CSC/ECE 517 Fall 2012/ch1 1w22 an: Difference between revisions

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'''method_missing in Ruby:''' 
=='''Introduction<ref>http://rubylearning.com/satishtalim/ruby_method_missing.html</ref>'''==
=='''Introduction<ref>http://rubylearning.com/satishtalim/ruby_method_missing.html</ref>'''==
A method of a class is called by creating an object of the class and passing the method name to the object as a message. The object then looks up into its [http://www.madebydna.com/all/code/2011/06/24/eigenclasses-demystified.html method lookup path] and tries to match the called method with the defined methods in the class. On success, the method is executed along with the parameters passed and the result is returned.
A method that belongs to a class is called by creating an object of the class and passing the method name to the object as a message. The object then looks up the [http://www.madebydna.com/all/code/2011/06/24/eigenclasses-demystified.html method lookup path] and tries to match the called method with the defined methods in the class. On success, the method is executed and the result is returned.


If the object does not find a match in its method lookup, in normal circumstances the [http://www.ruby-doc.org/core-1.9.3/NoMethodError.html NoMethodError Exception] is raised .
If the object does not find a match in its method lookup, in normal circumstances the [http://www.ruby-doc.org/core-1.9.3/NoMethodError.html NoMethodError Exception] is raised .


“method_missing” is defined as a method in the class and is invoked if a called method is not found.
In cases where the user wants to handle the methods which are not defined but are still called, “method_missing” can be defined and the user can handle the methods as he/she sees fit.


=='''Format for defining method_missing'''==
=='''Format for Defining method_missing'''==


  => def method_missing(m,*args,&block)
  => def method_missing(m,*args,&block)
Line 15: Line 12:
(i) '''m->''' accepts the symbol/name of the undefined method (ii) '''*args->''' accepts the array of arguments passed in the method call  (iii) '''&block->'''accepts a block passed to the method
(i) '''m->''' accepts the symbol/name of the undefined method (ii) '''*args->''' accepts the array of arguments passed in the method call  (iii) '''&block->'''accepts a block passed to the method


=='''Method Lookup Path'''==
=='''Ruby Method Lookup Flow'''==
The object of a class receives a method name to be executed, the following steps are carried out for matching and executing the method:
When the object of a class receives a method name to be executed, the following steps are carried out for matching and executing the method:


*First, the object looks in its own [http://docs.oracle.com/javase/tutorial/java/javaOO/classvars.html instance methods].
*First, the object looks in its own [http://docs.oracle.com/javase/tutorial/java/javaOO/classvars.html instance methods].
Line 26: Line 23:
*Finally, it calls method_missing (if defined in the class), else throws up the NOMethodError exception.
*Finally, it calls method_missing (if defined in the class), else throws up the NOMethodError exception.


This entire tracing that the object does is the method lookup path.
This entire tracing that the object does is called the method lookup path.


=='''Examples'''==
=='''Examples'''==
===Calling defined and undefined methods===
===Calling Defined and Undefined Methods===


  class A // creating a class 'A'
  class A // creating a class 'A'
Line 52: Line 49:
  NoMethodError: undefined method `sayhi' for #<A:0x2a082e0>  // the NoMethodError is raised
  NoMethodError: undefined method `sayhi' for #<A:0x2a082e0>  // the NoMethodError is raised


===method_missing implementation<ref>http://www.thirdbit.net/articles/2007/08/01/10-things-you-should-know-about-method_missing/</ref>===
===method_missing Implementation<ref>http://www.thirdbit.net/articles/2007/08/01/10-things-you-should-know-about-method_missing/</ref>===


  class A
  class A
Line 69: Line 66:
  => This method does not exist // this result returned when method_missing is executed
  => This method does not exist // this result returned when method_missing is executed


When the object 'a' traces its method lookup path for a matching method as 'sayhi', upon failure it resorts to method_missing and the body of method_missing is executed. 'method_missing' is the last resort.
When the object 'a' traces its method lookup path for a matching method 'sayhi', upon failure it resorts to method_missing and the body of method_missing is executed.




