Because I forgot an assignment, I read the undefined local variable hash before writing it. Surprise: Instead of getting a NameError, the value was read just fine: It was some FixNum and the program crashed much later.
Investigating on the problem, I did the following:
hash and press EnterWhy is that? Is it a bug or a feature? What am I reading here?
TL;DR – it's the hash value for Ruby's top-level object, equivalent to self.hash.
Here's a little debugging help:
irb(main):001:0> hash #=> 3220857809431415791 irb(main):002:0> defined? hash #=> "method" irb(main):003:0> method(:hash) #=> #<Method: Object(Kernel)#hash> You can now lookup Object#hash1 online:
http://ruby-doc.org/core-2.3.1/Object.html#method-i-hash
Or in IRB:
irb(main):004:0> help "Object#hash" = Object#hash (from ruby core) ------------------------------------------------------------------------------ obj.hash -> fixnum ------------------------------------------------------------------------------ Generates a Fixnum hash value for this object. This function must have the property that a.eql?(b) implies a.hash == b.hash. The hash value is used along with #eql? by the Hash class to determine if two objects reference the same hash key. Any hash value that exceeds the capacity of a Fixnum will be truncated before being used. The hash value for an object may not be identical across invocations or implementations of Ruby. If you need a stable identifier across Ruby invocations and implementations you will need to generate one with a custom method. #=> nil irb(main):005:0> 1 Object(Kernel)#hash actually means that hash is defined in Kernel, but as stated in the documentation for Object:
Although the instance methods of Object are defined by the Kernel module, we have chosen to document them here for clarity.
