I love jtbandes answer, but since it is pretty long, I will add my own compact answer:
==, ===, eql?, equal?
are 4 comparators, ie. 4 ways to compare 2 objects, in Ruby.
As, in Ruby, all comparators (and most operators) are actually method-calls, you can change, overwrite, and define the semantics of these comparing methods yourself. However, it is important to understand, when Ruby's internal language constructs use which comparator:
== (value comparison)
Ruby uses :== everywhere to compare the values of 2 objects, eg. Hash-values:
{a: 'z'} == {a: 'Z'} # => false
{a: 1} == {a: 1.0} # => true
=== (case comparison)
Ruby uses :=== in case/when constructs. The following code snippets are logically identical:
case foo
when bar; p 'do something'
end
if bar === foo
p 'do something'
end
eql? (Hash-key comparison)
Ruby uses :eql? (in combination with the method hash) to compare Hash-keys. In most classes :eql? is identical with :==.
Knowledge about :eql? is only important, when you want to create your own special classes:
class Equ
attr_accessor :val
alias_method :initialize, :val=
def hash() self.val % 2 end
def eql?(other) self.hash == other.hash end
end
h = {Equ.new(3) => 3, Equ.new(8) => 8, Equ.new(15) => 15} #3 entries, but 2 are :eql?
h.size # => 2
h[Equ.new(27)] # => 15
Note: The commonly used Ruby-class Set also relies on Hash-key-comparison.
equal? (object identity comparison)
Ruby uses :equal? to check if two objects are identical. This method (of class BasicObject) is not supposed to be overwritten.
obj = obj2 = 'a'
obj.equal? obj2 # => true
obj.equal? obj.dup # => false