dylemma / xml-spac   0.12.1

MIT License Website GitHub

Handle streaming XML and JSON with declarative, composable parsers

Scala versions: 3.x 2.13 2.12


Spac is a Scala library for building stream consumers in a declarative style, specialized for tree-like data types like XML and JSON. Parsers are:

  • Declarative - You write what you want to get, not how to get it
  • Immutable - Parsers may be shared and reused without worry
  • Composable - Combine and transform parsers to handle complex data structures
  • Fast - With minimal abstraction to get in the way, speed rivals any hand-written handler
  • Streaming - Parse huge XML/JSON documents from events, not a DOM
implicit val PostParser: XmlParser[Post] = (
  XmlParser.attr("date").map(LocalDate.parse(_, commentDateFormat)),
  Splitter.xml(* \ "author").as[Author].parseFirst,
  Splitter.xml(* \ "stats").as[Stats].parseFirst,
  Splitter.xml(* \ "body").text.parseFirst,
  Splitter.xml(* \ "comments" \ "comment").as[Comment].parseToList

   .xml("blog" \ "post")
   .parseTap { println }
   .parse(JavaxSource.fromFile { new File("./blog.xml") })

It delegates to a backend of your choice to obtain a stream of events:



Add (your choice of) the following to your build.sbt file:

val spacVersion = "0.12.1" 
libraryDependencies ++= Seq(
  "io.dylemma" %% "spac-core" % spacVersion,         // core classes like Parser and Transformer 
  "io.dylemma" %% "spac-interop-fs2" % spacVersion,  // adds interop with fs2.Stream and fs2.Pipe
  "io.dylemma" %% "xml-spac" % spacVersion,          // classes for XML-specific parsers
  "io.dylemma" %% "xml-spac-javax" % spacVersion,    // XML parser backend using javax.xml.stream
  "io.dylemma" %% "xml-spac-fs2-data" % spacVersion, // XML parser backend using fs2-data-xml

  "io.dylemma" %% "json-spac" % spacVersion,         // classes for JSON-specific parsers
  "io.dylemma" %% "json-spac-jackson" % spacVersion, // JSON parser backend using the Jackson library
  "io.dylemma" %% "json-spac-fs2-data" % spacVersion // JSON parser backend using fs2-data-json

Main Concepts

SPaC is about handling streams of events, possibly transforming that stream, and eventually consuming it.


A Parser[In, Out] consumes a series of In values to produce a single Out value.

Parsers don't necessarily have to consume the entire series to produce a result; for example Parser.firstOpt[A] can produce a result upon encountering the first value.

The xml-spac and json-spac modules define useful type aliases:

  • type XmlParser[Out] = Parser[XmlEvent, Out]
  • type JsonParser[Out] = Parser[JsonEvent, Out]

Parsers are Applicative with respect to the Out type, which means you can combine them:

val parser1: Parser[In, A] = /* ... */
val parser2: Parser[In, B] = /* ... */

import cats.syntax.apply._
val combined: Parser[In, (A, B)] = (parser1, parser2).tupled


A Transformer[In, Out] transforms a series of In values to a series of Out values.

Transformers are typically created as an intermediate step on the way to creating a Parser. You'll usually call one of a Transformer's parseTo* methods to convert it to a Parser.


A Splitter[In, Context] identifies "substreams" in a stream of In values, with each "substream" identified by a Context.

Think of Splitter like XPath; for example, if you have a series of XmlEvents representing an XML document, and you want to parse only a certain type of element, you use a Splitter to locate those elements. Each "substream" would represent the XmlEvents for one such element.

You can attach a Context => Parser[In, Out] function to the Splitter so that each "substream" is passed through the given Parser so they will each be treated as an Out, and every event outside of a "substream" will be ignored. Doing so creates a Transformer[In, Out].

  • Parser[In, Out] consumes a stream of In values, eventually producing an Out. Parsers are
  • Transformer[In, Out] transfrorms a stream of In values to a stream of Out values.
  • Splitter[In, Context] splits a stream of In events by selecting "substreams", e.g. only the events associated with some child element in the XML, or for a specific JSON field. Each substream is identified by a Context value. By attaching a Context => Parser[In, Out] function to each substream, you can create a Transformer[In, Out].


Instances of Transformer, Parser, and Splitter are immutable, meaning they can safely be reused and shared at any time, even between multiple threads.

It's common to define an implicit val fooParser: XmlParser[Foo] = /* ... */


XmlParser.attr("foo") is a parser which will find the "foo" attribute of the first element it sees.

<!-- file: elem.xml -->
<elem foo="bar" />
val xml = JavaxSource.fromFile { new File("elem.xml") }
val elemFooParser: XmlParser[String] = XmlParser.attr("foo")
val result: String = elemFooParser.parse(xml)
assert(result == "bar")

Suppose you have some XML with a bunch of <elem foo="..."/> and you want the "foo" attribute from each of them. This is a job for a Splitter. You write an XmlSplitter sort of like an XPATH, to describe how to get to each element that you want to parse.

With the XML below, we want to parse the <root> element, since it represents the entire file. We'll write our splitter by using the * matcher (representing the current element), then selecting <elem> elements that are its direct children, using * \ elem.

<!-- file: root.xml -->
  <elem foo="bar" />
  <elem foo="baz" />
val foos: List[String] = XmlSplitter
   .xml(* \ "elem")
   .parse(JavaxSource.fromFile { new File("root.xml") })
assert(foos == List("bar", "baz"))

Note that a Splitter has a handful of "attach a parser" methods. The one you use will depend on whether the parser is available implicitly, and whether you care about the Context value for each substream.

