How to work with XML documents in Elixir using xmerl

I am currently working on a project that needs to process a bunch of SVG files by applying a few predefined operations on them, for example:

• Find an element with id door and set its fill attribute to black,
• Find an element with id roof and set its class attribute to on-fire,
• Change the width, height, and viewport attributes of the root svg element to shrink the image on the x-axis by 50%,
• …and so on.

Afterward, I need to export the data back to a string representing a valid SVG document.

The Erlang xmerl library is capable of doing all of those operations. Unfortunately, it’s not that easy for an Elixir developer without a solid Erlang background to use.

Today I’m sharing with you what learned about using xmerl in your Elixir project: how to parse strings containing XML documents, modify them, and export back to string.

#Step 1: add :xmerl to your project

Edit extra_applications in mix.exs to add :xmerl:

def application do
  [
    extra_applications: [:logger, :xmerl]
  ]
end

#Step 2: add record definitions

Create a module for operating on XML documents, e.g. MyApp.Xml. In that document, define records :xmlElement and :xmlAttribute.

defmodule MyApp.Xml do
  require Record

  Record.defrecord(:xmlElement, Record.extract(:xmlElement, from_lib: "xmerl/include/xmerl.hrl"))
  Record.defrecord(:xmlAttribute, Record.extract(:xmlAttribute, from_lib: "xmerl/include/xmerl.hrl"))
end

Note: xmerl defines a few other record types, see xmerl/types. Depending on your use case, you might need some or all of them. In this blog post, I will only be using xmlElement and xmlAttribute.

But what exactly is going on in that code snippet? Using xmerl was the first time I encountered records in Elixir.

#What is a record?

A record is a tuple of an arbitrary length. Its first element is an atom - the record’s name. All other elements can be values of any type. Without a record definition, you don’t know what the values represent. For example, below is a :user record with two values, 13 and false.

{:user, 13, false}

To work efficiently with data in this format, you need a record definition. For example:

Record.defrecord(:user, comment_count: 0, wants_newsletter: nil)

Using Record.defrecord/2 adds meaning to the position of the elements in the record tuple. It also defines macros, named after the record, that allow you to create, update, and pattern match on the record.

# new user
some_user = user(comment_count: 13, wants_newsletter: false)
# => {:user, 13, false}

# update user with one more comment
user(comment_count: current_comment_count) = some_user
some_user = user(some_user, comment_count: current_comment_count + 1)
# => {:user, 14, false}

#Extracting xmerl record definitions

The xmerl library contains record definitions for the record types that it uses. You don’t have to copy-paste them into your Elixir code. You can extract them from the Erlang files like this:

Record.extract(:xmlAttribute, from_lib: "xmerl/include/xmerl.hrl")

It’s useful to actually read through the output of those macro calls. You will need to know the field names to read and modify them.

# xmlElement
[
  name: :undefined, # <-- attribute name, in SVGs e.g. 'circle', 'path', 'g'
  expanded_name: [],
  nsinfo: [],
  namespace: {:xmlNamespace, [], []},
  parents: [],
  pos: :undefined,
  attributes: [],
  content: [], # <-- field relevant for accessing nested elements
  language: [],
  xmlbase: [],
  elementdef: :undeclared
]

# xmlAttribute
[
  name: :undefined, # <-- attribute name, in SVGs e.g. 'id', 'fill', 'stroke'
  expanded_name: [],
  nsinfo: [],
  namespace: [],
  parents: [],
  pos: :undefined,
  language: [],
  value: :undefined, # <-- attribute value
  normalized: :undefined
]

#Step 3: parse a string to xmlElement

To parse a string containing an XML document to an xmlElement record, use the :xmerl_scan.string/1 function.

@spec parse_string(String.t()) :: record(:xmlElement)
def parse_string(content) do
  content = to_charlist(content)
  {doc, _rest} = :xmerl_scan.string(content)
  doc
end

Note: Erlang libraries often operate on charlists, not binaries.

