API Documentation style guide

This document describes the syntax and best practices for the documentation comments embedded in the source code.


This document is heavily inspired by the numpydoc docstring guide and follows its structure.

We employ the variants proposed by the Google style for compactness.

We aim to achieve the following middle ground:

  • The documentation comments can be processed easily by the embedded help function to present very readable and complete documentation to the matlab prompt.

  • The documentation comments can be parsed with Sphinx to produce stylish and naviguable documentation.

Documentation comments

MATLAB documentation calls the documentation strings that appears at the beginning of a class/function/method/property definition comments. To distinguish them from code comments, we call those special comments documentation comments, noting that the docstring terminology is specific to the Python language (standalone strings in the source code).

The documentation comments are written using a subset of the reStructuredText syntax. This style guide does not list which syntax elements are permitted: use common sense to employ the conventions that are compatible with reStructuredText, and display well standalone on the terminal (as is advocated by the numpydoc guide linked above).

In particular, we use single backticks for class, function, method, property names, and double backticks for verbatim text. Moreover, we include the + symbol in from of packages.

A guiding principle is that human readers of the text are given precedence over contorting docstrings so our tools produce nice output.

The length of comment lines should be kept to 75 characters; note that this applies to the comment string itself, not including the % character, whitespace, and source code tokens. RepLAB uses longer source code lines.

First part of a documentation comment

For a class, function or method (uniquely referred to as function from now on), a documentation comment starts with the following introductory content:

  1. First line with a concise description of the function. Names are not capitalized.

  2. Second line empty

  3. Third line starts an extended description (and/or discussion) of the function follows the third line. This should clarify the role played by the function, i.e. its functionality, not to discuss implementation details or background theory, which should rather be explored in the Notes section below. You may refer to the parameters and the function name (using single backticks), but the parameter descriptions belong in the Parameters section.

Here is an example of these first few lines of documentation comment:

function suite = fromMethod(testClass, testMethod, varargin)
% Create a suite from a single test method
% Creates a TestSuite from the test class described by testclass and the
% test method described by testmethod and returns it in suite. testclass
% is a meta.class instance which describes the desired test class. The
% ...

When the concise description is sufficiently clear to define the function, the complete description can be omitted. Here is an example:

function z = plus(x, y)
% Addition operation

When no one-line description of the function is present, the first line of the extended description will be used as a summary description (possibly incomprehensible).

After these initial informations come the sections described below, providing further systematic details.


The documentation comment can contain a number of sections separated by headings. Each heading is given by a keyword from the following list, with the section content indented.

We use the following section headings:

  • Args as an alias to Sphinx Parameters

  • Returns (for multiple return arguments, see the use of underlined section headers below).

  • Raises

  • Example

  • See Also

  • Notes

See also the Napoleon documentation for other possible section headers.

In order, the sections of a function comment are:

  1. Arguments/Parameters

    Description of the function arguments, keywords and their respective types, using Google style. Argument names are not capitalized.

    % Args:
    %   x (type): Description of parameter ``x``.
    %   y: Description of parameter ``y`` (with type not specified)

    If it is not necessary to specify an argument, use optional:

    %   x (type, optional): optional parameter of type ``type``

    When a parameter can only assume one of a fixed set of values, those values can be listed in braces. If the argument is optional, the default value appears first:

    %   field ({'R', 'C'}): Field name

    When two or more input parameters have exactly the same type, shape and description, they can be combined:

    %   a, b (double): Elements to sum

    Matlab/Octave encodes integer values using double floating point numbers. Our convention is to name those values integer. In the rare case a primitive integer type is needed, we write that type precisely (as in int32 or uint32). Big integers have type vpi, which is the name of the external library that supports them.

    Permutations are stored using row double vectors containing integers, and are documented as permutation. Same convention for signed permutations, that are documented as signed permutation.

    Strings represented as char arrays have type charstring (recent Matlab versions have a new string type which should not be confused with).