  • splitter.map(context => getParser(context))
  • splitter.joinBy(parser)
  • splitter.as[Out]

Running Parsers

A Parser[In, Out] allows several ways to consume a stream of data:

  • parse(Iterator[In]): Out pulls from the iterator until a result is available, or the iterator ends
  • parse(Source[In]): Out opens the source to obtain an iterator, pulls from the iterator to obtain a result, then closes the source
  • parseF(fs2.Stream[F, In]): F[Out] (provided by an implicit from the spac-interop-fs2 module) pulls from the stream until a result is available, or the stream ends, with evaluation performed in the F context
  • .toPipe[F]: fs2.Pipe[F, In, Out] (provided by an implicit from the spac-interop-fs2 module) allows you to transform an fs2.Stream
  • .start returns a low-level handler which can step through In values

Basic Usage

// build.sbt
libraryDependencies += "io.dylemma.spac" %% "spac-core" % spacVersion

// Main.scala
import io.dylemma.spac._

val parser: Parser[In, Out] = /* ... */
val events: Iterator[In] = /* ... */
val result: Out = parser.parse(events)

XML with Javax

// build.sbt
libraryDependencies ++= Seq(
   "io.dylemma" %% "xml-spac" % spacVersion,
   "io.dylemma" %% "xml-spac-javax" % spacVersion

// Main.scala
import io.dylemma.spac._
import io.dylemma.spac.xml._

val parser: XmlParser[Out] = /* ... */
val xml: Source[XmlEvent] = JavaxSource.fromFile(/* ... */)
val result: Out = parser.parse(xml)

JSON with Jackson

// build.sbt
libraryDependencies ++= Seq(
   "io.dylemma" %% "json-spac" % spacVersion,
   "io.dylemma" %% "json-spac-jackson" % spacVersion

// Main.scala
import io.dylemma.spac._
import io.dylemma.spac.json._

val parser: JsonParser[Out] = /* ... */
val json: Source[JsonEvent] = JacksonSource.fromFile(/* ... */)
val result: Out = parser.parse(json)

XML with fs2-data-xml

// build.sbt
libraryDependencies ++= Seq(
   "io.dylemma" %% "xml-spac" % spacVersion,
   "io.dylemma" %% "spac-interop-fs2" % spacVersion,
   "io.dylemma" %% "xml-spac-fs2-data" % spacVersion

// Main.scala
import cats.effect.IO
import io.dylemma.spac._
import io.dylemma.spac.xml._
import io.dylemma.spac.interop.fs2._

val parser: XmlParser[Out] = /* ... */
val rawXmlBytes: fs2.Stream[IO, Byte] = /* ... */
val xmlStream: fs2.Stream[IO, XmlEvent] = Fs2DataSource.fromRawXmlStream(rawXmlBytes)
val result: IO[Out] = parser.parseF(xmlStream)

JSON with fs2-data-json

// build.sbt
libraryDependencies ++ Seq(
   "io.dylemma" %% "json-spac" % spacVersion,
   "io.dylemma" %% "spac-interop-fs2" % spacVersion,
   "io.dylemma" %% "json-spac-fs2-data" % spacVersion

// Main.scala
import cats.effect.IO
import io.dylemma.spac._
import io.dylemma.spac.json._
import io.dylemma.spac.interop.fs2._

val parser: JsonParser[Out] = /* ... */
val rawJsonBytes: fs2.Stream[IO, Byte] = /* ... */
val jsonStream: fs2.Stream[IO, JsonEvent] = Fs2DataSource.fromRawJsonStream(rawJsonBytes)
val result: IO[Out] = parser.parseF(jsonStream)

Convenience constructors for Source[XmlEvent] and Source[JsonEvent] are available from the JavaxSource and JacksonSource objects, respectively. Those objects are considered "backends" that delegate to a separate library for the low-level parsing (i.e. the JVM's built-in javax.xml.stream package in the case of JavaxSource, and the jackson-core library in the case of JacksonSource). They are found in the xml-spac-javax and json-spac-jackson modules respectively.

Applying Transformers

A Transformer[In, Out] is applied by:

  • .transform to convert an Iterator[In] to an Iterator[Out]
  • .transform to convert a Source[In] to a Source[Out]
  • .toPipe[F] (provided by an implicit from the spac-interop-fs2 module) to convert an fs2.Stream[F, In] to an fs2.Stream[F, Out]

Under the Hood

Parser and Transformer both act as factories for their respective Handler traits. Whenever you consume a stream with a Parser's parse method, or transform a stream with a Transformer, a new Handler instance will be created and used to run the stream processing logic.

Handlers are internally-mutable, whereas the Parsers/Transformers that create them are not.

A Parser's Handler must respond to each input by either returning a result or a new handler representing the continuation of the parser logic. Once the stream of inputs ends, the handler must produce a result. Since Handlers are internally-mutable, it's acceptable (and even preferable) for the Handler to simply update its internal state and return a reference to itself instead of constructing a new handler.

object Parser {
  trait Handler[-In, +Out] {
    def step(in: In): Either[Out, Handler[In, Out]]
    def finish(): Out

A Transformer's Handler may respond to each input by any combination of; updating its internal state, emitting an output to a downstream handler, or signalling to the upstream that it no longer wants to receive any more inputs. It may also choose to emit some additional outputs in response to the end of the stream of inputs. It may also transform exceptions thrown by a downstream handler.

object Transformer {
  trait Handler[-In, +Out] {
    def push(in: In, out: HandlerWrite[Out]): Signal
    def finish(out: HandlerWrite[Out]): Unit
    def bubbleUp(err: Throwable): Nothing = throw err