In Ruby, how do I check if method "foo=()" is defined?
Solution 1
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
Solution 2
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.
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Alex Boisvert
Updated on March 31, 2021Comments
-
Alex Boisvert about 3 years
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.
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Alex Boisvert about 14 yearsThanks! This solves my problem. I'm still curious to know if there's a way to escape foo= such that it can be fed to defined? but at least now I can move on.
-
molf about 14 yearsThe problem here is that
foo=
is always used in assignments, so Ruby will return"assignment"
if you test fordefined? foo()
(see updated answer). -
0x4a6f4672 over 11 yearsIt is respond_to?, not responds_to?
-
Theo over 11 yearsAh, the spelling of Ruby core library...
respond_to?
,start_with?
,end_with?
. -
drewish over 9 yearsstackoverflow.com/questions/5280556/… has a nice explanation of the choice behind the naming TL;DR
you.knock if you.respond_to?(:knock)