Sometimes when a class has many methods that do generally the same kinds of things, and the programmer is not sure in advance which methods the user will call since there are so many of them, and they are all so similar, implementing all of them by hand seems futile. In these situations method_missing makes a new method that was previously not defined and adds it to the class. The below 'Generic Handler' example implements this.
Sometimes when a class has many methods that do generally the same kinds of functionality, and the programmer is not sure in advance which methods the user will call since there are so many of them, and all of them are similar, writing code for all of the methods seems futile. In these situations method_missing can be defined to take care of these cases. The below 'Generic Handler' example implements this.


===passing parameters to an undefined method call===
===Passing Parameters to an Undefined Method Call===


  class A
  class A
Line 101: Line 98:
The user made a genuine mistake by typing 'adds', but this method is not defined. When the 'adds' method with parameters is called, the object 'a' tries to match the method in the method lookup path. Upon failure it invokes method_missing, the args are passed, stored in the array 'args' and the body of method_missing is executed.
The user made a genuine mistake by typing 'adds', but this method is not defined. When the 'adds' method with parameters is called, the object 'a' tries to match the method in the method lookup path. Upon failure it invokes method_missing, the args are passed, stored in the array 'args' and the body of method_missing is executed.


===converting numbers from roman representation to integer representation<ref>http://www.rubyquiz.com/quiz22.html</ref>===
===Converting Numbers from Roman Representation to Integer Representation<ref>http://courses.ncsu.edu/csc517/common/lectures/notes/wk2.pdf</ref>===
  class Roman
  class Roman
  @@Roman_to_Numeric = {'i' => 1, 'v' => 5, 'x' => 10, 'l' => 50, 'c' => 100, 'd' => 500, 'm' => 1000}  
  DIGITS = {'I'=>1,'V'=>5,'X'=>10,'L'=>50,'C'=>100,'D'=>500,'M'=>1000,}
  def method_missing(method_var,*args,&block)
  def roman_to_integer(roman_string)
  numeric_value = 0
  last = nil
  roman_string = method_var.to_s.downcase
  roman_string.to_s.upcase.split(//).reverse.inject(0) do
for i in 0...roman_string.length-1
|memo, digit|
  if (cond) == nil
  if digit_value = DIGITS[digit]
if last && last > digit_value
memo -= digit_value
  else
  else
  puts "Roman string is invalid"
  memo += digit_value
return
  end
  end
last = digit_value
  end
  end
  while roman_string != ""
  memo
if roman_string[roman_string.length - 1] == 'x' && roman_string[roman_string.length - 2] == 'i'
numeric_value += 9
roman_string.chop!
roman_string.chop!
elsif
roman_string[roman_string.length - 1] == 'v' && roman_string[roman_string.length - 2] == 'i'
numeric_value += 4
roman_string.chop!
roman_string.chop!
else
numeric_value += @@Roman_to_Numeric[roman_string[(roman_string.length)-1]]
roman_string.chop!
  end
  end
  end
  end
  puts "Numeric Value for #{method_var} is: #{numeric_value}"
 
  def method_missing(method)
str=method.id2name
roman_to_integer(str)
  end
  end
  end
  end
cond = @@Roman_to_Numeric[roman_string[i]]-@@Roman_to_Numeric[roman_string[i+1]] == 0 || @@Roman_to_Numeric[roman_string[i]]-@@Roman _ to_                              Numeric [ roman _ string [ i + 1 ] ]  ==  - 9 || @@Roman_to_Numeric[roman_string[i]]-@@Roman_to_Numeric[roman_string[i+1]] == -4    ||    @@Roman_to_Numeric[roman_string[i]]-@@Roman_to_Numeric[roman_string[i+1]] >= 4) && (/.v.x/ =~ roman_string




Line 139: Line 128:
  r= Roman.new
  r= Roman.new
  r.vii
  r.vii
r.iClx
  r.xxix
  r.xxix
r.xxxi
  r.xxiv
  r.xxiv
  r.xxvi
  r.xxvi
  r.vx
   


The Output
The Output


  Numeric Value for vii is: 7
  => 7
  Roman string is invalid
  => 29
Numeric Value for xxix is: 29
  => 24
  Numeric Value for xxxi is: 31
Numeric Value for xxiv is: 24
Numeric Value for xxvi is: 26
Roman string is invalid