This implementation assumes that the string will always contain a valid XML document. If this is not the case for you, you will need to catch :exits when calling :xmerl_scan.string/1.

#xmlElement or xmlDocument?

By default, the function :xmerl_scan.string/1 uses the xmlElement record for the root element of the XML document. However, an xmlDocument record also exists.

If you want to use the xmlDocument record type for the root element of the XML document, pass the document option:

:xmerl_scan.string(content, document: true)

I don’t really know why you might need this - let me know if you know 🙂.

This blog post assumes that the root element of the document is a xmlElement record (the default behavior).

#Step 4: export the xmlElement back to a string

To turn an xmlElement back to a string, you can use the :xmerl.export_simple/1 function.

@spec export_string(record(:xmlElement)) :: String.t()
def export_string(doc) do
  [doc]
  |> :xmerl.export_simple(:xmerl_xml)
  |> to_string()
  |> Kernel.<>("\n")
end

I’m annoyed by the lack of a trailing newline, so I’m adding one.

#Step 5: modify the document

To figure out how to modify the XML document, let’s first see what a simple SVG file looks like after parsing to xmlElement:

"""
<?xml version="1.0"?>
<svg viewBox="0 0 100 100" xmlns="http://www.w3.org/2000/svg">
  <circle id="circle-1" cx="50" cy="50" r="50" fill="red"/>
  <circle id="circle-2" cx="50" cy="50" r="30" fill="orange"/>
  <circle id="circle-3" cx="50" cy="50" r="10" fill="yellow"/>
</svg>
"""

# becomes

{:xmlElement, :svg, :svg, [], {:xmlNamespace, :"http://www.w3.org/2000/svg", []}, [], 1,
    # the list of attributes of the <svg> element starts here
    [
      {:xmlAttribute, :viewBox, [], [], [], [svg: 1], 1, [], ~c"0 0 100 100", false},
      {:xmlAttribute, :xmlns, [], [], [], [svg: 1], 2, [], ~c"http://www.w3.org/2000/svg", false}
    ],
    # the content of the <svg> element starts here
    [
      {:xmlText, [svg: 1], 1, [], ~c"\n  ", :text},

      # first <circle> element
      {:xmlElement, :circle, :circle, [], {:xmlNamespace, :"http://www.w3.org/2000/svg", []}, [svg: 1], 2,
        # the list of attributes of the first <circle> element starts here
        [
          {:xmlAttribute, :id, [], [], [], [circle: 2, svg: 1], 1, [], ~c"circle-1", false},
          {:xmlAttribute, :cx, [], [], [], [circle: 2, svg: 1], 2, [], ~c"50", false},
          {:xmlAttribute, :cy, [], [], [], [circle: 2, svg: 1], 3, [], ~c"50", false},
          {:xmlAttribute, :r, [], [], [], [circle: 2, svg: 1], 4, [], ~c"50", false},
          {:xmlAttribute, :fill, [], [], [], [circle: 2, svg: 1], 5, [], ~c"red", false}
        ], [], [], ~c"", :undeclared},

      {:xmlText, [svg: 1], 3, [], ~c"\n  ", :text},

      # second <circle> element
      {:xmlElement, :circle, :circle, [], {:xmlNamespace, :"http://www.w3.org/2000/svg", []}, [svg: 1], 4,
        # the list of attributes of the second <circle> element starts here
        [
          {:xmlAttribute, :id, [], [], [], [circle: 4, svg: 1], 1, [], ~c"circle-2", false},
          {:xmlAttribute, :cx, [], [], [], [circle: 4, svg: 1], 2, [], ~c"50", false},
          {:xmlAttribute, :cy, [], [], [], [circle: 4, svg: 1], 3, [], ~c"50", false},
          {:xmlAttribute, :r, [], [], [], [circle: 4, svg: 1], 4, [], ~c"30", false},
          {:xmlAttribute, :fill, [], [], [], [circle: 4, svg: 1], 5, [], ~c"orange", false}
        ], [], [], ~c"", :undeclared},

