bill gate wrote in post #1064388:
> Why lot of Rails snippet code  use __FILE__ __LINE__

__FILE__ and __LINE__ are magic constants. They point to the file/line 
they're in.

In combination with class_eval these constants can be used to produce 
exact error messages. If you don't use them, an error message within the 
class_eval will only display the file and line of the "class_eval" 
(rather than the file and line of the actual string).

See
http://ruby-doc.org/core-1.9.3/Module.html#method-i-class_eval

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