===method_missing to log method calls<ref>http://expertiza.csc.ncsu.edu/wiki/index.php/CSC/ECE_517_Fall_2007/wiki1b_2_22</ref>===
===method_missing to Log Method Calls<ref>http://expertiza.csc.ncsu.edu/wiki/index.php/CSC/ECE_517_Fall_2007/wiki1b_2_22</ref>===
Another application that makes use of method_missing could be a simple logger used for debugging purposes. Many times, it may be required to log the trace of called methods and provide information such as: called method-name, arguments, return type. It can be tedious to repeat this part of code in every method. A simple solution to this problem can be obtained using method_missing as:
Another application that makes use of method_missing could be a simple logger used for debugging purposes. Many times, it may be required to log the trace of called methods and provide information such as: called method-name, arguments, return type. It can be tedious to repeat this part of code in every method. A simple solution to this problem can be obtained using method_missing as:


Line 173: Line 156:
This program makes use of method_missing in a way that it wraps around called method to output the logging information on entry and on exit, it logs the return type. Further, method_missing intercepts the method call and forward it to internal object with ‘send’ method of ruby. Hence, this use of method_missing acts as wrapper.
This program makes use of method_missing in a way that it wraps around called method to output the logging information on entry and on exit, it logs the return type. Further, method_missing intercepts the method call and forward it to internal object with ‘send’ method of ruby. Hence, this use of method_missing acts as wrapper.


===Generic Handler<ref>http://expertiza.csc.ncsu.edu/wiki/index.php/CSC/ECE_517_Fall_2007/wiki1b_2_22</ref>===
===Generic Handler===
  class NoBar
  class NoBar
  def method_missing(methodname, *args)
  def method_missing(methodname, *args)
Line 186: Line 169:


*In addition to specifying the error messages for the undefined methods, method_missing provides a more dynamic behavior in the programming environment.
*In addition to specifying the error messages for the undefined methods, method_missing provides a more dynamic behavior in the programming environment.
*If we are unfamiliar with the usage of the object we created, then using method_missing is a good implementation.
*If we are unfamiliar with the usage of the objects we created, then using method_missing is a good technique.
*Handles problems at runtime.
*Handles problems at runtime.
*Define's a generic method_missing and handle's any undefined method, a big advantage over Java. In Java, when you call an undefined method, the program will not compile.
*Define's a generic method_missing and handle's any undefined method, a big advantage over Java. In Java, when you call an undefined method, the program will not compile.
*method_missing falls under the general technique of meta-programming. Employ meta-programming in missing_function to write an another function to handle the call.
*method_missing falls under the general technique of [http://en.wikipedia.org/wiki/Metaprogramming meta-programming]. Employ meta-programming in missing_function to write an another function to handle the call.


=='''Disadvantages of method_missing'''==
=='''Disadvantages of method_missing'''==
Line 196: Line 179:
*Since the methods being called never actually exist—they are just intercepted at the last step of the method lookup process—they cannot be documented or introspected as conventional methods can.
*Since the methods being called never actually exist—they are just intercepted at the last step of the method lookup process—they cannot be documented or introspected as conventional methods can.


*Dynamic methods must go through the method_missing method, the body of that method can become quite large if there are many different aspects of the code that needs to add methods dynamically.
*method_missing restricts compatibility with future versions of an [http://en.wikipedia.org/wiki/Application_programming_interface API]. Introducing new methods in a future API version can break users' expectations.
 
=='''Key Points'''==
 
*In the following example, if within method_missing() we define an undefined method, we get a stack level too deep error message.
 
class A
@@i = 0
def method_missing(method_id)
puts "In Method Missing #{@@i}"
@@i += 1
self.fun
end
end
 
a = A.new
a.foo
 
Output


*method_missing restricts compatibility with future versions of an API. Introducing new methods in a future API version can break users' expectations.
The result is a 'stack level too deep' error.  


=='''Key Points'''==
When the 'foo' method is called, after no method match the method_missing is run and this block has a method 'self.fun' that is undefined.     Here when the program execution encounters 'self.fun' it once again calls method_missing. This goes on in an endless loop till the stack memory becomes full.
*Ruby knows method_missing( ) exisits, because it's a private instance method of 'BasicObject' that every object inherits. The BasicObject#method_missing( ) responds by raising the NoMethodError. Overriding this method_missing( ) allows you to call methods that don't really exist.  


