sherpal / url-dsl

Tiny dsl library for path et parameters of urls

Version Matrix


This is a tiny library for parsing and generating paths and parameters of urls.

Getting started

We represent the path and query parameters of a url as follows:

import urldsl.language.simpleErrorImpl._
import urldsl.vocabulary.{Segment, Param, UrlMatching}

val path = root / "hello" / segment[Int] / segment[String] / endOfSegments
val params = param[Int]("age") & listParam[String]("drinks")

val pathWithParams = path ? params

) should be(
  Right(UrlMatching((2019, "january"), (10, List("orange juice", "water"))))

path.matchPath("/hello/2019/january") should be(
  Right((2019, "january"))

params.matchQueryString("age=22&drinks=orange+juice&drinks=water") should be(
  Right((22, List("orange juice", "water")))

For more example usages, head over the tests, and in particular in the urldsl.examples package..


Add the following to your build.sbt (or wherever you add your dependencies):

libraryDependencies += "be.doeraene" %% "url-dsl" % "0.3.2"

or, for Scala.js,

libraryDependencies += "be.doeraene" %%% "url-dsl" % "0.3.2"

The biggest abstraction

The highest abstraction that this library provide is the UrlPart. A UrlPart represents a part of the URL and will be able to:

  • when applied to a URL, tries to extract information from that URL
  • when fed with information, generates a string that this piece of information represents.

The part that the UrlPart represents can be:

  • the path of the URL
  • the query of the URL
  • the fragment of the URL
  • a combination of any of the above.

This is a trait with two type parameters, T and E. The type T is the type of information represented in this UrlPart. For example, if T =:= (String, Int), it means that we will get a pair when extracting information from a URL, and we will need to feed that when generating the string part of the URL.

Matching a URL can fail. When it does, it will produce an element of type E. E can be any ADT of your choosing, however url-dsl comes with two default implementations, one particularly suited for debugging, and the other when you don't care about what error was produced (but you only care to know whether the test was successful).

The three important classes

There are essentially three classes that are important for using urldsl (plus two others when you combine them), all of them extending UrlPart. These three classes are

  • PathSegment: abstraction describinng the path part of the URL
  • QueryParameters: abstraction describing the query (search) part of the URL
  • Fragment: abstraction describing the fragment (ref) part of the URL

As said above, these classes come with two type parameters, one for the information, and one for the possible matching error. For example, in the case of the path, if T =:= (String, Int), the path contains the information about a String and an Int, as in the following segment

root / segment[String] / "foo" / segment[Int]


The path part of the URL is modelled by the urldsl.language.PathSegment[T, A] trait. At its core, a PathSegment is merely an object with two methods matchRawUrl and createPath. The matchRawUrl method takes as input the string containing a URL (well formed!) and returns Either an instance of T (the information contained in the input URL) or an error of type A (if the segment could not retrieve the information).

PathSegments are immutable objects that can be composed together with the / operator. This operator is associative and creates, given two PathSegments with type parameters T and U, will create a PathSegment with type parameter, roughly, (T, U) (with some additional rules described in the Tupler class that, without entering details, flattens the tuples and remove Units). So if T =:= (String, Int) and U =:= (String, Double), you'll get the type (String, Int, String, Double).

When a PathSegment matches a URL, it internally receives the list of Segments, consumes one or several of them, and passes the rest onto the following (when there are composed). For example, if you have a PathSegment that matches the string "foo", and an other that matches an Int, if you give the composition "foo/22", the first PathSegment consumes "foo" and passes "22" to the next one, which will them consume it.

Built in path segments

There are a bunch of PathSegments that are already defined, and should satisfy most of your basic needs. For example, the following things are implemented (you can look at the companion object of PathSegment to have the comprehensive list:

  • root: matches everything and passes all segments onto the next
  • segment[T] matches an element of type T, whose information is contained in only one segment, and passes the other segments onto the next
  • endOfSegments: which matches only the empty list of segments, and thus passes nothing onto the next (there should never be a next, though)
  • the list goes on...


Comprehensive examples can be found in here. Below we give a quick overview.

The following examples assume the following import:

import urldsl.language.PathSegment.simplePathErrorImpl._

Here are a bunch of things that you can do with the paths:

(root / "home" / "about").matchRawUrl("http://localhost:8080/home/about") // success, returns Unit
(root / "home" / "about").matchRawUrl("http://localhost:8080/home") // failure, returns MissingSegment

(root / segment[String] / segment[Int]).matchRawUrl("") // success, returns ("user", 22)
(root / segment[String] / segment[Int]).matchRawUrl("") // failure, returns SimpleError

(root / "home").matchRawUrl("") // failure, returns WrongValue("home", "about")

case class User(id: Int, name: String)

object User {
  implicit val userCodec: Codec[(Int, String), User] = new Codec[(Int, String), User] {
    def leftToRight(left: (Int, String)): User = User(left._1, left._2)
    def rightToLeft(right: User): (Int, String) = (,

val userPath = (root / "user" / segment[Int] / segment[String]).as[User]

userPath.matchRawUrl("") // success, returns User(5, "Alice")
userPath.createPath(User(5, "Alice")) // returns user/5/Alice

Note that starting with root is not strictly necessary (technically, root is the neutral of the / operator) but it allows to use the implicit conversion from elements to single segment matching. Also, it's good to really interpret it as the beginning of the path.

