Question

I am a Ruby beginner. From the book, I know that a Ruby method name should start with a lowercase letter or underscore. But I found different scenarios:

1. If a method is defined outside a class, it can only begin with lowercase letter, Ruby will complain with an error if you try to define a method which begins with an uppercase letter, for example:

   define sayHi
     puts "Hello" 
   end
   sayHi   # => Hello
   

   but, the following code does not work:

   define SayHi
     puts "Hello" 
   end
   SayHi 
   

   it will produce an error:

   :in `<main>': uninitialized constant SayHi (NameError)
   

2. If a method is defined inside a class, then it can begin with uppercase letter:

   class Test
     def SayHi
       puts "hello" 
     end
   end
   t = Test.new
   t.SayHi    # => hello
   

Does anyone know why #1 does not work while #2 work? What are the exact rules the ruby method name?

Answer 1

By convention, things that start with uppercase letters are constants. When you invoke SayHi, you’re telling Ruby to look for a constant with this name. Of course, there isn’t one, so it fails.

If you want to invoke the method, you’ll need to add a pair of parentheses. For example,

def S
  puts "shazam!"
end

S    #=> NameError: uninitialized constant S
S()  #=> "shazam!"

Inside of a class, the resolution rules are a little different. Let’s define a simple class with a constant and a method named to look like a constant:

irb(main):001:0> class C
irb(main):002:1>   A = "x"
irb(main):003:1>   def B
irb(main):004:2>     puts "B() invoked"
irb(main):005:2>   end
irb(main):006:1> end
=> nil

Now, A is certainly a constant. But what about B?

irb(main):008:0> C.const_defined?("A")
=> true    # A is a constant!
irb(main):009:0> C.const_defined?("B")
=> false   # B looks like a constant but isn't, because we
           # used "def" to create it. "def" is for methods,
           # not constants.

So it isn’t a constant, just a method with that name. When we try to access B from an instance of C, now Ruby’s looking for a method:

irb(main):011:0> C.new.B
B() invoked
=> nil

If we wanted to access a constant of C instead, we use the scope qualifier :::

irb(main):012:0> C::A
=> "x"