In Ruby, how do I check if method "foo=()" is defined?

ruby syntax-error
Alex Boisvert picture Alex Boisvert · Feb 27, 2010 · Viewed 67.8k times · Source

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.

Answer

molf picture molf · Feb 27, 2010

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

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