      {:xmlText, [svg: 1], 5, [], ~c"\n  ", :text},

      # third <circle> element
      {:xmlElement, :circle, :circle, [], {:xmlNamespace, :"http://www.w3.org/2000/svg", []}, [svg: 1], 6,
        # the list of attributes of the third <circle> element starts here
        [
          {:xmlAttribute, :id, [], [], [], [circle: 6, svg: 1], 1, [], ~c"circle-3", false},
          {:xmlAttribute, :cx, [], [], [], [circle: 6, svg: 1], 2, [], ~c"50", false},
          {:xmlAttribute, :cy, [], [], [], [circle: 6, svg: 1], 3, [], ~c"50", false},
          {:xmlAttribute, :r, [], [], [], [circle: 6, svg: 1], 4, [], ~c"10", false},
          {:xmlAttribute, :fill, [], [], [], [circle: 6, svg: 1], 5, [], ~c"yellow", false}
        ], [], [], ~c"", :undeclared},

      {:xmlText, [svg: 1], 7, [], ~c"\n", :text}
    ], [], ~c"", :undeclared}

The output is pretty long and hard to read. To read a record, you need to remember which position represents which value. For example, the 7th element of an xmlElement is its attributes, and the 8th element its content.

You can see that attributes are represented as lists of records, which poses a challenge with regard to setting an attribute on an element. Adding a new attribute to an element is a different operation than updating the value of an existing attribute. If you want to set attributes on elements without having to worry whether they already exist or not, you’ll have to write an “update or add” function yourself.

#Traversing and updating elements

xmlElement is a recursive record that contains a list of other xmlElements in its content field. To traverse and update elements in an XML document, you’ll need a function like this:

@typep mapper :: (:xmerl.element() -> :xmerl.element())
@spec traverse_and_update_elements(:xmerl.element(), mapper) :: :xmerl.element()
def traverse_and_update_elements(element, func) do
  updated_element = func.(element)

  case updated_element do
    xmlElement(content: content) ->
      updated_content =
        Enum.map(content, fn child -> traverse_and_update_elements(child, func) end)

      xmlElement(updated_element, content: updated_content)

    other ->
      other
  end
end

It runs a given mapper function on the current element and then recursively calls itself on all child elements.

Note: there is a difference between the record(:xmlElement) type and the :xmerl.element type. record(:xmlElement) is a subtype of :xmerl.element. :xmerl.element also includes text nodes, comment nodes, and a few other things. If you only need to add/remove attributes to XML elements, you can ignore all the other element types (return them unchanged).

#Finding attribute values

To write a useful mapper function, you need to be able to target the right elements that need updating.

I intend to target elements by id, so I wrote a function for finding the value of a specific attribute on an element.

@spec find_attribute_value(record(:xmlElement), atom) :: charlist | nil
def find_attribute_value(element, name) do
  xmlElement(attributes: attributes) = element

  attributes
  |> Enum.find_value(fn
    xmlAttribute(name: ^name, value: value) -> value
    _ -> nil
  end)
end

#Modifying attributes

When working with attributes, keep in mind that attribute names are atoms, and attribute values are charlists.

Below is the implementation of a few common operations on attributes that you might want.

