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Add Method Documentation Guide (#3399)
This documents how methods for core classes and classes in the standard library should be documented. Co-authored-by: Eric Hodel <drbrain@segment7.net>
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= Method Documentation Guide
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This guide discusses recommendations for documenting methods for Ruby core
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classes and classes in the standard library.
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== Goal
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The goal when documentating a method is to impart the most important
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information about the method in the least amount of time. A reader
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of the method documentation should be able to quickly understand
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the purpose of the method and how to use it. Providing too little
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information about the method is not good, but providing unimportant
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information or unnecessary examples is not good either. Use your
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judgment about what the user of the method needs to know to use the
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method correctly.
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== General Structure
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The general structure of the method documentation should be:
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* call-seq (for methods written in C)
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* Synopsis (Short Description)
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* Details and Examples
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* Argument Description (if necessary)
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* Corner Cases and Exceptions
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* Related Methods (optional)
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== call-seq (for methods written in C)
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For methods written in C, RDoc cannot determine what arguments
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the method accepts, so those need to be documented using a
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<tt>call-seq</tt>. Here's an example <tt>call-seq</tt>:
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* call-seq:
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* array.count -> integer
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* array.count(obj) -> integer
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* array.count {|element| ... } -> integer
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When creating the <tt>call-seq</tt>, use the form
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receiver_type.method_name(arguments) {|block_arguments|} -> return_type
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Omit the parentheses for cases where the method does not accept arguments,
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and omit the block for cases where a block is not accepted.
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In the cases where method can return multiple different types, separate the
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types with "or". If the method can return any type, use "object". If the
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method returns the receiver, use "self".
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In cases where the method accepts optional arguments, use a <tt>call-seq</tt>
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with an optional argument if the method has the same behavior when an argument
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is ommitted as when the argument is passed with the default value. For example,
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use:
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* obj.respond_to?(symbol, include_all=false) -> true or false
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Instead of:
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* obj.respond_to?(symbol) -> true or false
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* obj.respond_to?(symbol, include_all) -> true or false
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However, as shown above for <tt>Array#count</tt>, use separate lines if the
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behavior is different if the argument is ommitted.
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== Synopsis
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The synopsis comes next, and is a short description of what the
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method does and why you would want to use it. Ideally, this
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is a single sentence, but for more complex methods it may require
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an entire paragraph.
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For <tt>Array#count</tt>, the synopsis is:
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Returns a count of specified elements.
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This is great as it is short and descriptive. Avoid documenting
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too much in the synopsis, stick to the most important information
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for the benefit of the reader.
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== Details and Examples
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Most non-trivial methods benefit from examples, as well as details
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beyond what is given in the synopsis. In the details and examples
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section, you can document how the method handles different types
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of arguments, and provides examples on proper usage. In this
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section, focus on how to use the method properly, not on how the
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method handles improper arguments or corner cases.
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Not every behavior of a method requires an example. If the method
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is documented to return +self+, you don't need to provide an example
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showing the return value is the same as the receiver. If the method
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is documented to return +nil+, you don't need to provide an example
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showing that it returns +nil+. If the details mention that for a
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certain argument type, an empty array is returned, you don't need
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to provide an example for that.
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Only add an example if it provides the user additional information,
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do not add an example if it provides the same information given
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in the synopsis or details. The purpose of examples is not to prove
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what the details are stating.
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== Argument Description (if necessary)
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For methods that require arguments, if not obvious and not explicitly
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mentioned in the details or implicitly shown in the examples, you can
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provide details about the types of arguments supported. When discussing
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the types of arguments, use simple language even if less-precise, such
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as "level must be an integer", not "level must be an Integer-convertible
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object". The vast majority of use will be with the expected type, not an
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argument that is explicitly convertable to the expected type, and
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documenting the difference is not important.
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For methods that take blocks, it can be useful to document the type of
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argument passed if it is not obvious, not explicitly mentioned in the
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details, and not implicitly shown in the examples.
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If there is more than one argument or block argument, use an RDoc
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definition list:
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argument_name1 :: type and description
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argument_name2 :: type and description
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== Corner Cases and Exceptions
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For corner cases of methods, such as atypical usage, briefly mention
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the behavior, but do not provide any examples.
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Only document exceptions raised if they are not obvious. For example,
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if you have stated earlier than an argument type must be an integer,
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you do not need to document that a TypeError is raised if a non-integer
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is passed. Do not provide examples of exceptions being raised unless
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that is a common case, such as Hash#fetch raising KeyError.
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== Related Methods (optional)
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In some cases, it is useful to document which methods are related to
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the current method. For example, documentation for Hash#[] might
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mention Hash#fetch as a related method, and Hash#merge might mention
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#merge! as a related method. Consider which methods may be related
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to the current method, and if you think the reader would benefit it,
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at the end of the method documentation, add a line starting with
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"Related: " (e.g. "Related: #fetch"). Don't list more than three
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related methods. If you think more than three methods are related,
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pick the three you think are most important and list those three.
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== Methods Accepting Multiple Argument Types
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For methods that accept multiple argument types, in some cases it can
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be useful to document the different argument types separately. It's
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best to use a separate paragraph for each case you are discussing.
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== Use of English
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Readers of this documentation may not be native speakers of English.
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Documentation should be written with this in mind.
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Use short sentences and group them into paragraphs that cover a single
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topic. Avoid complex verb tenses, excessive comma-separated phrases,
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and idioms.
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When writing documentation, define unusual or critical concepts in
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simple language. Provide links to authoritative sources, or add a
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general description to the top-level documentation for the class or
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module.
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== Formatting
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Extraneous formatting such as headings and horizontal lines should be
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avoided in general. It is best to keep the formatting as simple as
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possible. Only use headings and other formatting for the most complex
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cases where the method documentation is very long due to the complexity
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of the method.
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Methods are documented using RDoc syntax. See the
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{RDoc Markup Reference}[https://docs.ruby-lang.org/en/master/RDoc/Markup.html#class-RDoc::Markup-label-RDoc+Markup+Reference]
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for more information on formatting with RDoc syntax.
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