I'm working on a ruby project in which we are planning to do some operations with ruby strings. Some operations are simple (like counting the number of words) and others more complex (like checking if a given string is in the correct language).
A possible way to implement this is by patching the String class with extra methods, without modifying any existing methods, and adding behaviors like "some string".word_count and "some string".cjk?.
Another approach, based on FileUtils is to create a class or module full of methods and always use string as parameters, like OddClassName.word_count("some string") and OddClassName.cjk?("some string"). We like the first better because of readability.
I understand that monkey patching a basic class as described in the first alternative can have name clashes. However, if this is the main application, not a library, should I worry with it at all?
So, the questions are:
Is adding methods to ruby base classes a bad practice? If yes, is that in all cases or only in some cases?
What is the best approach to accomplish this?
What could be the name of 'OddClassName'?
Please suggest any alternatives.
Monkey patching isn't considered to be a bad practice unless you are writing odd methods that do not have PatchedClass-related behavior (for example, String.monkeyPatchForMakingJpegFromString is rather bad, but Jpeg.fromString is good enough.)
But if your project is rather large, the libraries that you use in it may happen to have colliding patches, so you may have one more problem with all these patching stuffs. In Ruby 2.0, refinements come to an aid. They work as follows: you define a module, refine your (even core) class in it, and then use that module where it's necessary. So, in your code it works as:
YourClass.new.refinedMethodFromCoreClass #=> some result
But
CoreClass.refinedMethodFromCoreClass
produces undefined method exception.
That's all monkey patching stuff: monkey patching is useful and convenient, but refinements add some features, that make your code more secure, maintainable and neat.
I'd use a new class, call it Doc or something because getting the word count and checking languages sounds like operations for documents.
Let it take a string as a constructor parameter and have modifications chain to return a new Doc. Also give it a to_s method that returns the string.
class Doc
def initialize(str)
#str = str
end
def to_s
#str
end
define word_count, cjk?, etc.
end
Doc.new("Some document").word_count
# => 2
Related
Although I agree that extending native types and objects is a bad practice, inheriting from them should not be.
In a supposedly supporting gem (that I could not find), the way that the native types were to be used would be as follows:
require 'cool-unkown-light-gem'
class MyTypedArray < CoolArray # would love to directly < Array
def initialize(*args)
super(*args)
# some inits for DataArray
#caches_init = false
end
def name?(name)
init_caches unless !#caches_init
!!#cache_by_name[name]
end
def element(name)
init_caches unless !#caches_init
#cache_by_name[name]
end
private
# overrides the CoolArray method:
# CoolArray methods that modify self will call this method
def on_change
#caches_init = false
super
end
def init_caches
return #cache_by_name if #caches_init
#caches_init = true
#cache_by_name = self.map do |elem|
[elem.unique_name, elem]
end.to_h
end
end
Any method of the parent class not overridden by the child class that modifies self would call, let's say (in this case), the on_change function. Which would allow to do not have to re-define every single one of those methods to avoid losing track on changes.
Let's say the MyTypedArray would array Foo objects:
class Foo
attr_reader :unique_name
def initialize(name)
#unique_name = name
end
end
a short example of the expected behaviour of its usage:
my_array = MyTypedArray.new
my_array.push( Foo.new("bar") ).push( Foo.new("baz") )
my_array.element("bar").unique_name
# => "bar"
my_array.shift # a method that removes the first element from self
my_array.element("bar").unique_name
# => undefined method `unique_name' for nil:NilClass (NoMethodError)
my_array.name?("bar")
# => false
I understand that we should search for immutable classes, yet those native types support changes on the same object and we want a proper way to do an inheritance that is as brief and easy as possible.
Any thoughts, approaches, or recommendations are more than welcome, of course. I do not think I am the only one that have thought on this.
The reason why I am searching for a maintained gem is because different ruby versions may offer different supported methods or options for native types / classes.
[Edit]
The aim of the above is to figure out a pattern that works. I could just follow the rules and suggestions of other posts, yet would not get things work the way I am intended and when I see it proper (a coding language is made by and for humans, and not humans made for coding languages). I know everyone is proud of their achievements in learning, developing and making things shaped in a pattern that is well known in the community.
