Ruby Language Monkey Patching in Ruby
Remarks
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.