*If method_missing is only looking for certain method names, don't forget to call super if you haven't found what you're looking for, so that the other superclass' method_missing can handle it.
*Ruby knows method_missing( ) exists, because it's a private instance method of 'BasicObject' that every object inherits. The BasicObject#method_missing( ) responds by raising the NoMethodError. Overriding this method_missing( ) allows you to call methods that don't really exist.  


*obj.respond_to? function returns 'true' if the obj responds to the given method. So if you want to know whether your class will respond to a function you can use respond_to? to know the answer. But if method_missing() is used, the output may not be what you expect.
*If method_missing is only looking for certain method names, don't forget to call the [http://en.wikibooks.org/wiki/Java_Programming/Keywords/super super] keyword if you haven't found what you're looking for, so that the other superclass' method_missing can handle it.


*Below class A defines only method method_missing() and no other method. When a new object of class A tries to access a method say 'foo', the respond_to? will return the value 'false'. When you actually try to implement it using “a.foo”, the code will get executed, courtesy of method_missing(). Even though the respond_to? says that you cannot access 'foo' method using object of class A, if you have method_missing() defined, you can access the method.
*[http://www.prateekdayal.net/2007/10/16/rubys-responds_to-for-checking-if-a-method-exists/ obj.respond_to?] function returns 'true' if the obj responds to the given method. So if you want to know whether your class will respond to a function you can use respond_to? to know the answer. But if method_missing() is used, the output may not be what you expect.


class A
class A
  def method_missing(method_id)
  def method_missing(method_id)
  puts "In method_missing"
  puts "In method_missing"
Line 223: Line 224:
  In method_missing
  In method_missing


*If within method_missing() we define an undefined method.  
=='''Similar functionality in other languages'''==
Method missing, one of the dynamic features of Ruby, is not a feature that is unique to Ruby. It exists in Smalltalk, Python, Groovy, some Javascripts and most CLOS (Common Lisp Object System)extensions. In this section we look at the few such similar implementations in other languages. The table below gives different ways the functionality related to method_missing is handled in other languages.<ref>http://olabini.com/blog/2010/04/patterns-of-method-missing/</ref>
 
 
 
'''Construct'''                          '''Language '''
AUTOLOAD                          Perl
AUTOSCALAR, AUTOMETH, AUTOLOAD...  Perl6
__getattr__                        Python
method_missing                    Ruby
doesNotUnderstand                  Smalltalk
__noSuchMethod__(1)                CoffeeScript, JavaScript
unknown                            Tcl
no-applicable-method              Common Lisp
doesNotRecognizeSelector          Objective-C
TryInvokeMember(2)                C#
match [name, args] { ... }        E
the predicate fail                Prolog
forward                            Io
 
(1) supported by Firefox
 
(2) only for dynamic objects
 
Table Reference<ref>http://stackoverflow.com/questions/2865865/are-there-equivalents-to-rubys-method-missing-in-other-languages</ref>
 
*'''_getattr_ implementation in Python'''<ref>http://docs.python.org/reference/datamodel.html?highlight=__getattr__#object.__getattr__</ref>
 
object.__getattr__(self,name)
 
This method is called when an attribute lookup has not found the attribute in the usual places i.e. it is not an instance attribute nor is it found in the class tree for self. name is the attribute name. This method should return the (computed) attribute value or raise an AttributeError exception.
 
class Roman(object):
  def roman_to_int(self, roman):
    # implementation here
  end
 
  def __getattr__(self, name):
    return self.roman_to_int(name)
  end
 
>>> r = Roman()
 
>>> r.iv
 
4
 
 
*'''doesNotUnderstand in Smalltalk language'''<ref>http://c2.com/cgi/wiki?DoesNotUnderstand</ref>
 
In SmalltalkLanguage When a receiver is asked to perform a method that is unknown to it, then a run-time complaint is issued which is #doesNotUnderstand.
When a Smalltalk object is sent a message for a method it has not defined, the runtime system turns the message-send into an object and sends #doesNotUnderstand: to the original receiver with this message-send object as argument. By default the #doesNotUnderstand: method raises an exception, but the receiver can override it and implement it in a way that he sees fit.
 
*'''Other languages'''
JavaScript also has a method which has an implementation similar to that of method_missing and that is "noSuchMethod". The limitation of this method is that it is only supported by Firefox/Spidermonkey.
 