    Function handles have type function_handle.

  2. Returns

    Explanation of the returned values and their types. Return values are not capitalized. We distinguish two cases.

    First, when a single value is returned, we use the Google style:

    function c = sum(a, b)
    % Sums two numbers
    % Adds the value of a and b.
    % Returns:
    %   double: The sum of the parameters

    When there are multiple return values, we use another style:

    function [c d] = sorted2(a, b)
    % Sorts two numbers
    % Returns a and b as c and d so as to always satisfy the condition c <= d.
    % Returns
    % -------
    %   c: double
    %     Smallest number
    %   d: double
    %     Largest number
  3. Raises (optional)

    An optional section detailling which errors get raised and under what conditions.


  4. Warnings (optional)

    An optional section detailling which warnings get raised and under what conditions, formatted similarly to Raises.

  5. Examples (encouraged)

    A section with explicit commands illustrating as clearly as possible one or several ways of calling the function. These commands should be written in the doctest format. In partiular, they include the expected output.

  6. See Also (encouraged)

    An optional section used to refer to related code. This will allow for easily browsing the documentation. Fully qualified names should be used for class objects and methods.

  7. Notes (optional)

    This optional section can provide various additional information about the code of interest to the user, such as a discussion about the algorithm used by the function. Depreciation warnings can also be specified here. We haven’t yet specified a formal syntax for those. The numpydoc convention uses a Sphinx directive which we tend to avoid.

    Before the section, the comment should be broken by a single empty line without %. This should stop matlab from parsing, and thus allow us to use further formatting such as LaTeX. The content from now on would then only be presented in the Sphinx API. (TODO: check this)

  8. References (optional)

    (TODO: should we allow references for the Notes section here? see numpydoc guide)

Documenting functions

The documentation comment is given right after the function declaration.

Documenting classes

We document classes immediately after the classdef declaration. However, this class document does not address class properties or constructor parameters.

Class properties are documented by not adding a semicolon ; to each property, and following them by a comment as in below:

properties (SetAccess = protected)
    group % (`replab.Group`): Group being representation
    field % ({'R', 'C'}): 'R' for a representation on a real vector space, 'C' for a representation on a complex vector space
    dimension % (integer): Representation dimension

Property types are specified before a semicolon, in parenthesis, as for arguments.

Methods are documented as standalone functions. Do not include self in the list of parameters. The constructor is documented separately as any method.

Abstract methods should have a single code line in their body error('Abstract');.

Method blocks can be assigned to a method group, using the following syntax:

methods % Group operations

   function z = compose(self, x, y)



The group name Implementation or Implementations will be ignored: it concerns a block that implements abstract methods. The substring (Abstract) will be erased from group names: it can be used to mention that methods in a block need to be implemented in a subclass.

Documenting types

  • row cell vectors of a particular type: cell{1,*} of TYPE (typed as 'cell{1,\*}') or cell{1,n} of TYPE

  • row double vectors double(1,n) or double(1,*)

  • square matrices double(n,n)

  • matrices double(m,n) if m, n are used elsewhere, or double(*,*) if the sizes are not referenced elsewhere

  • charstring for row char vectors used as strings (as opposed to the new string Matlab type unsupported by Octave)

  • integer for integers stored as doubles

  • RepLAB own types using Sphinx reference syntax with backticks

Other points

  • Equations: we allow the use of LaTeX equations within $ delimitations, as the numpydoc guide stipulates (to be used reasonably since equations will not be interpreted in the REPL).

  • We use sparingly the reST conventions for italics, bold and monospace, but not for variable names. Only package, method, function and class names should be written between single backticks, so that the reference is valid for our infrastructure. One cannot use single backticks for argument names. Matlab/YALMIP objects can be referred with single backticks, as long as they are in the global scope.

  • Use fully qualified references in the first line comment, because that first line comment will be used in descriptions of subclasses in other packages.