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https://github.com/ilyapuchka/urlformat

Type safe url pattern matching without regular expressions and arguments type mismatches based on parser combinators.
https://github.com/ilyapuchka/urlformat

parser-combinators swift url-parsing

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Type safe url pattern matching without regular expressions and arguments type mismatches based on parser combinators.

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# URLFormat



Type safe url pattern matching without regular expressions and argument type mismatches based on parser combinators.



Example:



```swift

let format: URLFormat = ""/.users/.string/.repos/?.filter(.string)&.page(.int)

let url = URLComponents(string: "/users/apple/repos/?filter=swift&page=2")!

let request = URLRequestComponents(urlComponents: url)

let parameters = try format.parse(request)



_ = flatten(parameters) // ("apple", "swift", 2)

try format.print(parameters) // "users/apple/repos?filter=swift&page=2"

try format.template(parameters) // "users/:String/repos?filter=:String&page=:Int"

```



This library is based on [CommonParsers](https://github.com/ilyapuchka/common-parsers) which provides a common foundation for parser combinators and is heavily inspired by [swift-parser-printer](https://github.com/pointfreeco/swift-parser-printer) from [pointfreeco](https://twitter.com/pointfreeco). If you want to learn more about [parser combinators](https://www.pointfree.co/episodes/ep62-parser-combinators-part-1) and application of functional concepts in every day iOS development check out their [blog](https://www.pointfree.co).



URLFormat is used in [SwiftNIOMock](https://github.com/ilyapuchka/SwiftNIOMock) to implement URL router.



To use URLFormat with Vapor use a dedicated "vapor" branch ([Read more](#Vapor)) 



Also checkout [Interplate](https://github.com/ilyapuchka/Interplate) which provides a foundation for string templates using parser combinators and string interpolation together.



## Usage



URLFormat is a URL builder that allows you to describe URL in a natural manner and allows you to pattern match it in a type safe way.



The conventional way of representing URL patterns, i.e. for web server API routes, is using some kind of string placeholder for parameters, i.e. `/user/:name`. This is then parsed, and path and query parameters are aggregated into a collection. The issue is that this approach is error-prone (what if `:` is missed) and access to the parameters is not type safe - it's possible to access parameters as a wrong type or conversion must be implemented by the client, and it's possible to access parameter by the wrong key or index.



Another approach that Swift allows is to use enums pattern matching, as described in [this post](https://alisoftware.github.io/swift/pattern-matching/2015/08/23/urls-and-pattern-matching/) and implemented in [URLPatterns](https://github.com/johnpatrickmorgan/URLPatterns). While this approach allows type-safe access to parameters it's not very ergonomic and nice to read:



```swift

if case .n4("user", let userId, "profile", _) ~= url.countedPathElements() { ... }

```



Another downside of this approach is that it only allows to extract parameters of the same type, so most of the time you would extract all of them as `String` and convert to other types:



```swift

case chat(room: String, membersCount: Int)



case .n3("chat", let room, let membersCount):

  self = .chat(room: room, membersCount: number) // Cannot convert value of type 'String' to expected argument type 'Int'

```



In Vapor routes are defined as a collection of path components:



```swift

router.get("users", String.parameter) { req in

    let name = try req.parameters.next(String.self)

    return "User #\(name)"

}

```



You can as well use string placeholders for parameters:



```swift

router.get("users", ":name") { request in

    guard let userName = request.parameters["name"]?.string else {

        throw Abort.badRequest

    }



    return "You requested User #\(userName)"

}

```



This is nicer to write and read, but it's even less type safe - the parameters must be fetched in the order they appear in the path and their types should match but the compiler won't ensure that and you would need to make sure that the pattern definition and parameter access are always in sync.



You also can't describe query parameters in the route, they are instead accessed in the route handler either via `request.data["key"]?.string` or `request.query?["key"]?.stirng` which is also not type safe.



With URLFormat you would describe URLs as follows:



```swift

let urlFormat: URLFormat = ""/.users/.string/.repos/?.filter(.string)&.page(.int)

let url = URLComponents(string: "/users/apple/repos/?filter=swift&page=2")!

let request = URLRequestComponents(urlComponents: url)

let parameters = urlFormat.parse(request)

print(flatten(parameters)) // ("apple", "swift", 2)

```



This pattern will match URL with path like `/users/apple/repos/?filter=swift&page=1` (first and last `/` are optional). The fully qualified type of `urlFormat` in this case would be  `ClosedQueryFormat<((String, String), Int)>` (most of the time using base class type `URLFormat` is sufficient). The type of generic parameter describes the types of all captured parameters. To extract them from the actual URL you'd use `parse` method and one of `flatten` functions to "flatten" nested tuples, i.e. `((A, B), C) -> (A, B, C)` which makes it more convenient to access parameters.



Note that it's not necessary to specify a generic type parameter manually as the compiler can infer it from the declaration[1](#f1). And the compiler ensures that pattern and types of captured parameters are always in sync.



