Objects and Aspects: What we

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Objects and AspectsWhat weve seen so far

• 15-819
• Jonathan Aldrich

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In-place vs. Functional Extension

• In-place extension
• Add new methods, fields, superclasses to existing
  ADTs
• Cecil, EML, Aspects
• Strengths
• Can evolve system without planning ahead
• Direct, first-order composition
• My opinion Cant ignore need for in-place
  extension in real world due to unplanned
  evolution/reuse

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In-place vs. Functional Extension

• Functional extension
• Produce new classes that enhance old ones
• Inheritance, Mixins, Scala
• Strengths
• Can have different versions of classes
  side-by-side
• Simpler semantics and typechecking
• My opinion (1) Can get better reuse this way,
  and(2) Many systems need side-by-side versions

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Whats Unique About(Disclaimer the systems
where we first saw a features are notnecessarily
the first systems with that features in the
research literature)

• Self
• Dynamic inheritance
• Singleton objects (copied by Cecil, Scala)
• Shared state (copied by Cecil)
• Overridable fields (copied by Cecil, C)
• Cecil (much is actually also in CLOS)
• Multi-methods
• In-place extensibility methods, fields,
  inheritance
• Flexible type system
• F-bounded poly structural (partially copied by
  Scala)
• Conditional subtyping
• EML
• Clean unification of ML-like FP OOP
• Combination of pattern matching and multi-methods
• Modular typechecking of in-place method extension

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Whats Unique About

• Units/Mixins
• Side-by-side base classes and extensions
• Extensions of related groups of classes
• More flexible reuse through mixins
• Dynamic, recursive module linking
• Scala
• Clean integration of fns and objects
• But more object-like vs. EMLs design
• Nice implementation choices
• XML support, control structure syntax, local type
  inference
• Clean version of mixins/multiple inheritance
• Traits no state, flexible use
• Mixins richer but less flexible
• Rich parameteric types
• Variance for type parameters, F-bounded poly.,
  Family Polymorphism, ADTs
• Views partial unplanned extensibility
• Vs. Cecil views are a new object, not an
  extended version of the old one

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Whats Unique About

• F-bounded polymorphism
• Binary methods like comparable
• Hierarchies of families of related types
• Graph Node
• Family polymorphism
• Keeps different Graphs of same type separate
• Nested Inheritance
• Way to refer to surrounding Graph type from
  Nodes, Edges
• Typestate for Objects
• Enforcing client-level protocols in OO setting
• Correlating typestate to internal invariants
• Ownership
• Enforcing encapsulation properties in presence of
  aliasing, state, and inheritance

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Open Problems

• Typechecking
• Dynamic inheritance
• Allow more info. hiding in EML
• Advanced in-place extensibility (e.g.,
  subclassing)
• Full flexibility of Units/Mixins
• Combining family poly. nested inheritance
• Combining in-place and functional extensibility
• Typestate, ownership, etc.
• Still wide open increasing expressiveness
  checking

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Expressiveness Challenges

• Grouped extension
• Scala, Nested Inheritance
• Late-bound inheritance
• Parameterized modules with mixins (Units)
• Dynamic inheritance (Self)
• Other idioms
• Multiple dispatch/pattern match (Cecil, EML)
• Singleton objects (Self, Cecil, Scala)
• Shared state (Self, Cecil)
• My opinion necessary, nice,
  interesting

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Checking Challenges

• In-place extension
• Modular typechecking for open classes (EML)
• Do they have the rules right yet?
• Typechecking extensibility features
• Hierarchies of related classes (F-bounded
  poly)
• Grouped by object instance (Family poly)
• With ways of talking about group from within
  (Nested Inheritance)
• Merging structural nominal (Cecil)
• Can this be simulated by in-place inheritance
  extension?
• Checking properties in presence of inheritance
• Typestate correct use impl. of library objects
• Ownership encapsulation w/ state, aliasing,
  inheritance
• Later effects and object invariants
• My opinion necessary, nice,
  interesting

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The Best of Both Worlds?

• In-place extensibility
• Add methods, fields, subclasses to ADTs
• Multi-methods pattern matching
• Other AOP features
• Functional extensibility
• Parameterized modules singleton objects
• Combine family poly. nested inheritance
• Dynamic inheritance
• Combining these is probably non-trivial!
• What does it mean to extend a parametric module
  in place?

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The Road Ahead

• AOP
• Advanced in-place extensibility techniques
• Cleaner separation of concerns
• Empirical validation
• Modular reasoning
• In presence of inheritance and AOP