Slax

Slax is a SAX parser for XML documents, built atop the erlsom parser.

Usage

To begin parsing, pass an IO object or String, along with a parser module and (optional) initial state to the Slax.parse/3 function.

The parser module should include a handle/2 function head for any event it wishes to handle. The function will receive an event struct containing relevant data for the current portion of the XML being parsed, as well as whatever ancillary state has been accumulated. The return value of the function call will be passed to the following invocation of handle/2 as the new state. If no function clause matches the supplied arguments, the previous state will be preserved and supplied to the next invocation of handle/2.

Parser Modules

Use of the Slax.Parser module can simplify the construction of parser modules:

• The @behaviour Slax.Parser declaration will ensure that your module defines the proper functions with the correct arity.
• The use Slax.Parser declaration will provide default implementations of the required functions, allowing your parser to only define the handlers it needs.

An example parser that will write all text in a XML document to standard out looks like:

defmodule StdoutParser do
  @behaviour Slax.Parser

  use Slax.Parser

  def handle(%Slax.Event.Characters{characters: chars}, _), do: IO.puts chars
end

Initialization

The parser module's init/1 function will be called prior to start of parsing. The value returned by the init/1 function will be passed to the first invocation of handle/2. An optional state keyword can be used in conjunction with the use declaration to provide an argument to the init function:

defmodule StateParser do
  @behaviour Slax.Parser

  use Slax.Parser, state: 55

  def init([value]), do: value * 2 # Will set initial state to 110
end

Events

Slax defines a struct for each kind of event that can possibly be generated, which can be used to pattern match in your handle/2 function clauses. Slax will emit some or all of the following events when processing a document:

• Slax.Event.StartDocument
• Slax.Event.EndDocument
• Slax.Event.StartPrefixMapping
• Slax.Event.EndPrefixMapping
• Slax.Event.StartElement
• Slax.Event.EndElement
• Slax.Event.Characters
• Slax.Event.IgnorableWhitespace
• Slax.Event.ProcessingInstruction
• Slax.Event.Error
• Slax.Event.InternalError

Concurrency

Slax does not spawn a new processs, the parser runs in the same process that calls it and no progress will be made on parsing until the handle/3 call returns. Should you wish to process events concurrently with parsing the document, it is advised that your handle/3 functions call out (asynchronously) to a GenServer or some other such mechanism.

Installation

Slax is available in Hex and can be installed by adding slax to your list of dependencies in mix.exs:

def deps do
  [
    {:slax, "~> 0.1.0"}
  ]
end