Why are methods in Ruby documentation preceded by a hash sign?

Solution 1:

Note that the convention is:

Class#method

rather than

object#method

In code you would have object.method, if object was an instance of class. The # convention is not used in code.

From the RDoc documentation:

Use :: for describing class methods, # for describing instance methods, and use . for example code.

Solution 2:

The # notation is used to refer to the canonical instance method, like String#upcase. The . notation is used to refer to the method of a particular instance, like mystring.upcase. The distinction is made to not imply that a class method 'upcase' exists.

Solution 3:

I just realized that none of the other answers touch the most trivial aspect of the question: why the # sign?

I have two theories:

1. It might come from Smalltalk, where symbols are written #sym (instead of :sym) as they are in Ruby. So, if you want to refer to a Method object (as opposed to calling a method), then you would call something like Array >> #new. (The >> is itself a method that returns the method passed to it. So, in Ruby that would be Array.method :new.) In Smalltalk documentation, methods are generally referred to as Class>>method, but in Ruby Class:method would have made more sense, except that it is easily confused with Class::method. Therefore, Class#method was chosen.
2. My other theory is that it simply was chosen because # is the comment character in Ruby.

A definitive answer can only be given by whoever invented that convention. If it was invented for the Programming Ruby book, that would be either Dave Thomas or Andy Hunt, but I kind of doubt that. The book came out in 2001, Ruby started in 1993, how were they referring to methods before then?

Solution 4:

From the rdoc docs (emphasis mine):

Names of classes, source files, and any method names containing an underscore or preceded by a hash character are automatically hyperlinked from comment text to their description.