Method Shelters : Another Way to Resolve Class Extension Conflicts

Method Shelters :
Another Way to Resolve Class
Extension Conflicts
Classboxes でも Refinements でもない別のやり方

Shumpei Akai / 赤井駿平
@flexfrank

What I talk about today
Open Class causes conflicts
of methods
Method Shelters resolve it!

% whoami
 Shumpei Akai / 赤井駿平
 @flexfrank
 Ph.D. Student / 学生(博士課程)
◦ 東工大の千葉研
◦ Programming Languages and Moduralization
◦ プログラミング言語とモジュール化

 Today’s topic is my current research

Open Class
 Ruby’s one of the important features
 You can (re)define methods in existing
classes
◦ including Object, Integer, Array, …

 Frequently used in Ruby world

in open-uri
 open-uri redefines “open” method
◦ It accepts URI
open("http://penguindrum.jp/"){|f|f.read}
# => Errno::ENOENT: No such file or directory

require "open-uri”
open("http://penguindrum.jp/"){|f|f.read}
# => "<!DOCTYPE html PUBLIC "-//W3C//DTD
XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-
transitional.dtd”…”

in Ruby on Rails
 ActiveSupport adds various convenient
methods to core classes
◦ e.g.

10.megabytes # => 10485760
1.day.ago # => Sat Jul 16 16:15:00 +0900 2011
"survival strategy".pluralize #=> "survival strategies"

Problem of Open Class
 Methods may conflict
 When One library adds a method
◦ and another library adds a method with the
same name in the same class
 The method defined first is vanished!
◦ depends on the order of “require”
◦ Confusing!

Method conflicts
 I encountered about five years ago
◦ Ruby on Rails and flvtool added a method to
String
◦ they used String as binary/array of int
 Difficult to collaborate these two libraries
◦ I separated processes
◦ communicating via dRuby

Another example of conflicts
 “mathn” redefines the “Integer#/”
◦ returns a Rational object
◦ Ordinary code expects “/” returns a integer
 Programs get broken

Existing Solutions
 Several module systems are proposed to
resolve conflicts
◦ Selector namespaces (for Smalltalk)
◦ Classboxes (for Smalltalk and Java)
◦ Refinements (for Ruby)
◦ …

Refinements
 Proposed by Shugo Maeda [ruby-core:33322]

class Foo
module MathN using MathN
refine Fixnum do def foo
def /(other) p1/2
quo(other) end
end end
end
end f = Foo.new
f.foo #=> (1/2)
p1/2

Refinements (cont.)
 Refinements changes behavior of
methods in a lexical scope
◦ methods defined by Refinements are not
enabled in indirectly called methods
◦ No local rebinding

 I need local rebinding
◦ e.g. scoped monkey patching

Classboxes
 You might have known via matz’s diary

Classboxes
 A classbox restrict the scope of methods
 A classbox can import a class in another
classbox
◦ You can use an imported class
◦ You can add/redefine methods to the
imported class
◦ A redefined method can be called from the
imported class
 Local rebinding property

Classboxes

Cited from
“Classbox/J: Controlling the Scope of Change in Java”

The Problem in Classboxes
 Importing overwrites internally used
classes
◦ Importing causes another conflict

The Problem in
Classboxes

Redefines Original
Integer#div Integer

Uses redefined Use original
Integer#div Integer
returns rational

Oops!
returns integer

I need another module system
 A new module should:
◦ have local rebinding
◦ provide a way to resolve conflicts cause by
importing
◦ not depends on the order of load

Key concept

Hide your methods
Hide your imports

What is a method shelter
 A method shelter is a module which
provides a scope of methods
◦ define methods in a method shelter
◦ import other method shelters
 You can call methods in the imported shelter
 You can call methods in the shelter which is
importing the current shelter
 for local rebinding
call importee

call
importer

A Code with Method Shelters
shelter :MathN do
class Fixnum # fixed size integer in Ruby
def /(x)
Rational(self,x)
end
end
end
shelter :Average do
class Array
def avg
sum = self.inject(0){|r,i|r+i}
sum / self.size
end
end
hide
import :MathN
end

What conforms a method shelter
 A method shelter is separated into tow
parts
◦ An exposed chamber and a hidden chamber
◦ in order to protect internally used methods
◦ Each chamber can define methods and import

