Documentation Guide

This guide discusses recommendations for documenting classes, modules, and methods in the Ruby core and in the Ruby standard library.

Generating documentation

Most Ruby documentation lives in the source files and is written in RDoc format.

Some pages live under the doc folder and can be written in either .rdoc or .md format, determined by the file extension.

To generate the output of documentation changes in HTML in the {build folder}/.ext/html directory, run the following inside your build directory:

make html

Then you can preview your changes by opening {build folder}/.ext/html/index.html file in your browser.


The goal of Ruby documentation is to impart the most important and relevant in the shortest time. The reader should be able to quickly understand the usefulness of the subject code and how to use it.

Providing too little information is bad, but providing unimportant information or unnecessary examples is not good either. Use your judgment about what the user needs to know.

General Guidelines


Use only US-ASCII-compatible characters in a C source file. (If you use other characters, the Ruby CI will gently let you know.)

If want to put ASCII-incompatible characters into the documentation for a C-coded class, module, or method, there are workarounds involving new files doc/*.rdoc:


Ruby is documented using RDoc. For information on RDoc syntax and features, see the RDoc Markup Reference.

Output from irb

For code examples, consider using interactive Ruby, irb.

For a code example that includes irb output, consider aligning # => ... in successive lines. Alignment may sometimes aid readability:

a = [1, 2, 3] #=> [1, 2, 3]
a.shuffle!    #=> [2, 3, 1]
a             #=> [2, 3, 1]


Organize a long discussion with headers.

Blank Lines

A blank line begins a new paragraph.

A code block or list should be preceded by and followed by a blank line. This is unnecessary for the HTML output, but helps in the ri output.


In general, RDoc’s auto-linking should not be suppressed. For example, we should write Array, not \Array.

We might consider whether to suppress when:

Documenting Classes and Modules

The general structure of the class or module documentation should be:


The synopsis is a short description of what the class or module does and why the reader might want to use it. Avoid details in the synopsis.

Common Uses

Show common uses of the class or module. Depending on the class or module, this section may vary greatly in both length and complexity.

What’s Here Summary

The documentation for a class or module may include a “What’s Here” section.


Documenting Methods

General Structure

The general structure of the method documentation should be:

Calling Sequence (for methods written in C)

For methods written in Ruby, RDoc documents the calling sequence automatically.

For methods written in C, RDoc cannot determine what arguments the method accepts, so those need to be documented using RDoc directive {:call-seq:}.


*  call-seq:
*    array.count -> integer
*    array.count(obj) -> integer
*    array.count {|element| ... } -> integer

When creating the call-seq, use the form

receiver_type.method_name(arguments) {|block_arguments|} -> return_type

Omit the parentheses for cases where the method does not accept arguments, and omit the block for cases where a block is not accepted.

In the cases where method can return multiple different types, separate the types with “or”. If the method can return any type, use “object”. If the method returns the receiver, use “self”.

In cases where the method accepts optional arguments, use a call-seq with an optional argument if the method has the same behavior when an argument is omitted as when the argument is passed with the default value. For example, use:

obj.respond_to?(symbol, include_all=false) -> true or false

Instead of:

obj.respond_to?(symbol) -> true or false
obj.respond_to?(symbol, include_all) -> true or false

However, as shown above for Array#count, use separate lines if the behavior is different if the argument is omitted.

Omit aliases from the call-seq, but mention them near the end (see below).

A call-seq block should have {|x| ... }, not {|x| block } or {|x| code }.

A call-seq output should:


The synopsis comes next, and is a short description of what the method does and why you would want to use it. Ideally, this is a single sentence, but for more complex methods it may require an entire paragraph.

For Array#count, the synopsis is:

Returns a count of specified elements.

This is great as it is short and descriptive. Avoid documenting too much in the synopsis, stick to the most important information for the benefit of the reader.

Details and Examples

Most non-trivial methods benefit from examples, as well as details beyond what is given in the synopsis. In the details and examples section, you can document how the method handles different types of arguments, and provides examples on proper usage. In this section, focus on how to use the method properly, not on how the method handles improper arguments or corner cases.

Not every behavior of a method requires an example. If the method is documented to return self, you don’t need to provide an example showing the return value is the same as the receiver. If the method is documented to return nil, you don’t need to provide an example showing that it returns nil. If the details mention that for a certain argument type, an empty array is returned, you don’t need to provide an example for that.

Only add an example if it provides the user additional information, do not add an example if it provides the same information given in the synopsis or details. The purpose of examples is not to prove what the details are stating.

Argument Description (if necessary)

For methods that require arguments, if not obvious and not explicitly mentioned in the details or implicitly shown in the examples, you can provide details about the types of arguments supported. When discussing the types of arguments, use simple language even if less-precise, such as "level must be an integer", not "level must be an Integer-convertible object". The vast majority of use will be with the expected type, not an argument that is explicitly convertible to the expected type, and documenting the difference is not important.

For methods that take blocks, it can be useful to document the type of argument passed if it is not obvious, not explicitly mentioned in the details, and not implicitly shown in the examples.

If there is more than one argument or block argument, use a labeled list.

Corner Cases and Exceptions

For corner cases of methods, such as atypical usage, briefly mention the behavior, but do not provide any examples.

Only document exceptions raised if they are not obvious. For example, if you have stated earlier than an argument type must be an integer, you do not need to document that a TypeError is raised if a non-integer is passed. Do not provide examples of exceptions being raised unless that is a common case, such as Hash#fetch raising a KeyError.


Mention aliases in the form

// Array#find_index is an alias for Array#index.

Related Methods (optional)

In some cases, it is useful to document which methods are related to the current method. For example, documentation for Hash#[] might mention Hash#fetch as a related method, and Hash#merge might mention Hash#merge! as a related method. Consider which methods may be related to the current method, and if you think the reader would benefit it, at the end of the method documentation, add a line starting with “Related: ” (e.g. “Related: fetch”). Don’t list more than three related methods. If you think more than three methods are related, pick the three you think are most important and list those three.

Methods Accepting Multiple Argument Types

For methods that accept multiple argument types, in some cases it can be useful to document the different argument types separately. It's best to use a separate paragraph for each case you are discussing.