Similarly, Perl has an AUTOLOAD method which works on subroutines & class/object methods.
 
=='''Patterns of method missing<ref>http://olabini.com/blog/2010/04/patterns-of-method-missing/</ref>'''==
Now that we have covered all the important areas about method missing including the advantages, the disadvantages and key points related to its functionality, it would be appropriate to know about the different ways method_missing is used and what are the consequences of its use.
 
* Providing Debug information on Failure
Well as method missing is called when there is no object to handle the method being called, we can use method_missing to include more information about the reasons for it being called, i.e. to say that we can provide users with more information about the error messages and hence make the life of the programmers easy and provide a faster way to solve the bugs.
 
* Encode parameters in method name
Instead of sending method as explicit parameters, another method is to use the name to encode parameters. Find below a Rails-style find expression:
Person.find_by_name_and_age("ABC",30)
Another way of writing the same:
Person.find_by(:name => "ABC", :age => 30)
 
The disadvantage of this is that creating such kind of API's make it difficult to debug and maintain the application.
 
* Builders
The idea of a builder is that you use Ruby’s blocks and method_missing to make it easy to create any kind of output structure. You create a builder object and then send it messages and it responds to the messages by building up a data structure based on those messages. For example, the following code
 
  builder = Builder::XmlMarkup.new("", 2)
  puts builder.person {
    name("ABC")
    phone("12345", "local"=>"yes")
    address("Raleigh")
  }
 
will print


class A
  <person>
@@i = 0
    <name>ABC</name>
def method_missing(method_id)
    <phone local="yes">12345</phone>
puts "In Method Missing #{@@i}"
    <address>Raleigh</address>
@@i += 1
  </person>
self.fun
end
end


a = A.new
Here we have defined a method_missing method and it handles any undefined method and adds the name of the method to the XML markup that is being built. Code blocks are used to capture the nested nature of the XML. The result is a very natural way to programmatically generate XML markup. Also the cumbersome task of closing the tags and escaping rules are taken care of for us.
puts a.respond_to?(:foo) // just checking whether the receiver 'a' responds to the method 'foo'
 
a.foo


Output
* Accessors
The inversion of the builder pattern is to use a parser that goes through the XML document and then allow access to the elements by using method_missing.


The expected result would be a 'stack level too deep' error.  
* Test Helpers
Different kind of test helpers can be created using method_missing. Many of the open source Ruby projects implementations of method missing are found in the tests.


When the 'foo' method is called, after no method match the method_missing is run and this block has a method “self.fun” that is undefined. Here when the program execution encounters 'self.fun' it once again calls method_missing. This goes on in an endless loop till the stack memory is full.
=='''Conclusion'''==
method_missing is a very powerful feature of Ruby and as is the way with powerful things it can help you a great deal if used properly but it can make things a lot harder for you if implemented incorrectly. It is a kind of a feature which should be used sparingly, if not at all. And there are things that needs to be taken into consideration, as mentioned in the Key Points section above, if method_missing is to be implemented.


=='''References'''==
=='''References'''==
Line 250: Line 330:
=='''Further Suggested Reading'''==
=='''Further Suggested Reading'''==
*http://www.rubyinside.com/  
*http://www.rubyinside.com/  
*https://www.youtube.com/watch?v=SAEiCkixrdE&feature=relmfu
*http://blog.jayfields.com/2008/02/ruby-replace-methodmissing-with-dynamic.html  
*http://blog.jayfields.com/2008/02/ruby-replace-methodmissing-with-dynamic.html  
*http://liquiddevelopment.blogspot.com/2006/04/twisting-and-shaping-dsls-using-ruby.html  
*http://liquiddevelopment.blogspot.com/2006/04/twisting-and-shaping-dsls-using-ruby.html  
*http://blog.rubybestpractices.com/posts/gregory/031-issue-2-method-lookup.html
*http://blog.rubybestpractices.com/posts/gregory/031-issue-2-method-lookup.html
*http://phrogz.net/RubyLibs/RubyMethodLookupFlow.pdf
*http://phrogz.net/RubyLibs/RubyMethodLookupFlow.pdf(Diagram)

Latest revision as of 03:59, 20 September 2012

Introduction<ref>http://rubylearning.com/satishtalim/ruby_method_missing.html</ref>

A method that belongs to a class is called by creating an object of the class and passing the method name to the object as a message. The object then looks up the method lookup path and tries to match the called method with the defined methods in the class. On success, the method is executed and the result is returned.