#Add attribute

@spec add_attribute(record(:xmlElement), atom, charlist) :: record(:xmlElement)
def add_attribute(element, name, value) do
  xmlElement(attributes: attributes) = element

  new_attribute = xmlAttribute(name: name, value: value)
  updated_attributes = [new_attribute | attributes]

  xmlElement(element, attributes: updated_attributes)
end

#Update attribute

@spec update_attribute(record(:xmlElement), atom, charlist) :: record(:xmlElement)
def update_attribute(element, name, value) do
  xmlElement(attributes: attributes) = element

  updated_attributes =
    attributes
    |> Enum.map(fn
      xmlAttribute(name: ^name) = attribute -> xmlAttribute(attribute, value: value)
      attribute -> attribute
    end)

  xmlElement(element, attributes: updated_attributes)
end

#Update or add attribute

If you want to be able to set attributes on elements without worrying whether they already exist, you need something like this:

@spec has_attribute?(record(:xmlElement), atom) :: boolean
def has_attribute?(element, name) do
  xmlElement(attributes: attributes) = element

  attributes
  |> Enum.any?(fn
    xmlAttribute(name: ^name) -> true
    _ -> false
  end)
end

@spec update_or_add_attribute(record(:xmlElement), atom, charlist) :: record(:xmlElement)
def update_or_add_attribute(element, name, value) do
  if has_attribute?(element, name) do
    update_attribute(element, name, value)
  else
    add_attribute(element, name, value)
  end
end

#Working with numerical values

Numerical values can be a bit annoying to work with. All attribute values returned by xmerl are charlists, so you need to transform them to numbers and back to charlists yourself.

Beware of calling to_string() on floats - it might format them in a scientific notation!

to_string(1000000.00)
# => "1.0e6"

I needed to read the width of the svg element, scale it by some number, and then set that new value as the width. I didn’t care if I turn integers into floats. I was also fine with a hardcoded precision of 2 decimal places.

Here’s the code that I used to transform charlists to numbers and back:

@spec charlist_to_number(charlist) :: float
def charlist_to_number(x) do
  x |> to_string() |> Float.parse() |> elem(0)
end

@spec number_to_charlist(integer | float) :: charlist
def number_to_charlist(x) do
  if is_float(x) do
    :erlang.float_to_binary(x, decimals: 2) |> to_charlist()
  else
    Integer.to_string(x) |> to_charlist()
  end
end

#Combining things together

Here’s an example that takes my test image of 3 circles and changes warm colors (red, orange, yellow) to cold colors (green, blue, purple). It also decreases the radius of the last circle by half.

input = """
<?xml version="1.0"?>
<svg viewBox="0 0 100 100" xmlns="http://www.w3.org/2000/svg">
  <circle id="circle-1" cx="50" cy="50" r="50" fill="red"/>
  <circle id="circle-2" cx="50" cy="50" r="30" fill="orange"/>
  <circle id="circle-3" cx="50" cy="50" r="10" fill="yellow"/>
</svg>
"""

mapper = fn
  xmlElement() = element ->
    case Xml.find_attribute_value(element, :id) do
      nil ->
        element

      ~c"circle-1" ->
        Xml.update_or_add_attribute(element, :fill, "green")

      ~c"circle-2" ->
        Xml.update_or_add_attribute(element, :fill, "blue")

      ~c"circle-3" ->
        element = Xml.update_or_add_attribute(element, :fill, "purple")

        current_radius =
          element |> Xml.find_attribute_value(:r) |> Xml.charlist_to_number()

        new_radius = Xml.number_to_charlist(0.5 * current_radius)
        Xml.update_attribute(element, :r, new_radius)
    end

  other ->
    other
end

input
|> Xml.parse_string()
|> Xml.traverse_and_update_elements(mapper)
|> Xml.export_string()

# returns:
"""
<?xml version="1.0"?>
<svg viewBox="0 0 100 100" xmlns="http://www.w3.org/2000/svg">
  <circle id="circle-1" cx="50" cy="50" r="50" fill="green"/>
  <circle id="circle-2" cx="50" cy="50" r="30" fill="blue"/>
  <circle id="circle-3" cx="50" cy="50" r="5.00" fill="purple"/>
</svg>
"""

#Example repository

TL;DR: Here’s the repository with the full example from this blog post.

#What could be improved?

Working with xmerl still feels a bit clumsy to me. Did I miss anything that would make it easier? Did I get something wrong? Let me know!

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