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rubyobjectshort-circuitingrespond-to

Ruby 1.9.2 Object.respond_to? :hello && Object.hello gives error, why?

While stepping through code today, I noticed something unexpected. This statement:

if Object.respond_to? :hello && Object.hello #stuff

gives an undefined method error. But why? Obviously hello is not a valid method of Object, however given the short-circuit evaluation, shouldn't Object.hello be ignored whenever Object.respond_to? :hello is false?

I noticed this while playing with Authlogic, trying to figure out exactly why the UserSession class must define persisted? in Rails 3.

Thanks

Solution

  • The lack of parentheses is leading to a precedence issue:

    >> Object.respond_to? :hello && Object.hello
NoMethodError: undefined method `hello' for Object:Class
    from (irb):2
    from /Users/john/.rvm/rubies/ruby-1.9.2-p136/bin/irb:16:in `<main>'
>> Object.respond_to?(:hello) && Object.hello
=> false