If the object does not find a match in its method lookup, in normal circumstances the NoMethodError Exception is raised .

In cases where the user wants to handle the methods which are not defined but are still called, “method_missing” can be defined and the user can handle the methods as he/she sees fit.

Format for Defining method_missing

=> def method_missing(m,*args,&block)

(i) m-> accepts the symbol/name of the undefined method (ii) *args-> accepts the array of arguments passed in the method call (iii) &block->accepts a block passed to the method

Ruby Method Lookup Flow

When the object of a class receives a method name to be executed, the following steps are carried out for matching and executing the method:

  • First, the object looks in its own instance methods.
  • Second, it looks in the list of instance methods that all objects of that class share.
  • Third, in each of the included modules of that class, in reverse order of inclusion.
  • Fourth, it looks in that class’s superclass.
  • Fifth, in the superclass’s included modules, all the way up until it reaches the class Object.
  • Sixth, if it still can’t find a method, the very last place it looks is in the Kernel module, included in the class Object.
  • Finally, it calls method_missing (if defined in the class), else throws up the NOMethodError exception.

This entire tracing that the object does is called the method lookup path.

Examples

Calling Defined and Undefined Methods

class A		// creating a class 'A'
def say		// defining a method 'say'
puts " say Hi " // body of method say
end
end

Creating the object of the class

 a=A.new	       // object of the class
 => #<A:0x2a082e0>     //object id

Calling the defined method

a.say                  // defined method
=> say Hi	       // returned result

Calling the undefined method

a.sayhi                // undefined method sayhi
NoMethodError: undefined method `sayhi' for #<A:0x2a082e0>   // the NoMethodError is raised

method_missing Implementation<ref>http://www.thirdbit.net/articles/2007/08/01/10-things-you-should-know-about-method_missing/</ref>

class A
def say
puts " say hi "
end
def method_missing(m,*args,&block)	// defining method_missing
puts " This method does not exist"	// body of method_missing
end
end

Calling a method that is not defined

a=A.new
a.sayhi                                // calling the undefined method sayhi with no arguments
=> This method does not exist		// this result returned when method_missing is executed

When the object 'a' traces its method lookup path for a matching method 'sayhi', upon failure it resorts to method_missing and the body of method_missing is executed.


Sometimes when a class has many methods that do generally the same kinds of functionality, and the programmer is not sure in advance which methods the user will call since there are so many of them, and all of them are similar, writing code for all of the methods seems futile. In these situations method_missing can be defined to take care of these cases. The below 'Generic Handler' example implements this.

Passing Parameters to an Undefined Method Call

class A
def add(a,b)
a+b
end
def method_missing(name,*args,&block)    // the method_missing is defined and the *args parameter accepts all the parameters passed during 								                                 
                                            the method call   
puts “You have typed the method name wrong and these were the parameters passed ; #{args[0]}, #{args[1]}”								
end                         			
end

The passed parameters are stored in the array 'args' and can be accessed like a normal array


Calling the defined method

a.add(1,2)		        // calling the defined method add and passing the parameters (1,2)
=> 3                           // result

Calling the undefined method

a.adds(4,2) 			// calling the undefined method adds and passing the parameter (4,2)
=> You have typed the method name wrong and these were the parameters passed; 4, 2

The user made a genuine mistake by typing 'adds', but this method is not defined. When the 'adds' method with parameters is called, the object 'a' tries to match the method in the method lookup path. Upon failure it invokes method_missing, the args are passed, stored in the array 'args' and the body of method_missing is executed.

