Surfing Ada for ESP Part 2

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1 Surfing Ada for ESP Part 2 C. Montangero Dipartimento d Informatica Corso di ESperienze di Programmazione a.a. 2012/13 1

2 Table of contents 2

3 CHAPTER 9: Packages Packages allow the programmer to define groups of logically related items wide variety of uses collections of common declarations groups of subprograms encapsulated data types basis for a powerful structuring tool for complex programs clear separation between information that is usable by the rest of the program, and information that must remain purely internal to the package the internal information is hidden, and thereby protected from deliberate or inadvertent use by other programmers. easier to replace one implementation of (the services offered by) a package by another. 3

4 Packages Packages are an essential tool for program modularity supporting program verification and information hiding Parnas, D.L., Information distribution aspects of design methodology, Information Processing 71, pp As Steve McConnell has pointed out, good object oriented design requires that objects hide something. Unfortunately, most of the O-O code that I have seen hides nothing. The programs look like COBOL programs with a new syntax. ACM Fellow Profile: David Lorge Parnas Facilities for modularization in many languages Simula, Clu, and Alphard provide dynamic facilities may entail large run-time overhead Ada is more static in the spirit of previous solutions offered in Lis, Euclid, Mesa, and Modula it retains the best aspects of earlier solutions in Fortran and Jovial. 4

5 Kinds of modularization Named collections of entities Logically related constants, variables, and types to be used in other program units The package exposes all of its declarations Groups of related subprograms commonly called a software package Logically related functions and procedures Sharing of internal data, types, and subprograms The package exposes the declarations of the externally usable subprograms The package hides their implementations and the declarations of the shared internal entities 5

6 Kinds of modularization Encapsulated data types - Private types definition of new types and associated operations the user does not know how they are implemented the user need not care, either The package exposes the type name the declarations of applicable operations The package hides all details of structure, representation, and implementation of the operations several related types may be encapsulated in the same package. 6

7 Named collections of entities: examples Group of constants Groups of entities 7

8 Groups of Related Subprograms Actually, an object ITEM_TABLE, better than TABLE_MANAGER Package specification -> subprogram specs only The implementation is hidden in the package body 11

9 Package body complete definition body elaboration after that of specification it includes execution of the final statements cfr object constructor 12

10 Private Types the name of a type is public the internal properties are available only to the subprogram bodies in the package body the type name is declared within the visible part at the same time specifying the type to be private the full definition of the type is provided following the visible part 13

11 Private Types: example Why not two syntactic pieces? Difficult file management... 14

12 Private and body 15

13 Private: visibility Client: logical interface Compiler: physical interface body 16

14 Eg: simple Java objects Need to split the public methods getters /setters To allow access to any private attribute/operation No information needed on the implementation structure Even for compilation Possibly: REPRESENTATION CLAUSE for configuration 17

15 CHAPTER 10: Separate Compilation and Libraries Ada program = collection of compilation units Compilation unit library unit package declaration subprogram declaration generic declaration or instantiation secondary unit (related to another compilation unit) the body of a library unit a package body a subprogram body the body of a generic unit a subunit of another compilation unit Each compilation unit may have a context clause with clauses that mention the names of the needed library units Compilation units can be submitted individually to a compiler But they depend on each other as indicated by with clauses (partial order) the compilation units that form a given program belong to a program library 19

16 Bottom-Up Program Development programmers develop libraries of generally usable packages separately compiled and made available in the program library and used 20

17 Hierarchical Program Development or stepwise refinement Wirth, N., Program development by stepwise refinement, Comm. ACM 14,4 (1971) Top level: formulation of the program in terms of operations to be supplied by the next level Each such operation further defined in terms of operations of another lower level, and so on. In support of stepwise refinement, Ada offers compilation units that are subunits of other compilation units. (in)formal specification are paramount 21

18 An example 22

19 Here comes the secondary unit Visibility: TABLE is seen as if the body of the separate unit were textually nested at the place of the stub Secondary units may have their own with clauses 23

20 CHAPTER 12: Generic Units A generic subprogram is a template for ordinary subprograms obtained by associating specific actual parameters with the generic formal parameters Similarly a generic package is a template for ordinary packages A generic declaration vs type declaration 24

21 An example : sort -- possibile default ITEM qualunque purché abbia un operazione ITEM x ITEM -> BOOL ROW array di ITEM, con qualunque indice purché discreto, naturalmente 25

22 Uso di SORT (animaz) -- usa le procedura generica -- struttura da ordinare -- il tipo degli elementi -- tipo dell oggetto da ordinare -- oggetto da ordinare -- chiamata istanza di SORT 26

23 mapcar mapcar restituisce la lista dei risultati di ACTION su ogni elemento di T 28

24 DATA STRUCTURE (PLUS A B) 29

25 IMPLEMENTATION Poco dinamismo, più efficienza a run-time 30

26 CHAPTER 14: Exception Handling Eccezione predefinita: Funzione capitozzata Attributo predefinito 31

27 Ada vs Java Possible effect: exception Not in the function spec cfr Java s throws clause 32

28 Previous models PL/1: on condition possibility to raise the exception again possibility to continue with a value More powerful, but problems with reasoning on programs without resumption it is easier to distinguish the normal flow and the exceptional one two different assertions 33

29 Ada 2005: Object Orientation tagged record types implicit access, dereferencing and dot notation the first argument is «this»: note the «in» mode Distance to be inherited, but Area as to be redefined 35

30 with extension clause as extends Inheritance and redefines Area how to be sure to call the right Area? 36

31 Polimorphic functions CLASS attribute to declare polimorphic variables/parameters il tipo preciso di OC è staticamente ignoto la chiamata di Area si usa il binding dinamico se OC fosse di tipo Object, il risultato sarebbe sempre zero (binding statico) 37

32 Ada 2005: OO & maintenance Hiererachical library child packages Piecewise package specification No effect on old programs, no re-compilation needed Similarly for bodies and private parts (private) package child 38

33 The Object class 39

34 The Circle «class».cir is Obj s public child.cir implies with Obj 40

35 piecewise private part More in general... private package Parent.Child to structure the implementation 41

36 42

10. Abstract Data Types

10. Abstract Data Types 11.1 The Concept of Abstraction The concept of abstraction is fundamental in programming Nearly all programming languages support process abstraction with subprograms Nearly all