1. INF 212 Modularity Instructors: Crista Lopes Copyright © Instructors.

2. Outline  The 2 sides of Modularity  Today:  Modularity and Style  Style in the other Arts  Some Styles

3. Modules as physical components

4. Modules as conceptual units

5. Software Systems – Your Brain...

6. Software Systems – Your Code...

7. Your brain and Your code Expression of thoughts using PLSuggestion for thoughts

8. How to Divide the Problem? ? (*) (*) This is not a trivial question, and you only face it much later, possibly after years of experience in programming. (*) Some people never even face this question, having learnt only one way of thinking.

9. PLs and Modularization  PLs are the materials of expression  Suggest ways of thinking, ordering, dividing  English suggests placing adjectives before nouns  E.g. “An inconvenient truth”  Latin languages suggest placing adjectives after nouns  E.g. “Uma verdade inconveniente”  Try doing this in English...

10. A very old issue  Expressing one’s thoughts using some language and materials  Understanding the deeper structures of artifacts, after analyzing them  One Word: STYLE

11. PLs and Styles  PLs encourage styles of expression, sometimes quite strongly  Often the style one learns first is the one that sticks to our brain, like our mother natural language  Language wars are style wars  “English is better than French”  “Haskell is better than Java”  More on utilitarian perspectives later on

12. Styles in the other Arts  Pachelbel Canon in D  Original Original  18 styles (piano) 18 styles (piano)  A few more (guitar) A few more (guitar)  Exercises in Style, R. Queneau

13. When CS rediscovered the issue  Parnas “On the criteria to be used in decomposing systems into modules”  Concrete examples and guidelines for different ways to modularize  Primarily how to deal with state  We can do it in many ways, now what? Which is better?  Parnas suggestion: information hiding should play a role in deciding that  Functional programming had been suggesting it all along without formulating the question

14. Modularity  Easier comprehension of complex problems  Systematic resolution: Divide-and-conquer  Loose coupling  Division of labor  Enables independent evolution and maintenance  Reuse  High Cohesion  Module has single responsibility/purpose

15. Example - KWIC  Problem  Input: Ordered set of lines, with ordered set of words, with ordered set characters.  Any line may be circularly shifted (removing the last word of the line, appending at the beginning)  Output: All circular shifts of all lines in alphabetical order Q u i c k 1 2 3 4 5 Q u i c k 1 2 3 4 5 B r o w n 1 2 3 4 5 B r o w n 1 2 3 4 5 F o x 1 2 3 F o x 1 2 3 123

16. Example - KWIC  Shared Data Master Control Text array Word index Alphabetized index

17. Example - KWIC Read input Shift Sort Display output Display output Pipe-and-filter (functional)

18. Example - KWIC Read input Shift Sort Display output Display output Monadic

19. Example - KWIC  OOP-ish (data abstraction) Line Storage Input Circular Shifter Alphabetizer Output Master Control

20. Example - KWIC Input agent Input agent Shift agent Shift agent Sort agent Sort agent Output agent Output agent Pub-Sub Event Manager listen/announce