Converting Numbers from Roman Representation to Integer Representation<ref>http://courses.ncsu.edu/csc517/common/lectures/notes/wk2.pdf</ref>

class Roman
DIGITS = {'I'=>1,'V'=>5,'X'=>10,'L'=>50,'C'=>100,'D'=>500,'M'=>1000,}
def roman_to_integer(roman_string)
last = nil
roman_string.to_s.upcase.split(//).reverse.inject(0) do
|memo, digit|
if digit_value = DIGITS[digit]
if last && last > digit_value
memo -= digit_value
else
memo += digit_value
end
last = digit_value
end
memo
end
end
def method_missing(method)
str=method.id2name
roman_to_integer(str)
end
end


Calling the undefined methods

r= Roman.new
r.vii
r.xxix
r.xxiv
r.xxvi

The Output

=> 7
=> 29
=> 24

method_missing to Log Method Calls<ref>http://expertiza.csc.ncsu.edu/wiki/index.php/CSC/ECE_517_Fall_2007/wiki1b_2_22</ref>

Another application that makes use of method_missing could be a simple logger used for debugging purposes. Many times, it may be required to log the trace of called methods and provide information such as: called method-name, arguments, return type. It can be tedious to repeat this part of code in every method. A simple solution to this problem can be obtained using method_missing as:

 class SimpleCallLogger
 def initialize(o)
 @obj = o
 end
 def method_missing(methodname, *args)
 puts "called: #{methodname}(#{args})"
 a = @obj.send(methodname, *args)
 puts "\t-> returned: #{a}"
 return a
 end
 end

This program makes use of method_missing in a way that it wraps around called method to output the logging information on entry and on exit, it logs the return type. Further, method_missing intercepts the method call and forward it to internal object with ‘send’ method of ruby. Hence, this use of method_missing acts as wrapper.

Generic Handler

class NoBar
def method_missing(methodname, *args)
define_method(:bar) if "bar" == methodname.to_s
define_method(:nobar) if "nobar" == methodname.to_s
end
end

This is an example of using method_missing as a generic handler to handle when a calling method is not exist. You can use missing_method to dynamically create a method at a runtime.

Advantages of method_missing

  • In addition to specifying the error messages for the undefined methods, method_missing provides a more dynamic behavior in the programming environment.
  • If we are unfamiliar with the usage of the objects we created, then using method_missing is a good technique.
  • Handles problems at runtime.
  • Define's a generic method_missing and handle's any undefined method, a big advantage over Java. In Java, when you call an undefined method, the program will not compile.
  • method_missing falls under the general technique of meta-programming. Employ meta-programming in missing_function to write an another function to handle the call.

Disadvantages of method_missing

  • Slower than conventional method lookup. Simple tests indicate that method dispatch with method_missing is at least two to three times as expensive in time as conventional dispatch.
  • Since the methods being called never actually exist—they are just intercepted at the last step of the method lookup process—they cannot be documented or introspected as conventional methods can.
  • method_missing restricts compatibility with future versions of an API. Introducing new methods in a future API version can break users' expectations.

Key Points

  • In the following example, if within method_missing() we define an undefined method, we get a stack level too deep error message.
class A
@@i = 0
def method_missing(method_id)
puts "In Method Missing #{@@i}"
@@i += 1
self.fun
end
end
a = A.new
a.foo 

Output

The result is a 'stack level too deep' error. 

When the 'foo' method is called, after no method match the method_missing is run and this block has a method 'self.fun' that is undefined. Here when the program execution encounters 'self.fun' it once again calls method_missing. This goes on in an endless loop till the stack memory becomes full.

  • Ruby knows method_missing( ) exists, because it's a private instance method of 'BasicObject' that every object inherits. The BasicObject#method_missing( ) responds by raising the NoMethodError. Overriding this method_missing( ) allows you to call methods that don't really exist.
  • If method_missing is only looking for certain method names, don't forget to call the super keyword if you haven't found what you're looking for, so that the other superclass' method_missing can handle it.
  • obj.respond_to? function returns 'true' if the obj responds to the given method. So if you want to know whether your class will respond to a function you can use respond_to? to know the answer. But if method_missing() is used, the output may not be what you expect.

class A

def method_missing(method_id)
puts "In method_missing"
end
end
a = A.new
puts a.respond_to?(:foo)
a.foo

Output

false
In method_missing

Similar functionality in other languages

Method missing, one of the dynamic features of Ruby, is not a feature that is unique to Ruby. It exists in Smalltalk, Python, Groovy, some Javascripts and most CLOS (Common Lisp Object System)extensions. In this section we look at the few such similar implementations in other languages. The table below gives different ways the functionality related to method_missing is handled in other languages.<ref>http://olabini.com/blog/2010/04/patterns-of-method-missing/</ref>


