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Method Documentation Guide

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


The goal when documenting a method is to impart the most important information about the method in the least amount of time. A reader of the method documentation should be able to quickly understand the purpose of the method and how to use it. Providing too little information about the method is not good, but providing unimportant information or unnecessary examples is not good either. Use your judgment about what the user of the method needs to know to use the method correctly.

General Structure

The general structure of the method documentation should be:

call-seq (for methods written in C)

For methods written in C, RDoc cannot determine what arguments the method accepts, so those need to be documented using a call-seq. Here’s an example 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.


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 an RDoc definition list:


type and description


type and description

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 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 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.

Use of English

Readers of this documentation may not be native speakers of English. Documentation should be written with this in mind.

Use short sentences and group them into paragraphs that cover a single topic. Avoid complex verb tenses, excessive comma-separated phrases, and idioms.

When writing documentation, define unusual or critical concepts in simple language. Provide links to authoritative sources, or add a general description to the top-level documentation for the class or module.


Extraneous formatting such as headings and horizontal lines should be avoided in general. It is best to keep the formatting as simple as possible. Only use headings and other formatting for the most complex cases where the method documentation is very long due to the complexity of the method.

Methods are documented using RDoc syntax. See the RDoc Markup Reference for more information on formatting with RDoc syntax.