Why does `defined?` keyword not return boolean?

Question

In ruby, most methods or keywords that end with ? return boolean values. And we except them to behave like this. Why does defined? keyword return somethings else? Or why is there ? at the end of it?

Answer 1

Developers chose to return something more meaningfull than true or false because the only case that breaks by not having boolean returned is explicit comparison:

defined?(:x) == true
# => always `false`

Such comparison is something you should usually not do, as logical operators like || and && are just as likely to return some truthy object instead of true. This is barely needed for anything.

Answer 2

This question can be understood in two ways:

1. Why doesn't it simply return true or false?

It's because it encodes more information than simply if something is defined or not:

defined? Class # => "constant"
defined? 42    # => "expression"
defined? nil   # => "nil"
defined? x     # => nil

2. Why does it have ? at the end since as the convention goes, the question mark is reserved for predicates?

You are right that this is inconsistent. The most likely reasons are:

• Almost always, you will use it as predicate anyway

  if defined? x
    # do something
  end
  

• The shortest alternative, which doesn't sound like a predicate I can think of is definition_type_of. Generally, you want to keep the reserved words in your language short

Answer 3

The "defined?"-method can return more than "true" or "false". It tells you what type of variable it is if it's defined at all.

Check Checking if a variable is defined? and http://ruby-doc.org/docs/keywords/1.9/Object.html#method-i-defined-3F