local variables - Ruby: method inexplicably overwritten and set to nil

Question

If I execute this ruby code:

def foo
  100
end

p defined?(foo), foo
if false
  foo = 200
end
p defined?(foo), foo

The output I get is:

"method"
100
"local-variable"
nil

Can someone explain to me why foo is set to nil after not executing the if? Is this expected behavior or a ruby bug?

Answer 1

Names on the left hand side of assignments get set to nil, even if the code can't be reached as in the if false case.

>> foo
NameError: undefined local variable or method `foo' for main:Object
...
>> if false
..   foo = 1
..   end #=> nil
>> foo #=> nil

When Ruby tries to resolve barewords, it first looks for local variables (there's a reference to that in the Pickaxe book, which I can't seem to find at the moment). Since you now have one called foo it displays nil. As Mischa noted, the method still can be called as foo().