The target of the above is because all the methods of Array are more than welcome. I do not care if in the version 20 of Ruby they remove some methods of Array. By then my application will be obsolete or someone will achieve the same result in far less code.
Why Array?
Because the order matters.
Why an internal Hash?
Because for the usage I want to make of it, in overall, the cost of building the hash compensates the optimization it offers.
Why not just include Enumerable?
Because we just reduce the number of methods that change the object, but we do not actually have a pattern that allows to change #caches_init to false, so the Hash is rebuilt on next usage (so same problem as with Array)
Why not just whitelist and include target Array methods?
Because that does not get me where I want to be. What if I want anyone to still use pop, or shift but I do not want to redefine them, or even having to bother to manage my mixins and constantly having to use responds_to?? (perhaps that exercise is good to improve your skills in coding and read code from other people, but that is not what it should be)
Where I want to be?
I want to be in a position that I can re-use / inherit any, I repeat, any class (no matter if it is native or not). That is basic for an OOP language. And if we are not talking about an OOP language (but just some sugar at the top of it to make it appear as OOP), then let's keep ourselves open to analyse patterns that should work well (no matter if they are odd - for me is more odd that there are no intermediate levels; which is symptom of many conventional patterns, which in turn is symptom of poor support for certain features that are more widely required than what is accepted).
Why should a gem offer the above?
Well, let's humble it. The above is a very simple case (and even though not covered). You may gain in flexibility at some point by using what some people want to call the Ruby way. But at a cost when you move to bigger architectures. What if I want to create intermediate classes to inherit from? Enriched native classes that boost simple code, yet keeping it aligned with the language. It is easier to say this is not the Ruby way than trying to make the language closer to something that escalates well from the bottom.
I am not surprised that Rails and Ruby are almost "indistinctly" used by many. Because at some point, without some Rails support, what you have with Ruby is a lot of trouble. As, consequently, I am not surprised that Rails is so maintained.
Why should I redefine a pop, or a last, or first methods? For what? They are already implemented.
Why should I whitelist methods and create mixins? is that a object or method oriented programming?
Anyway... I do not expect anyone to share my view on this. I do see other patterns, and I will keep allowing my mind to find them. If anyone is open enough, please, feel free to share. Someone may criticize the approach and be right, but if you got there is because it worked.
To answer your question as it is written, no, there is no gem for this. This is not a possibility of the language, either in pure Ruby or in C which is used internally.
There is no mechanism in detect when self is changed, nor any way to detect if a method is pure (does not change self) or impure (does change self). It seems you want a way to "automatically" be able to know when a method is one or the other, and that, to put simply, is just not possible, nor is it in any language that I am aware of.
Internally (using your example) an Array is backed by a RArray structure in C. A struct is simple storage space: a way to look at an arbitrary block of memory. C does not care how you choose to look at memory, I could just as easily cast the pointer of this struct and say it is a now a pointer to an array of integers and change it that way, it will happily manipulate the memory as I tell it to, and there is nothing that can detect that I did so. Now add in the fact that anyone, any script, or any gem can do this and you have no control over it, and it just shows that this solution is fundamentally and objectively flawed.
This is why most (all?) languages that need to be notified when an object is changed use an observer pattern. You create a function that "notifies" when something changes, and you invoke that function manually when needed. If someone decides to subclass your class, they need only continue the pattern to raise that function if it changes the object state.
There is no such thing as an automatic way of doing this. As already explained, this is an "opt-in" or "whitelist" solution. If you want to subclass an existing object instead of using your own from scratch, then you need to modify its behavior accordingly.
That said, adding the functionality is not as daunting as you may think if you use some clever aliasing and meta-programming with module_eval, class_eval or the like.
# This is 100% untested and not even checked for syntax, just rough idea
def on_changed
# Do whatever you need here when object is changed
end
# Unpure methods like []=, <<, push, map!, etc, etc
unpure_methods.each do |name|
class_eval <<-EOS
alias #{name}_orig #{name}
def #{name}(*args, &block)
#{name}_orig(*args, &block)
on_changed
end
EOS
end
It's very simple. Here's what I want to do with a date that is formatted as YYYYMMDD:
month = datestring[0:2]
day = datestring[2:2]
year = datestring[4:4]
return "#{month}/#{day}/#{year}"
The problem is, and I've never understood this about Ruby, do I do:
a module?
a mixin?
something else?
