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suhlig/defined.rb

Created June 1, 2016 11:08
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Side effect of defined?
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defined.rb
$ pry
pry> foo
NameError: undefined local variable or method `foo' for main:Object
pry> if defined?(foo)
* bar = 1
* end
=> nil
pry> if defined?(bar)
* puts "oops"
* end
oops
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suhlig commented Jun 13, 2016

@avdi answered this in RubyTapas' Episode #412: Autovivification:

What we've run into here is a very special rule in how Ruby evaluates code. In a nutshell, the rule says that at a given line in the code, if there is any possibility that a local variable will exist, then that variable is guaranteed to always exist. Even if Ruby has to quietly assign it a nil value to ensure its existence.

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