- Exposed - Hidden

Exposed Chambers S0
- Obj#m0
 for public API
 Exposed methods
◦ Visible from importer
◦ Importer can call or
redefine exposed S1
methods
 Exposedly import
◦ Transitive importing
◦ Imported methods are
S2
also visible from importer

Hidden chamber S0
- Obj#m1 - Obj#m0
 for internally used
methods
 Hidden method S1
◦ invisible in importing
shelter
 Hiddenly import
S2
◦ Imported methods are not
exposed

Global Methods
 Ordinal methods not in shelters
◦ Callable from any shelter
◦ Global methods can call methods in a shelter
if the caller is in the shelter
Global

- Obj#g0

S0
obj.g0()

No Ambiguity
 If 2+ methods are found in imported
shelters S3
- C#m0
◦ Error!

S1 S2
- C#m0

S0
Error!

Syntax
 I don’t want to edit parse.y
 define methods in a block
shelter :ShelterName do
class Foo
def hoge # <- defined in the method shelter
end
end
end

Syntax: Import
import :AnotherShelterName
end

Syntax: hide
 “hide” method switches a chamber
◦ do def or import below “hide”

# exposed chamber
hide
# hidden chamber
end

Evaluate
 Evaluate within a shelter

shelter_eval :ShelterName do
#shelter is enabled
end

Method Lookup Algorithm
 1. look up hidden-chamber
 2. look up exposed-chamber
 3. look up global methods

 If not found, go to superclass

8 3

7 6 2

5 1
start

Global 9

4

Found!
1

Start

Global

Implementation
 Based on Ruby 1.9.2
 Add one implicit argument to method:
◦ A node of method shelter tree

Optimization: Method Cache
 Shelter node cache
◦ Caches method entry in a node of shelter
 Extend inline cache
◦ Size of an inline cache : 3 word -> 4word
(per method call)
◦ Stores the found shelter node

Performance: empty methods
 Call a empty method 10,000,000 times
◦ Less than 5% overhead when shelters are
used

Performance: Fibonacci
 fib(33) in a method shelter
◦ Up to 20% overhead

Performance: Ruby on Rails
 Enabled in an action method
◦ Numeric#/.*bytes?/ methods are in a shelter
 In the action
◦ 1. Call one method in shelter
◦ 2. One access to SQLite via ActiveRecord
 on WEBRick
 Rails3

Performance:
Ruby on Rails (result)
 4% overhead on production env.
 50% on development
◦ Method caches are invalidated per req.
production

development

Cache hit ratio on rails
 Count shelter’s cache hit

Performance: tDiary 3.0.1
 defined
String#to_a, String#each, String#method_
missing in a shelter
◦ These are used for compatibility of 1.8 & 1.9
 Ran on CGI with apache
 Method shelter improved performance !!
◦ Why?

Why shelter made tDiary fast
 String#method_missing issue
 “require” calls its arg’s to_path method if
defined
◦ If arg’s method_missing is defined, try to call
it
◦ String#method_missing slows “require”
 Method shelter restrict its negative effect

Other Usage:
protect optimized methods
 In ruby, + - * / … are optimized
◦ only if they are not redefined
◦ Redefinition slows programs
 Method shelters can confine effect of
redefinition
 Method shelter can improve performance

Other Usage:
shelter-private accessor
 Ruby has no private instance variables
 A method shelter can mimic private ivars
◦ by generating unique name
◦ Accessible within the defined shelter
and visible shelters

class Module
def shelter_accessor(name)
define_method name do
ivname=get_unique_name(name)
self.instance_variable_get(ivname)
end
define_method( (name.to_s+"=").to_sym) do|val|
ivname= get_unique_name(name)
self.instance_variable_set(ivname,val)
end
end
end

Conclusion
 Open class is dangerous
 Method shelters resolving conflicts
◦ With hidden methods, hiddenly importing
 I implemented in Ruby
◦ Not so slow (個人的な感覚)

 For more details or the source code,
◦ wait for the acceptance of my paper
 Deadline: 2.days.since

 In my lookup algorithm,
◦ Shelter must have up to one parent

 For simpler semantics
 For efficient implementation

after
C’ C’’

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Method Shelters : Another Way to Resolve Class Extension Conflicts

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