What is `hash` in ruby?

Question

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:

• Open up irb
• type hash and press Enter
• Surprise! the answer is -1831075300640432498 (and surprisingly not NameError, nor 42)

Why is that? Is it a bug or a feature? What am I reading here?

Answer 1

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>

---

¹Object(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.