE wouldn’t know
> how to enforce correctly.
> -Ben
> On Dec 19, 2016, at 1:41 AM, David Sweeris via swift-evolution <
> swift-evolution at swift.org> wrote:
> On Dec 17, 2016, at 1:12 PM, Micah Hainline via swift-evolution <
> swift-evolution at swift.org> wrote:
> I'd love a fleshed out elegant example for URL that shows what a complete
> implementation of that special init method would look like.
> Sorry this took so long… the weekend kinda got away from me.
> Anyway, I was thinking something like this (which has been very simplified
> on account of my regexing being sub-sketchy, and me not knowing exactly
> what’s valid in an URL anyway):
> #literalpatterns += (name: “URLLiteralType”, components: (name: url,
> type: StringLiteralType, pattern: “(http|https)://(www.)?[a-z|A-Z|0-9]+
> .(com|org|net)(/[a-z|A-Z|0-9]+)*(/[a-z|A-Z|0-9]+.[a-z|A-Z|0-9]+)?”),
> protocol: ExpressibleByURLLiteral)
> This would let the compiler know pretty much everything it needs to know…
> that the “new” type is called “URLLiteralType", that it starts out life as
> young StringLiteralType with a bright future in the computer industry, that
> in order to succeed it has to match a given pattern, and what protocol a
> type has to conform to in order to use an URLLiteral. In practice, the
> compiler would synthesize a struct containing the specified members and
> validate the literal with the specified pattern before making an “instance”
> of it (since we’re talking about literals and compile-time code here, I’m
> pretty sure that “instance" the wrong terminology… pardon my ignorance)
> struct URLLiteralType: {
>     let url: StringLiteralType
> }
> A tuple would be better, IMHO, but according to the playground,
> single-element tuples can’t have element labels. As for the
> implementation of the init function:
> init(urlLiteral value: URLLiteralType) {
>     let urlString = value.url
>     //Do whatever URL is doing now, except there’s no need to check for
> errors since the compiler pre-validated it for us
> }
> If it’d be more useful, the pattern could be split into multiple pieces:
> #literalpatterns += (name: “URLLiteralType”,
>                      components: ((name: “`protocol`", type:
> StringLiteralType, pattern: “(http|https)”),
>                                   (name: _,            type:
> StringLiteralType, pattern: “://”),
>                                   (name: “domain",     type:
> StringLiteralType, pattern: “(www.)?[a-z|A-Z|0-9]+.(com|org|net)”),
>                                   (name: “path”,       type:
> StringLiteralType, pattern: "(/[a-z|A-Z|0-9]+)*(/[a-z|A-Z|0-9]+.
> [a-z|A-Z|0-9]+)?”))
>                      protocol: ExpressibleByURLLiteral)
> This would result in URLLiteralType looking like this:
> struct URLLiteralType: {
>     let `protocol`: StringLiteralType
>     let domain: StringLiteralType
>     let path: StringLiteralType
> }
> And in the init would start out like this:
> init(urlLiteral value: URLLiteralType) {
>     let protocolType = value.protocol
>     let domain = value.domain
>     let path = value.path
>     //Do whatever with the components
> }
> The “base” types of literals like Int or String that don’t refine
> pre-existing literal types would still need a bit of compiler magic (or at
> least a different mechanism for becoming actual types), but as long as a
> type doesn’t take advantage of reference semantics in its stored properties
> or something, I *think* pretty much any data type could become
> “literalizeable” with something like this. Oh, and there’s nothing
> particularly magical about regular expressions as far as this idea is
> concerned; they’re just usually the first thing that comes to mind when I
> think of pattern matching in a string.
> I know this looks like a lot of code, but the scary-looking parts with the
> regex stuff only has to be written once for each “type” of literal… types
> that want to be expressible by such a literal just have to write an init
> function.
> It doesn’t have to be regex per se… instead of
> #literalpatterns += (name: “URLLiteralType”, components: (name: url,
> type: StringLiteralType, pattern: “(http|https)://(www.)?[a-z|A-Z|0-9]+
> .(com|org|net)(/[a-z|A-Z|0-9]+)*(/[a-z|A-Z|0-9]+.[a-z|A-Z|0-9]+)?”),
> protocol: ExpressibleByURLLiteral)
> I probably should’ve written something more like:
> #literalpatterns += (name: “URLLiteralType”, components: (name: url,
> type: StringLiteralType, matching: Regex(“(http|https)://
> (www.)?[a-z|A-Z|0-9]+.(com|org|net)(/[a-z|A-Z|0-9]+)*(/[a-z|A-Z|0-9]+.
> [a-z|A-Z|0-9]+)?”)), protocol: ExpressibleByURLLiteral)
> where the `matching` argument can be anything that can (“@purely”-ly) use
> some specified mechanism (I’d vote for the ~= operator) with a literal to
> test whether it matches. Also, there is no existing `Regex`
> struct/class/mechanism in Swift, unless you count `NSRegularExpression`. I
> didn’t want to use that for a couple reasons… 1) I don’t think it’s part of
> the stdlib, and 2) it doesn’t have a non-failable init that just takes a
> string, so using it unmodified would kinda put us in a “it’s turtles all
> the way down” kind of situation. What I’d started doing was to look for the
> existing mechanism for specifying literals in the compiler so I could use
> the existing name for it (somehow I doubt there’s a actually an array of
> patterns called “literalpatterns" in the compiler) and copy the existing
> methods for specifying a valid literal. After being unsuccessful for some
> amount of time, I decided I was getting too tired and made up what I sent
> last night.
> The more I think about it, the more I’m starting to be of the opinion that
> we really ought to have two mechanisms here… One for specifying what
> constitutes a “base” literal (like `43`, `[“foo”, “bar”]`, or `true`), and
> one for types that merely need to perform some sort of validation on
> existing “base” literals. The first mechanism probably should be fairly
> arcane and involved, because you’d essentially be able to create new
> syntaxes, which *should* be kinda scary and hard to understand because
> it’s most certainly not an area beginners should be in. The second
> mechanism — something like that `ExpressibleByValidatedStringLiteral`
> idea — isn’t nearly as complicated. In the case of URLs, I’d vote the
> second approach. We only really need two extra features to implement it
> (“@constexpr” and the compiler being able to use the REPL to evaluate
> @costexpr statements), and both of them have more uses other than just
> getting a few more compile-time checks or allowing for more inits to be
> non-failable. With both of those in place, getting an url “literal” becomes
> just this:
> protocol ExpressibleByValidatedStringLiteral {
>     init?(stringLiteral value: StringLiteralType)
> }
> struct URL : ExpressibleByValidatedStringLiteral {
>     //Stuff
>     //...
>     init?(stringLiteral value: StringLiteralType) {
>         //Perform validation here; return nil if it fails
>     }
>     //...
>     //Things
> }
> var lru: URL = "foo" //Compiler throws this to the init? function, it
> returns nil, the compiler raises a syntax error
> var url: URL = "http://www.some.valid.url.com" //Compiler throws this to
> the init? function, it returns an optional URL, the compiler unwraps it and
> does the assignment
> I still very much want a way to define custom literals (precisely because
> it’d let me make new syntaxes), but I’m starting to think that something
> like the second, disappointingly easy idea, is probably the way to go in
> this case.
> - Dave Sweeris
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> swift-evolution at swift.org
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