Defining singleton methods

Question

The following code is from the book Exceptional Ruby:

starts_with_a = Object.new

def starts_with_a. ===(e)
    /^A/ =~ e.name
end

If I comment out the first line where a new object is assigned to starts_with_a, then I get this error:

`<main>': undefined local variable or method `starts_with_a' for main:Object (NameError)

Question 1: why do I need to assign a new object to starts_with_a to avoid that error?

Also, the method definition starts_with_a has a . before the ===, although the variable starts_with_a does not. There's an error if I leave out that . in the method definition.

Question 2: what is happening with that .? why is it necessary etc.

See also http://stackoverflow.com/questions/13706373/what-does-def-self-function-name-mean/13709911#13709911 — BernardK, Dec 29 '12 at 08:46

score 0 · Accepted Answer · answered Dec 29 '12 at 06:30

With def starts_with_a. ===(e), you are defining a singleton method === on the object starts_with_a. The . between the object and the method indicates that it is a method defined on that object. If you do not have such object already created, a singleton method cannot be defined on it, and will return an error.

Defining singleton methods

1 Answers1