method_vs_operator.rb

# Ruby's `=` can be a part of a method name, but is not a method on its own. Consider:
class MyClass
  def initialize(value)
    @a = value
  end
  
  def a=(new_value) # `=` is part of a method name
    @a = new_value  # `=` is an operator
  end
  
  def a; @a; end
end

temp = MyClass.new(7)
puts temp.a           # => 7
temp.a = 9
puts temp.a           # => 9

# Above, `temp.a = 9` is indeed a call to `temp`'s instance method `a=`.
# Thus, writing it as `temp.a= 9` is syntactically correct, if one is so inclined.
# This is why we can do the following

temp.send(:a=, 11)
puts temp.a           # => 11

# However, lone `=` is not a method in Ruby, unlike e.g. `+`:
$ irb
:001> a = 7           # => 7
:002> a.send(:+, 2)   # => 9
:003> a.send(:=, 11)  # => SyntaxError, unexpected '='
:004> a.send('=', 11) # => NameError, undefined method `=` for 9:Fixnum

# Nor is `a=` a method call:

:005> send(:a=, 11)  # => NoMethodError: undefined method `a=` for main:Object
:006> send('a=', 11)  # => NoMethodError: undefined method `a=` for main:Object

# Which is why I can grudgingly accept the missing space in `instance.method= 99`, but not in `a= 99` :)