A Seminar on Encapsulation http://www.cs.tau.ac.il/~msagiv/courses/encapsulate.html

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A Seminar on Encapsulationhttp//www.cs.tau.ac.il
/msagiv/courses/encapsulate.html

• Noam Rinetzky
• Mooly Sagiv

Summary
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Outline

1. Formalisms in programming languages
2. What is encapsulation
3. Some model of encapsulation
4. Feedback

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Programming Language Formalisms

• Regular languages
• Context free grammars
• Attribute grammars
• Operational Semantics
• Axiomatic Semantic/ Deductive Verification
• Denotational semantics
• Type Checking
• Abstract Interpretation

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Example The While Programming Language
S x a skip S1 S2 if b then S1 else
S2 while b do S
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Natural Semantics

• Notations
• ltS, sgt - the program statement S is executed on
  input state s
• s representing a terminal (final) state
• For every statement S, write meaning rulesltS, igt
  ? oIf the statement S is executed on an input
  state i, it terminates and yields an output state
  o
• The meaning of a program P on an input state i is
  the set of outputs states o such that ltP, igt ? o
• The meaning of compound statements is defined
  using the meaning of immediate constituent
  statements

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Natural Semantics for While
assns ltx a, sgt ? sx ?A?a?s skipns ltskip,
sgt ? s
axioms
rules
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Natural Semantics for While(Loop rules)
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Hoare Proof Rules for Partial Correctness
p skip p
p/v? e ve p
p c0 r r c1 q p c0c1q
p?b c0 q p ??b c1 q p if b then c0 else
c1q
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Hoare Proof Rules for Partial Correctness
i?b c i i while b do ci??b
?p ? p p c q ? q ? q p c q
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Type Checking Rules

• ? ? intconst int

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Type Checking Rules

• Enforce consistency of usages
• Many satisfying type assignments
• Most principle type
• Sound type system
• Every type assignment describes a superset of
  expression values

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Even/Odd Abstract Interpretation

• Determine if an integer variable is even or odd
  at a given program point

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Example Program
/ x? /

• while (x !1) do
• if (x 2) 0
• x x / 2
• else
• x x 3 1
• assert (x 2
  0)

/ x? /
/ xE /
/ x? /
/ xO /
/ xE /
/ xO/
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Abstract Interpretation
Concrete
Sets of stores
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What is encapsulation?
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Common Answer(1)

• Encapsulation is the ability of an object to
  place a boundary around its properties (ie. data)
  and methods (ie. operations)
• Programs written in older languages suffered
  from side effects where variables sometimes had
  their contents changed or reused in unexpected
  ways. Some older languages even allowed branching
  into procedures from external points
• This led to 'spaghetti' code that was difficult
  to unravel, understand and maintain
• Encapsulation is one of three basic principles
  within object oriented programming languages

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Common Answer(2)

• Object variables can be hidden completely from
  external access
• These private variables can only be seen or
  modified by use of object accessor and mutator
  methods
• Access to other object variables can be allowed
  but with tight control on how it is done
• Methods can also be completely hidden from
  external use
• Those that are made visible externally can only
  be called by using the object's front door (ie.
  there is no 'goto' branching concept).

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Is it that simple?

• Does private guarantee encapsulation?

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What is encapsulation?

• Desired feature of programs/systems
• Not well defined

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What is encapsulation?

• Restrictions on aliasing
• Desired features
• Modular reasoning
• Representation independence
• Simplified specifications
• Predictable side-effects
• Simplified concurrent programming
• Not too restrictive
• Enforced
• Programming disciplines
• Programming language syntax
• Runtime checks
• Type system
• Abstract interpretation

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Towards a Model of Encapsulation

• J. Noble, R. Biddle, E. Tempero, A. Potanin, D.
  Clark

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Dan Ingalls

• No component in a complex system should depend
  on the internal details of any other components

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Access Graph

• Models the topology of the system
• Nodes N
• Edges E between nodes represent accesses between
  objects
• Either directed or undirected
• Abstract
• Simple example Storage graph

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Encapsulation Function

• A system S ? N
• An encapsulated component C
• Can be a package
• A set of objects
• Map C into three sets
• B the Boundary nodes
• Y the Inside nodes
• R the External nodes referred
• O the Outside nodes
• O S (B ? I)
• R ? O
• e C ? B, I, O

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An encapsulated component
O
I
R
B
?
?
?
?
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Encapsulation Constraints

• Disjointness of sets
• B ? I ?
• B ? R ?
• I ? R ?
• Connectivity
• ?I ? (B ? I)
• ? o ? O, ? i ? I ? p ? paths(o, i) ?b ?p
  s.t. b ?B
• (B ? I)? ? (B ? I ?R)

No component in a complex system should depend
on the internal details of any other components
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Nested Encapsulation
O
Ic
Rc
Id
Bd
Bc
?
?
Rd
?
?
?
?
?
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Nested Encapsulation

• d is nested c (d ? c) when
• Id ? Ic
• Bd ? Bc ? Ic
• Rd ? Bc ? Ic ? Rc

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Islands Full EncapsulationHoggAharon
I
B
R
?
?
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Islands Full EncapsulationHoggAharon

• Protects static references only

• Syntactic enforcement

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Uniqueness
R
I
B
unique
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Uniqueness
where ?u 1
Can be enforced by 1. Runtine 2. Type system 3.
Abstract Interpretation
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External UniquenessClarkeWrigstrad Ziv
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External Uniqueness ClarkeWrigstrad Ziv
B v
external_unique(u)
I u ?o u ? o
where v ?u I v 1
R O
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Balloon TypesAlemida
Iouter
Router
Iinner
Rinner
Binner
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Flexible Alias ProtectionNoble, Vitek, Potter

• More permissive than Islands and Balloon on
  static alias protection
• Adds requirements on dynamic aliases
• Two concepts representation and arguments

rep(o)
arg(o)
o
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Ownership TypesClarke, Potter, Noble Shay,
Sharon, Ziv

• Alias protection via ownership
• Every object has an owner
• ownership forms a tree
• Only references from parent to decedents
• Nested encapsulation
• Enforced by the type system

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Ownership
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Formalizing Simple Ownership

• owner N ? N
• owner(root)root
• i ownedby o ?k ownerk(i) o
• o owns i ?k ownerk(i) o
• Invariant s ? t ? s ownedby owner(t)

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Other Ownership Models

• Parameterized ownership
• The set R is explicitly parameterized by
  ownerships
• Inner Ownership Boyapati

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Confined TypesBokawskiVitek Guy
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Confined TypesBokawskiVitek Guy

• package N ?C

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Lecture Summary
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Feedback