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Method Shelters: Avoiding Conflicts among Class Extensions Caused by Local Rebinding

Shumpei Akai , Shigeru ChibaTokyo Institute of Technology

Class ExtensionsDestructively change method definitions

in the existing classes◦Write new definitions in a separate file

Available in ◦Smalltalk, Objective-C, AspectJ …◦and Ruby

(called Open Class)

String

JSON lib

+ to_ json

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Class Extensions are popular in Ruby

Ruby on Rails aggressively adds methods to built-in classes◦Class extensions are used in real applications◦e.g.

10.kilobytes # => NoMethodError: undefined method

require “active_record” #load rails’ lib10.kilobytes # => 10240

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Conflicts among Class Extensions

Class extensions are dangerous◦Cause conflicts, of course

Ruby allows class extensions◦Ruby on Rails aggressively use◦Serious issue in Ruby◦Scope of class extensions are needed

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What is the appropriate scope of class extensions?Global?

◦Same as RubyLexical?

◦Redefinitions are available in a lexical scopeLocal rebinding property (of Classbox *)?

◦Redefine methods in another module by importing

◦Redefinition is limited in imported module

5* [‘05 Bergel et al.]

Global Scopes

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List

avg(): average of elems using div

Integerdiv(): return rational

Integerdiv(): return integerplus(): …minus(): …

1.div(2) #=> (1/2)

Redefine destructively

Module 1

Module 2

Module 3

No one can use original

div()

Lexical Scopes

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List

avg(): average of elems using div

Integerdiv(): return rational

Integerdiv(): return integerplus(): …minus(): …

Redefinition in Lexical scope

Module 1

Module 2

Module 3

Cannot reuse redefined div()

[1, 2].avg() #=> (3/2)1.div(2) #=> 0[1, 2].avg() #=> (3/2)1.div(2) #=> 0

Local rebinding(Classbox)

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List

avg(): average of elems using div

Integerdiv(): return rational

Integerdiv(): return integerplus(): …minus(): …

1.div(2) #=> 0

Module 1

Module 2

Module 3

import

import

[1, 2].avg() #=> (3/2)1.div(2) #=> (1/2)

Module 4

Redefinition in importing chain

Original div()Redefined div()

Local rebinding(Classbox)

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List

avg(): average of elems using div

Integerdiv(): return rational

Integerdiv(): return integerplus(): …minus(): …

[1, 2].avg() #=> (3/2)1.div(2) #=> Conflicts

Module 1

Module 2

Module 31.div(2) #=> (1/2)[1, 2].avg() #=> (3/2)

Module 4

Our proposal: Method SheltersA method shelter is a module which

provides a way to control scopes of class extensions◦2 mechanisms

Preserves local rebinding Or redefine methods in limited scope

Based on Ruby

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

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Chambers in a method shelterA method shelter has two parts

◦An exposed chamber and a hidden chamber◦Each chamber contains methods and

“import”s◦Exposed : for public APIs (similar to public)

◦Hidden : for internal use (similar to protected)

- Obj#m0S0

Exposed Hidden

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Import

Exposed Chambersfor public API

◦Local rebindingMethods

◦Visible from importerImport

◦ Imported methods are also visible from importer

- Obj#m0S0

S1

S2

import

import

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Call/redefine

Call/redefine

Hidden chamberfor internally used

methods◦Not called/redefined from

importerMethod

◦Visible only from the same shelter

Import◦ Imported methods are not

visible from importer

- Obj#m1 - Obj#m0S0

S1

S2

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Call

Call

import

Method Lookup AlgorithmContexts :

◦(class, method name, current shelter)Search for methods as follows:

◦1. look up the current shelter’s hidden-chamber and its importing shelters

◦2. look up the current shelter’s exposed-chamber and its importing shelters

◦3.If not found, go to the superclass

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- m1- m0

current First, look up this group

Second,

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*Look up from importer

Detect AmbiguityIf you use exposed chambers, it may

cause conflicts◦Detects and raises an error

S0

- C#m0S1 S2

Error!

- C#m0S3

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Syntax We have not modified syntax

◦Ruby has powerful syntax◦Use ordinal methods with a block

shelter :ShelterName do class Foo def hoge # <- defined in the method shelter end endend

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Syntax: Import

shelter :ShelterName do import :AnotherShelterNameend

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Syntax: hide“hide” method switches a chamber

◦Methods and imports below “hide” are in the hidden chamber

shelter :ShelterName do # exposed chamber hide # hidden chamberend

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Example of method shelters

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List

avg(): average of elems using div

Integerdiv(): return rational

Integerdiv(): return integerplus(): …minus(): …

Shelter1

Sheleter 2

Sheleter 3

[1, 2].avg() #=> (3/2)1.div(2) #=> 0

Shelter 4

Example of method shelters

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List

avg(): average of elems using div

Integerdiv(): return rational

Integerdiv(): return integerplus(): …minus(): …

Shelter1

Sheleter 2

Sheleter 3

[1, 2].avg() #=> (3/2)1.div(2) #=> (1/2)

Shelter 4

ImplementationBased on Ruby 1.9.2 Add one implicit argument to method:

◦Current method shelter Optimize method-lookup caches

◦Shelter node cache Caches method body in a shelter

◦Extend inline cache Stores the found shelter Size of an inline cache : 3 word -> 4word

(per method call)

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Micro benchmark : empty methods

Call an empty method in shelter 10,000,000 times◦Less than 5% overhead when shelters are

used

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Micro benchmark : Ruby on RailsEnabled shelters in an action method

◦Numeric#kilobytes method in a shelterIn the action method

◦1. Call kilobytes method in shelter◦2. One access to SQLite

4% overhead

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Related WorkRefinements (for Ruby)

◦Provide a scope of methods◦Redefined methods are available in lexical

scope No local rebinding

Classboxes [‘05 Bergel et al.]

◦A classbox provides the scope of class extensions

◦ Introduce Local rebinding property

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ConclusionMethod shelters provide 2 mechanisms for

implementing scope of class extensions◦Exposed and Hidden◦You can control scopes of class extensions by

combining them◦Avoid conflicts

Implementation on Ruby◦reasonable overhead by caches

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