Question

I’m kind of confused about reserved words in Ruby.

“The Ruby Programming Language”, co-authored by Matz, says that begin and end are reserved words of the language. They’re certainly used syntactically to mark out blocks.

However, range objects in the language have methods named begin and end, as in

(1..10).end
=> 10 

Now, testing this out, I find that, indeed, I can define methods named “begin” and “end” on objects, though if I try to name a variable “begin” it fails. (Here’s a sample of using it as a method name, it actually works…:)

class Foo
  def begin
    puts "hi"
  end
end

Foo.new.begin

So, I suppose I’m asking, what actually is the status of reserved words like this? I would have imagined that they couldn’t be used for method names (and yet it seems to work) or that at the very least it would be terrible style (but it is actually used in the core language for the Range class).

I’m pretty confused as to when they’re allowed to be used and for what. Is there even documentation on this?

Answer 1

Yes, they are reserved words. Yes, they can be used for method names. No, you can’t call them without an explicit receiver. It’s probably not a good idea anyway.

class Foo
  def if(foo)
    puts foo
  end
end

Foo.new.if("foo") # outputs foo, returns nil

Update: Here’s a quote from “The Ruby Programming Language”, by Matz (the creator of Ruby) himself:

In most languages, these words would be called “reserved words” and
they would be never allowed as identifiers. The Ruby parser is
flexible and does not complain if you prefix these keywords with @,
@@, or $ prefixes and use them as instance, class, or global variable
names. Also, you can use these keywords as method names, with the
caveat that the method must always be explicitly invoked through an
object.