Design for Ease in Contraction and Extension

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Design for Ease in Contraction and Extension

• CSE870 Discussion
• April 16, 2003

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Key Ideas

• Design for change
• Maintainability
• Identified characteristics of non-adaptable
  programs
• Undesirable properties
• Distributing related information over numerous
  programs
• Chaining of data transformations
• Designing multi-function components
• Circular-uses
• Subset
• Smallest useful collection of functions
• High cohesion among the functions

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Key Ideas (contd)

• Uses hierarchy
• Degree in which one program uses other programs
• Levels
• 0 programs that do NOT use any other programs
• Single-purpose programs
• I programs that use at least one program at
  (I-1) level, not above I-1.
• Multi-purpose
• Virtual Machines
• SW abstraction/instructions for instructions
  provided by the HW.
• May have several layers of abstractions that can
  be customized for specific applications

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Key Ideas (contd)

• General vs Specific
• General
• SW could work without modification
• Not as much concern for designing for flexibility
• Run-time costs customization can be expensive
• Specific
• Solution to specific problem
• SW footprint can be reduced because it does not
  contain extraneous functionality
• Program can be more efficient
• Narrow focus of program

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Key Ideas (contd)

• Flexible design
• Takes specific approach
• Provide general services with WELL-DEFINED
  interfaces
• Have specific implementations for these services
  that are kept secret
• Information Hiding
• Implementation details of modules (or functions
  within modules)

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Comparison to Program Families

• Extension/Contraction paper gives specific
  techniques for how to build program families

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Impact on Modern Technology

• Frameworks
• Possible relationship to Graybox (have ability to
  extend) while adhering closely to superclass
  interfaces

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Impact on Modern Tech (contd)

• Design Patterns
• Iterator (operations can be independent of data
  type)
• Model View Controller
• Bridge decouples the abstract services from its
  implementation
• Strategy pattern
• Add strategies
• Decouple interface for a strategys purpose from
  specific algorithm.
• Factory Pattern
• Composes the abstract components that can then be
  realized by concrete components
• Chain of responsibility
• Each unit should have minimal functionality that
  can then be extended to provide additional
  capabilities.

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Impact on Modern Day Tech (contd)

• Distinguishes specification from implementation
• Specification
• Implementation
• Later clarified by Lamport, Lynch, Abadi
• Potential impact on Aspect-Oriented Programming
• Advocate collection of like-purpose
  functionalities into single module
• Interdependence with compiler (def-use) and
  Russell and Whiteheads mathematical formulation
  for set theory (early 19XXs)