I know what I want to do, I just have NO idea what kind of file or structure to put it in. And if it's a module, do I prefix the method name with the name of the module:
module DateHelper
def DateHelper.transform(datestring)
...
end
end
Why or why wouldn't I do this? Thanks a lot for helping to clear something up that's represented a mental block for me.
Instead of modules, consider using objects with composition:
class DateString
def initialize(date)
#date_string = date
end
def format
...
end
end
Now you can do:
my_date_string = DateString.new("20121203")
puts my_date_string.format # => "12/03/2012"
This answer covers the ruby-philosophical part of your question, but if you're doing a lot of date formatting, consider using a gem that already does it for you.
The code in your question is suggestive of a mixin, which is one possible way to go. It is more idiomatically written like this:
module DateHelper
def date_to_some_format(date)
...
end
end
Notice that the method is an instance method, and that it has a name that won't potentially clash with other methods in an arbitrary class. If you wanted the method available at the class-level, one way you could do this would be:
class SomeClass
class << self
include DateHelper
...
end
end
That is one way to handle class-level mixins; there are others as well.
Whether you want a mixin will depend upon the context. Does the date formatting involve code that you find repeating all over the place in unrelated classes? Is it a one-off that you will never use again? Is remembering state required? Is the date format so useful it might be made into an extension to a core library? The answer to these questions will suggest whether the code should be handled in a mixin or in some other way. (Mixins are generally implemented in Ruby with modules.) Part of really learning ruby is getting a sense of the answers to these questions in different contexts; there are few simple answers in this regard, and to a certain extent things depend upon convention, personal preference and house style.
As an aside, one way to get the date format you're looking for is this (following your input date format of YYYYMMDD):
Date.strptime("20100101", "%Y%m%d").strftime("%m/%d/%Y")
You would not necessarily chain it together in this way in your code—Brendan Benson's response provides a good approach, for example—although you might not need something fancy if you're only doing this in one place.
What are the things to be careful about while redefining methods in Ruby? Is redefining core library methods okay?
the problems IMHO are
You'll forget about the change.
You'll copy paste a snippet from the internet, which will trigger an error the altered behavior and you'll scratch your head until you get hairless patches.
Another developer will come after your and fight a bug for 3 months, until he finds it's in one of the monkey patches. He'll go to HR, get your address and and show you why not to do monkey patches.
Now, sometimes you need to monkey patch a class (even in the core library, why not). My suggestion is
Put all of your monkey-patches in ONE source folder.
The second thing you say to a new developer after "Hello my name is ..." is the location of that folder and a detailed explanation of what each monkey patch does.
I don't do much monkeypatching myself, but I've heard that rather than doing
class String
def improved_method
# teh codes
end
end
It's better to put the new method into a module, and then include the module
module ImprovedString
def improved_method
# teh codes
end
end
class String
include ImprovedString
end
it makes it easier to find where a method has been defined, and the old version still exists without having to do alias chaining.
I like the other answers. Though, I have to add that:
Sometimes you may only want to redefine methods only for certain instances. You can do this, and it makes it somehow more controlled than changing the functionality for all objects of a certain class - as long as a proper debugger is used for debugging:
class << object_instance
def method_redefinition
return "method_redefinition"
end
end
object_instance.method_redefinition => "method redefinition"
The metioned set of functionalities can also be encapsulated in a mix-in in order to avoid too much nesting and messy "code definition inside code execution":
module M
def method_redefinition
"method_redefinition"
end
end
object_instance.extend M
object_instance.method_redefinition => "method_redefinition"
You're talking about monkey patching and it's dangerous for the following reasons according to wikipedia
Carelessly written or poorly
documented monkey patches can lead to
problems:
They can lead to upgrade problems when the patch makes assumptions about
the patched object that are no longer
true; if the product you have changed
changes with a new release it may very
well break your patch. For this reason
monkey patches are often made
conditional, and only applied if
appropriate.
If two modules attempt to monkey-patch the same method, one of
them (whichever one runs last) "wins"
and the other patch has no effect,
unless monkeypatches are written with
pattern like alias_method_chain
They create a discrepancy between the original source code on disk and
the observed behaviour that can be
very confusing to anyone unaware of
the patches' existence.