Construct                          Language 
AUTOLOAD                           Perl
AUTOSCALAR, AUTOMETH, AUTOLOAD...  Perl6
__getattr__                        Python
method_missing                     Ruby
doesNotUnderstand                  Smalltalk
__noSuchMethod__(1)                CoffeeScript, JavaScript
unknown                            Tcl
no-applicable-method               Common Lisp
doesNotRecognizeSelector           Objective-C
TryInvokeMember(2)                 C#
match [name, args] { ... }         E
the predicate fail                 Prolog
forward                            Io

(1) supported by Firefox

(2) only for dynamic objects

Table Reference<ref>http://stackoverflow.com/questions/2865865/are-there-equivalents-to-rubys-method-missing-in-other-languages</ref>

object.__getattr__(self,name)

This method is called when an attribute lookup has not found the attribute in the usual places i.e. it is not an instance attribute nor is it found in the class tree for self. name is the attribute name. This method should return the (computed) attribute value or raise an AttributeError exception.

class Roman(object):

 def roman_to_int(self, roman):
   # implementation here
 end
 def __getattr__(self, name):
   return self.roman_to_int(name)
 end

>>> r = Roman()

>>> r.iv

4


In SmalltalkLanguage When a receiver is asked to perform a method that is unknown to it, then a run-time complaint is issued which is #doesNotUnderstand. When a Smalltalk object is sent a message for a method it has not defined, the runtime system turns the message-send into an object and sends #doesNotUnderstand: to the original receiver with this message-send object as argument. By default the #doesNotUnderstand: method raises an exception, but the receiver can override it and implement it in a way that he sees fit.

  • Other languages

JavaScript also has a method which has an implementation similar to that of method_missing and that is "noSuchMethod". The limitation of this method is that it is only supported by Firefox/Spidermonkey.

Similarly, Perl has an AUTOLOAD method which works on subroutines & class/object methods.

Patterns of method missing<ref>http://olabini.com/blog/2010/04/patterns-of-method-missing/</ref>

Now that we have covered all the important areas about method missing including the advantages, the disadvantages and key points related to its functionality, it would be appropriate to know about the different ways method_missing is used and what are the consequences of its use.

  • Providing Debug information on Failure

Well as method missing is called when there is no object to handle the method being called, we can use method_missing to include more information about the reasons for it being called, i.e. to say that we can provide users with more information about the error messages and hence make the life of the programmers easy and provide a faster way to solve the bugs.

  • Encode parameters in method name

Instead of sending method as explicit parameters, another method is to use the name to encode parameters. Find below a Rails-style find expression: Person.find_by_name_and_age("ABC",30) Another way of writing the same: Person.find_by(:name => "ABC", :age => 30)

The disadvantage of this is that creating such kind of API's make it difficult to debug and maintain the application.

  • Builders

The idea of a builder is that you use Ruby’s blocks and method_missing to make it easy to create any kind of output structure. You create a builder object and then send it messages and it responds to the messages by building up a data structure based on those messages. For example, the following code

 builder = Builder::XmlMarkup.new("", 2)
 puts builder.person {
   name("ABC")
   phone("12345", "local"=>"yes")
   address("Raleigh")
 }

will print

 <person>
   <name>ABC</name>
   <phone local="yes">12345</phone>
   <address>Raleigh</address>
 </person>

Here we have defined a method_missing method and it handles any undefined method and adds the name of the method to the XML markup that is being built. Code blocks are used to capture the nested nature of the XML. The result is a very natural way to programmatically generate XML markup. Also the cumbersome task of closing the tags and escaping rules are taken care of for us.

  • Accessors

The inversion of the builder pattern is to use a parser that goes through the XML document and then allow access to the elements by using method_missing.

  • Test Helpers

Different kind of test helpers can be created using method_missing. Many of the open source Ruby projects implementations of method missing are found in the tests.

Conclusion

method_missing is a very powerful feature of Ruby and as is the way with powerful things it can help you a great deal if used properly but it can make things a lot harder for you if implemented incorrectly. It is a kind of a feature which should be used sparingly, if not at all. And there are things that needs to be taken into consideration, as mentioned in the Key Points section above, if method_missing is to be implemented.

References

<references/>

Further Suggested Reading