In Ruby, I can define a method foo=(bar):
irb(main):001:0> def foo=(bar) irb(main):002:1> p "foo=#{bar}" irb(main):003:1> end => nil Now I'd like to check if it has been defined,
irb(main):004:0> defined?(foo=) SyntaxError: compile error (irb):4: syntax error, unexpected ')' from (irb):4 from :0 What is the proper syntax to use here? I assume there must be a way to escape "foo=" such that it is parsed and passed correctly to the defined? operator.
The problem is that the foo= method is designed to be used in assignments. You can use defined? in the following way to see what's going on:
defined? self.foo=() #=> nil defined? self.foo = "bar" #=> nil def foo=(bar) end defined? self.foo=() #=> "assignment" defined? self.foo = "bar" #=> "assignment" Compare that to:
def foo end defined? foo #=> "method" To test if the foo= method is defined, you should use respond_to? instead:
respond_to? :foo= #=> false def foo=(bar) end respond_to? :foo= #=> true You can check if a method exists by using the respond_to? method, and you pass it a symbol, e.g. bar.respond_to?(:foo=) to see if the object bar has a method foo=. If you want to know if instances of a class respond to a method you can use method_defined? on the class (or module), e.g. Foo.method_defined?(:bar=).
defined? isn't a method, but an operator which returns a description of the operand (or nil if it is not defined, which is why it can be used in an if statement). The operand can be any expression, i.e. a constant, a variable, an assignment, a method, a method call, etc. The reason why it doesn't work when you do defined?(foo=) is because of the parentheses, skip them and it should work more or less as expected. That being said, defined? is a pretty weird operator, and no one uses it to test for the existence of methods.