Even if monkey patching isn't used,
some see a problem with the
availability of the feature, since the
ability to use monkey patching in a
programming language is incompatible
with enforcing strong encapsulation,
as required by the object-capability
model, between objects.
There's talk of a safer way of monkey
patching coming in ruby 2 called
refinements
This tiny gem might be useful if you're finding yourself running into monkey-patching issues: (https://github.com/gnovos/ctx). I originally wrote it to create more expressive DSLs by allowing alterations to the base objects without doing too much damage elsewhere, but it can probably be put to any number of uses. It gets around some of the monkey-patching issues by scoping method re-definition into arbitrary contexts that can be swapped out as needed.
If I want to redefine a method in some core class (for example, in String, etc), I use ctx_define instead of "def" and then later wrap the section of code that should use the new definition in a ctx block, like so:
class ::String
ctx_define :dsl, :+ do |other|
"#{self[0].upcase}#{self[1..-1]}#{other.capitalize}"
end
end
puts "this" + "is" + "normal" + "text" + "concatination"
# => thisisnormaltextconcatination
ctx(:dsl) { puts "this" + "is" + "special" + "text" + "concatination" }
# => ThisIsSpecialTextConcatination
I only threw it together in a few minutes, so I can't make any guarantees about how robust it is in any number of complicated situations, but it seems to work fine for simple needs. Give it a look if you're interested and see if it is of any help. :)
I wonder if one should validate that the arguments passed to a method is of a certain class.
eg.
def type(hash = {}, array = [])
# validate before
raise "first argument needs to be a hash" unless hash.class == Hash
raise "second argument needs to be an array" unless array.class == Array
# actual code
end
Is it smart to do this or is it just cumbersome and waste of time to validate all passed in arguments?
Are there circumstances when you would like to have this extra security and circumstances when you won't bother?
Share your experiences!
I wouldn't recommend this specific approach, as you fail to accommodate classes that provide hash or array semantics but are not that class. If you need this kind of validation, you're better off using respond_to? with a method name. Arrays implement the method :[], for what it's worth.
OpenStruct has hash semantics and attribute-accessor method semantics, but won't return true for the condition hash.class==Hash. It'll work just like a hash in practice, though.
To put it in perspective, even in a non-dynamic language you wouldn't want to do it this way; you'd prefer to verify that an object implements IDictionary<T>. Ruby would idiomatically prefer that, when necessary, you verify that the method exists, because if it does, the developer is probably intending their object to act alike. You can provide additional sanity with unit tests around the client code as an alternative to forcing things to be non-dynamic.
There's usually no need to validate that arguments are of a certain class. In Ruby, you're encouraged to use Duck Typing.
I have found that validating that the input parameters meet your preconditions is a very valuable practice. The stupid person that its saves you from is you. This is especially true for Ruby as it has no compile time checks. If there are some characteristics of the input of your method that you know must be true it makes senes to check them at run time and raise errors with meaningful error messages. Otherwise, the code just starts producing garbage out for garbage in and you either get a wrong answer or an exception down the line.
I think that it is unnecessary. I once read on a blog something like "If you need to protect your code from stupid people, ruby isn't the language for you."
If you want compiler/runtime-enforced code contracts, then Ruby isn't for you. If you want a malleable language and easy testability, then Ruby is.
According to Wikipedia, a monkey patch is:
a way to extend or modify the runtime
code of dynamic languages [...]
without altering the original source
code.
The following statement from the same entry confused me:
In Ruby, the term monkey patch was
misunderstood to mean any dynamic
modification to a class and is often
used as a synonym for dynamically
modifying any class at runtime.
I would like to know the exact meaning of monkey patching in Ruby. Is it doing something like the following, or is it something else?
class String
def foo
"foo"
end
end
The best explanation I heard for Monkey patching/Duck-punching is by Patrick Ewing in RailsConf 2007
...if it walks like a duck and talks like a duck, it’s a duck, right? So
if this duck is not giving you the noise that you want, you’ve got to
just punch that duck until it returns what you expect.
