Why does `defined?` keyword not return boolean?

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?

Solution

This question can be understood in two ways:

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

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