Query parameters

The query parameters part of the URL is modelled by the urldsl.language.QueryParameters[Q, A]. As for the PathSegment class, this is essentially a class that has two methods matchRawUrl and createParams. Analogously to the / operator of path segments, the query parameters have an operator & to compose them and build more complicated query parameters.

The tupling of the type parameters works the same way as for path.

Built in query parameters

There are two main built in query parameters that you can use as building blocks for most of yours needs:

  • param[T](paramName: String): represents the value of the parameter with name paramName as a type T element
  • listParam[T](paramName: Strig): same as param but for lists.

Query parameters examples

Comprehensive examples can be found in here. Below we give a quick overview.

The following examples assume the following import:

import urldsl.language.QueryParameters.simpleParamErrorImpl._

Here are a bunch of things that you can do with the query parameters:

(param[String]("foo") & param[Int]("bar")).matchRawUrl(
) // returns ("hello", 3)
(param[Int]("bar") & param[String]("foo")).matchRawUrl(
) // returns (3, "hello")

) // returns List(1, 2) (however you should assume that it could be List(2, 1) in a non predictible way)

Note on commutativity of &

The significant difference between / for paths and & for parameters is that & is commutative. That is, the string "foo=hello&bar=3" is equivalent to "bar=3&foo=hello" (this is obviously not the case for /). The & operator of QueryParameters is however not commutative, but it essentially is (we say that it is quasi-commutative).

Let's expose what this means. Suppose we have two instances foo and bar respectively of types QueryParameters[String, A] and QueryParameters[Int, A]. Then foo & bar and bar & foo will both match the two strings above, but the first one has type QueryParameters[(String, Int), A], while the other one has type QueryParameters[(Int, String), A].


The last class is urldsl.language.Fragment. It represents the fragment (sometimes called ref) part of the URL (which is the part after "#" at the end).

The fragment part is very basic compared to the other two. It is either present or absent, and you can invoke it with

import urldsl.language.simpleErrorImpl._

fragment[String] // extract the fragment information when it's there, failing otherwise
fragment[Int]    // extract the fragment information when it's there, casting to int, failing otherwise

maybeFragment[String] // extract the fragment information wrapped in Some when it's there, or None otherwise (never failing)
empty                 // imposes that the fragment is absent, failing otherwise

Examples for Fragment

Head over this test file.

Error mechanism

Both PathSegments and QueryParameters contain the type information of their content and the type information of the errors they return. This allows the user to define its own ADT of errors that can be used to easily manage the errors if needed.

There are however two kind of errors that are built in, see below.

Simple errors

The first kind of built in errors are the SimplePathMatchingError and SimpleParamMatchingError implementations. These simple errors form a basic error system that contains error message as strings. The (probably) best usage of these errors is to debug and begin to play with the library.

In order to use the simple errors, you need the following imports:

import urldsl.language.PathSegment.simplePathErrorImpl._
import urldsl.language.QueryParameters.simpleParamErrorImpl._

Custom errors

If you want to create your own ADT for errors, you need to do two things:

  • first, you need an implementation of the urldsl.errors.PathMatchingError and urldsl.errors.ParamMatchingError type classes, that you can set implicit for ease with the following
  • second, you create an instance of urldsl.language.PathSegmentImpl and urldsl.language.QueryParametersImpl simply by use their constructor

Implementing these two type classes simply requires you to give a concrete implementation to some special errors that are needed for the default paths and segments built in helpers.

Then, wherever you want to use the library, you should import the contents of your implimentation, as shown in the "Simple errors" section above.


What about if you simply want to know whether something matches, but don't really care about the reason why? This would for example be the case when implementing a Router.

For that scenario, there is a type of error, called DummyError that only has one instance, which is returned for every failure.

In some methods, this error type also adds sugar on some methods (see, e.g., the filter method of PathSegment).

In order to use the dummy error, you need the following imports:

import urldsl.language.dummyErrorImpl._

A router example

url-dsl was thought with one possible goal in mind, the one of creating a routing system. An example of how to do that can be found here.

Moving from 0.2.0 to 0.3.x

If you come from version 0.2.0, here are a few things that you should pay attention to.

The new UrlPart trait

If you need/want to abstract over all possible way to extract information from a URL, this is the way to go. Asking your user to give a UrlPart[T, E] will allow them to provide any of the above mentioned classes, as well as their combinations.

Import all syntax

Previously, if you needed to mix PathSegment and QueryParameters, you had to make the two imports:

import urldsl.language.QueryParameters.simpleParamErrorImpl._
import urldsl.language.PathSegment.simplePathErrorImpl._

With the addition of the fragment feature, we decided to create an instance for importing everythin at once:

import urldsl.language.simpleErrorImpl._
// or
import urldsl.language.dummyErrorImpl._

Error type is now covariant

This should not impact you very much.


The project is decomposed in four packages:

  • url: it contains parsing and rendering of urls (with encodings)
  • vocabulary: it contains the little blocks on which the dsl is built
  • language: it contains the actual implementation of the dsl
  • errors: it contains the implementation of the matching errors logic.

In order to flatten the tuples that are generated by the operators / and &, we use the Tupler mechanism built by @julienrf and orginiated from here.