Monkey patching, while convenient, has some pitfalls that aren't immediately obvious. Most notably, a patch like that in the example pollutes the global scope. If two modules both add
Hash#symbolize, only the last module required actually applies its change; the rest are erased.
Furthermore, if there's an error in a patched method, the stacktrace simply points to the patched class. This implies that there's a bug in the
Hash class itself (which there is now).
Lastly, because Ruby is very flexible with what containers to hold, a method that seems very straightforward when you write it has lots of undefined functionality. For instance, creating
Array#sum is good for an array of numbers, but breaks when given an array of a custom class.
A safer alternative is refinements, available in Ruby >= 2.0.
You can add a method to any class in Ruby, whether it's a builtin or not. The calling object is referenced using
class Fixnum def plus_one self + 1 end def plus(num) self + num end def concat_one self.to_s + '1' end end 1.plus_one # => 2 3.plus(5) # => 8 6.concat_one # => '61'