A nice caveat is that `URLFormat` can be used to print actual URLs and their readable templates if you provide it values for its parameters (again the compiler makes sure that they are always in sync):



```swift

let parameters = parenthesize("apple", "swift", 2)

urlFormat.print(parameters) // "users/apple/repos?filter=swift&page=2"

urlFormat.template(parameters) // "users/:String/repos?filter=:String&page=:Int"

```



Note that there are no string literals involved in declaring this URL except the first one. This is because under the hood `URLFormat` implements `@dynamicMemberLookup`, so an expression like `.users` is converted to the parser that parses `"users"` string from the path components.



You can either leave the first string component empty[2](#f2) or use it to specify the HTTP method of the request if you use URLFormat with HTTP requests and not just URLs:



```swift

let urlFormat: URLFormat = "GET"/.users/.string/.repos/?.filter(.string)&.page(.int)

let url = URLComponents(string: "/users/apple/repos/?filter=swift&page=2")!

let request = URLRequestComponents(method: "GET", urlComponents: url)

let parameters = urlFormat.parse(request)

urlFormat.print(parameters) // "GET users/apple/repos?filter=swift&page=2"

```



Path parameters are parsed using `.string` and `.int` operators. Query parameters are parsed with a combination of these operators and dynamic member lookup, so `.filter(.string)` will parse a string query parameter named `"filter"`, `.page(.int)` will parse an integer query parameter named `"page"`.



URLFormat also makes sure that URL is composed of path and query components correctly by allowing usage of `/`, `/?`, `&`, `*` and `*?` operators only in the correct places. This is done by using different subclasses of `URLFormat` to keep track of the builder state. It is similar to using phantom generic type parameters but allows to implement dynamic member lookup only for specific states of the builder.



1: an exeption here is when pattern does not capture any parameters, i.e. `_ = URLFormat = ""/.helloworld` . `Prelude.Unit` here is a type, similar to `Void`, but unlike `Void` it is an actual empty struct type. [↩](#a1)



2: String in the beginning of the pattern is needed because static `dynamicMemberLookup` subscript calls can't be infered without explicitly specifying type in the beginning of expression (see [this discussion](https://forums.swift.org/t/static-dynamicmemberlookup/33310/5) for details) [↩](#a2)



## Parameters types



Following parameters types are supported:



- `String` with `.string` operator 

- `Character` with `.char` operator

- `Int` with `.int` operator

- `Double` with `.double` operator

- `Bool` with `.bool` operator

- `UUID` with `.uuid` operator

- `Any` with `.any` operator (unlike `*` this will match only single path component, `*` will capture all trailing path components into one string)

- `LosslessStringConvertible` types with `lossless(MyType.self)` operator

- `RawRepresentable` with `String`, `Character`, `Int` and `Double` raw value types with `raw(MyType.self)` operator



In rare cases where your URL path components collide with these operator names you can use a `.const` operator to define path component as a string literal and not a typed parameter:



```swift

/.users/.const("uuid")/.uuid

```



You can add support for your own types by implementing `PartialIso`:



```swift

import CommonParsers



extension URLPartialIso where A == String, B == MyType {

    static var myType: URLPartialIso { ... }

}

extension OpenPathFormat where A == Prelude.Unit {

    var myType: ClosedPathFormat {

        return ClosedPathFormat(parser %> path(.myType))

    }

}

extension OpenPathFormat {

    var myType: ClosedPathFormat<(A, MyType)> {

        return ClosedPathFormat(parser <%> path(.myType))

    }

}

```



With that you can use your type as a path or a query parameter:



`""/.users/.myType/.repos/?.filter(.myType)&.page(.int)`



## Operators



`/` - concatenates two path components

`/?` - concatenates path with a query component

`&` - concatenates two query components

`*` - allows any trailing path components

`*?` - concatenates path with any trailing path components and a query component



## Vapor



To use URLFormat with Vapor you need to install it from the "vapor" branch . Then you can use `import VaporURLFormat` instead of `import URLFormat` and register routes using `router` method instead of `get`, `post`, `put` etc.:



```swift

router.route(GET/.hello/.string) { (request, string) in

    print(string) // "vapor"

    ...

}

try app.client(.GET, "/hello/vapor")

```



With Swift 5.2 you can use router as a function directly (using Swift static callable feature):



```swift

router(GET/.hello/.string) { (request, string) in

    print(string) // "vapor"

    ...

}

try app.client(.GET, "/hello/vapor")

``` 



## Installation



```swift

import PackageDescription



let package = Package(

    dependencies: [

        .package(url: "https://github.com/ilyapuchka/URLFormat.git", .branch("master")),

    ]

)

```



For using URLFormat with Vapor:



```swift

import PackageDescription



let package = Package(

    dependencies: [

        .package(url: "https://github.com/ilyapuchka/URLFormat.git", .branch("vapor")),

    ]

)

```