[swift-evolution] (core library) modern URL types

Dave DeLong delong at apple.com
Tue Aug 22 14:01:57 CDT 2017

I suppose, if you squint at it weirdly.

My current Path API is a “Path” protocol, with “AbsolutePath” and “RelativePath” struct versions. The protocol defines a path to be an array of path components. The only real difference between an AbsolutePath and a RelativePath is that all file system operations would only take an AbsolutePath. A URL would also only provide an AbsolutePath as its “path” bit.

public enum PathComponent {
    case this // “."
    case up   // “..” 
    case item(name: String, extension: String?)

public protocol Path {   
    var components: Array<PathComponent> { get }
    init(_ components: Array<PathComponent>) // used on protocol extensions that mutate paths, such as appending components

public struct AbsolutePath: Path { }
public struct RelativePath: Path { }

By separating out the concept of an Absolute and a Relative path, I can put additional functionality on each one to make semantic sense (you cannot concatenate two absolute paths, but you can concat any path with a relative path, for example). Or all file system operations must take an AbsolutePath. 

One of the key things I realized is that a “Path” type should not be ExpressibleByStringLiteral, because you cannot statically determine if a Path should be absolute or relative. However, one of the initializers for an AbsolutePath would handle things like expanding a tilde, and both types try to reduce a set of components as much as possible (by filtering out “.this” components, and handling “.up” components where possible, etc). Also in my experience, it’s fairly rare to want to deal with a known-at-compile-time, hard-coded path. Usually you’re dealing with paths relative to known “containers” that are determined at runtime (current user’s home folder, app’s sandboxed documents directory, etc).

Another thing I’ve done is that no direct file system operations exist on AbsolutePath (like “.exists” or “.createDirectory(…)” or whatever); those are still on FileManager/FileHandle/etc in the form of extensions to handle the new types. In my app, a path is just a path, and it only has meaning based on the thing that is using it. An AbsolutePath for a URL is used differently than an AbsolutePath on a file system, although they are represented with the same “AbsolutePath” type.

I’m not saying this is a perfect API of course, or even that a hypothetical stdlib-provided Path should mimic this. I’m just saying that for my use-case, this has vastly simplified how I deal with paths, because both URL and String smell really bad for what I’m doing.


> On Aug 22, 2017, at 12:37 PM, Taylor Swift <kelvin13ma at gmail.com> wrote:
> So are you saying we need three distinct “URI” types for local-absolute, local-relative, and remote? That’s a lot of API surface to support.
> On Tue, Aug 22, 2017 at 12:24 PM, Dave DeLong <delong at apple.com <mailto:delong at apple.com>> wrote:
> I completely agree. URL packs a lot of punch, but IMO it’s the wrong abstraction for file system paths.
> I maintain an app that deals a lot with file system paths, and using URL has always felt cumbersome, but String is the absolute wrong type to use. Lately as I’ve been working on it, I’ve been experimenting with a concrete “Path” type, similar to PathKit (https://github.com/kylef/PathKit/ <https://github.com/kylef/PathKit/>). Working in terms of AbsolutePath and RelativePath (what I’ve been calling things) has been extremely refreshing, because it allows me to better articulate the kind of data I’m dealing with. URL doesn’t handle pure-relative paths very well, and it’s always a bit of a mystery how resilient I need to be about checking .isFileURL or whatever. All the extra properties (port, user, password, host) feel hugely unnecessary as well.
> Dave
>> On Aug 20, 2017, at 11:23 PM, Félix Cloutier via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> I'm not convinced that URLs are the appropriate abstraction for a file system path. For the record, I'm not a fan of existing Foundation methods that create objects from an URL. There is a useful and fundamental difference between a local path and a remote path, and conflating the two has been a security pain point in many languages and frameworks that allow it. Examples include remote file inclusion in PHP and malicious doctypes in XML. Windows also had its share of issues with UNC paths.
>> Even when loading an arbitrary URL looks innocuous, many de-anonymizing hacks work by causing a program to access an URL controlled by an attacker to make it disclose the user's IP address or some other identifier.
>> IMO, this justifies that there should be separate types to handle local and remote resources, so that at least developers have to be explicit about allowing remote resources. This makes a new URL type less necessary towards supporting file I/O.
>> Félix
>>> Le 20 août 2017 à 21:37, Taylor Swift via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> a écrit :
>>> Okay so a few days ago there was a discussion <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170814/038923.html> about getting pure swift file system support into Foundation or another core library, and in my opinion, doing this requires a total overhaul of the `URL` type (which is currently little more than a wrapper for NSURL), so I’ve just started a pure Swift URL library project at <https://github.com/kelvin13/url <https://github.com/kelvin13/url>>.
>>> The library’s parsing and validation core (~1K loc pure swift) is already in place and functional; the goal is to eventually support all of the Foundation URL functionality.
>>> The new `URL` type is implemented as a value type with utf8 storage backed by an array buffer. The URLs are just 56 bytes long each, so they should be able to fit into cache lines. (NSURL by comparison is over 128 bytes in size; it’s only saved by the fact that the thing is passed as a reference type.)
>>> As I said, this is still really early on and not a mature library at all but everyone is invited to observe, provide feedback, or contribute!
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