The short answer is that there is no "exact" meaning, because it's a novel term, and different folks use it differently. That much at least can be discerned from the Wikipedia article. There are some who insist that it only applies to "runtime" code (built-in classes, I suppose) while some would use it to refer to the run-time modification of any class.
Personally, I prefer the more inclusive definition. After all, if we were to use the term for modification of built-in classes only, how would we refer to the run-time modification of all the other classes? The important thing to me is that there's a difference between the source code and the actual running class.
In Ruby, the term monkey patch was
misunderstood to mean any dynamic
modification to a class and is often
used as a synonym for dynamically
modifying any class at runtime.
The above statement asserts that the Ruby usage is incorrect - but terms evolve, and that's not always a bad thing.
Monkey patching is when you replace methods of a class at runtime (not adding new methods as others have described).
In addition to being a very un-obvious and difficult to debug way to change code, it doesn't scale; as more and more modules start monkey patching methods, the likelihood of the changes stomping each other grow.
You are correct; it's when you modify or extend an existing class rather than subclass it.
This is monkey patching:
class Float
def self.times(&block)
self.to_i.times { |i| yield(i) }
remainder = self - self.to_i
yield(remainder) if remainder > 0.0
end
end
Now I imagine this might be useful sometimes, but imagine if you saw routine.
def my_method(my_special_number)
sum = 0
my_special_number.times { |num| sum << some_val ** num }
sum
end
And it breaks only occasionally when it gets called. To those paying attention you already know why, but imagine that you didn't know about the float type having a .times class-method and you automatically assumed that my_special_number is an integer. Every time the parameter is a whole number, integer or float, it would work fine (whole ints are passed back except when there is a floating-point remainder). But pass a number with anything in the decimal area in and it'll break for sure!
Just imagine how often this might happen with your gems, Rails plugins, and even by your own co-workers in your projects. If there's one or two little methods in there like this and it could take some time to find and correct.
If you wonder why it breaks, note that sum is an integer and a floating-point remainder could be passed back; in addition, the exponential sign only works when types are the same. So you might think it's fixed, because you converted bother numbers to floats ... only to find that the sum can't take the floating-point result.
In Python monkeypatching is referred to a lot as a sign of embarrassment: "I had to monkeypatch this class because..." (I encountered it first when dealing with Zope, which the article mentions). It's used to say that it was necessary to take hold of an upstream class and fix it at runtime instead of lobbying to have the unwanted behaviors fixed in the actual class or fixing them in a subclass. In my experience Ruby people don't talk about monkeypatching that much, because it's not considered especially bad or even noteworthy (hence "duck punching"). Obviously you have to be careful about changing the return values of a method that will be used in other dependencies, but adding methods to a class the way that active_support and facets do is perfectly safe.
Update 10 years later: I would amend the last sentence to say "is relatively safe". Extending a core library class with new methods can lead to problems if somebody else gets the same idea and adds the same method with a different implementation or method signature, or if people confuse extended methods for core language functionality. Both cases often happen in Ruby (especially regarding active_support methods).
Explanation of the concept without code:
It means you can "dynamically" modify code. Wanna add a method "dynamically" to a particular class known only at "runtime"? No problem. It's powerful, yes: but can be misused. The concept "dynamically" might be a little too esoteric to understand, so I have prepared an example below (no code, I promise):
How to monkey patch a car:
Normal Car Operations
How do you normally start a car? It’s simple: you turn the ignition, the car starts!
Great, but how can we "monkey patch" the car class?
This is what Fabrizzio did to poor Michael Corleone. Normally, if you want to change how a car operates, you would have to make those changes in the car manufacturing plant (i.e. at "compile" time, within the Car class ^**). Fabrizzio ain't got no time for that: he monkey patches cars by getting under the bonnet to surreptitiously and sneakily rewire things. In other words, he re-opens the Car class, makes the changes he wants, and he's done: he's just monkey patched a car. he done this "dynamically".
You have to really know what you are doing when you monkey patch otherwise the results could be quite explosive.
“Fabrizzio, where are you going?”
Boom!
Like Confucius Say:
"Keep your source code close, but your monkey patches closer."
It can be dangerous.
^** yes i know, dynamic languages.
Usually it is meant about ad-hoc changes, using Ruby open classes, frequently with low quality code.
Here's a good follow-up on the subject.