Eolian Documentation Guide

Eolian files (.eo) are plain text files which describe an Application Programming Interface (API) in a language-independent way. EFL uses them to automatically generate the development files required for each language such as header files for C.

Eolian files also allow documentation tags, which are translated both into the development files (as comments) and into the auto-generated EFL API reference guide.

This guide shows how to add documentation tags to existing .eo files and describes the style to be used.

You can find a thorough description of the file format in the Eolian File Format Specification but if you're writing documentation you shouldn't need to visit this very often.

Documentation Tags

Documentation tags go inside double square brackets: [[ Documentation text ]].

On some target documentation systems (like Doxygen), the first paragraph is handled differently. Normally, it is used as a short summary, followed by a more in-depth description. So, if you need to add a lot of text, use a single-sentence first paragraph, then add the rest.

Note that Eolian also supports comments. These are intended to help understand the Eo file itself and will NOT be translated into documentation. Eolian comments follow the C/C++ style /* comment */ and // comment.


You can place any text inside a documentation tag. However, since it will be copied over to different kinds of development files using varying syntaxes, you shouldn't use any Markup whatsoever.

Broken lines are automatically brought together. Indentation and long runs of spaces are automatically removed. Leave a blank line if you need to insert line break.

A limited amount of styling is possible however. The following are guaranteed to work across different documentation platforms:

  • A $ before any word will mark it as a keyword and most probably render it in monospace. Use this to highlight method parameters or special values like true, false or null, for example.

    [[If the $foo parameter is $true amazing things will happen]]
  • An @ before any word creates a reference to another part of the API, which will probably be rendered as a link to the appropriate page. The syntax is @Class.Attribute. You can omit the Class part if you want to refer to an attribute of the current class: @.Attribute.

    [[See for instance the @.border_scale method in this class or the


Documentation tags can only appear after open braces { or semicolons ;. When placed after a brace, the tag refers to its entire content (e.g., a class). When placed after a semicolon, the tag refers to whatever the semicolon ends (e.g., a parameter)

class Foo {
    [[This is the documentation for class Foo.]]
    methods {
        bar {
            [[This is the documentation for method bar.]]
            return: bool; [[This documents the return value of method bar.]]

Bear in mind that documentation tags placed elsewhere will be ignored and may even interfere with compilation of EFL.

Style Guide

  • Start all your sentences with Upper Case.
  • End all your sentences with periods.
  • Keep your lines under 80 characters.
  • Document properties at the @property ... { level. In other words, do not document the property's get and set methods (accessors) independently. If the methods really do something other than setting and getting a property, then put as much common text at the @property level and only put whatever's different in the accessors.
  • Start the description of methods with a verb in singular form, that is, Destroys the object and not Destroy the object.
  • Methods returning a Boolean (like is_object_a_box) should start with the phrase Returns $true if (or $false). For example: Returns $true if the $object parameter is of the box type..

For anything else not specified here, follow the Style Guide.

Complete Example

This is a corrected and stripped-down version of the efl_file.eo file actually used by EFL. Use it for reference purposes.

mixin Efl.File {
   [[Efl file interface]]
   methods {
      @property file {
         [[Source file backing the real image data for an image object (it may
           be an Eet file, besides pure image ones).
           If the file supports storage of multiple data (as Eet files do),
           you can specify the key to be used as the index of the image in
           this file.]]
         set {
            return: bool; [[$true on success, $false otherwise]]
         get {
         values {
            file: string; [[The image file path.]]
            key: string; [[The image key in $file (if its an Eet one), or
            $null, otherwise.]]
      save @const @pure_virtual {
         [[Saves the given image object's contents to an (image) file.
           The extension suffix on $file will determine which saver
           module Evas is to use when saving, thus the final file's
           format. If the file supports multiple data storage (Eet ones),
           you can specify the key to be used as the index of the image in it.
           You can specify some flags when saving the image. Currently
           acceptable flags are $quality and $compress. Eg.:
           "quality=100 compress=9"
           Use the @.file property to set the file name.]]
         return: bool;  [[$true on success, $false otherwise]]
         params {
            @in file: string @nonull; [[The filename to be used to save the image (extension obligatory).]]
            @in key: string; [[The image key in the file (if an Eet one), or $null,otherwise.]]
            @in flags: string; [[String containing the flags to be used ($null for none).]]