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Surfing Ada for ESPPart 2

C. Montangero

Dipartimento d’Informatica

Corso di ESperienze di Programmazione

a.a. 2012/13

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Table of contents

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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.

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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. 339-344

• 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.

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 http://www.sigsoft.org/SEN/parnas.html

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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

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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.

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Named collections of entities: examples

• Group of constants

• Groups of entities

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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

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Package body

• body elaboration after that of specification

• it includes execution of the final statements– cfr object constructor

complete definition

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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 followingthe visible part

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Private Types: example

• Why not two syntactic pieces?

• Difficult file management...

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Private and body

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Private: visibility

Client: logical

interface

body

Compiler:

physical interface

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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

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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

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Bottom-Up Program Development

• programmers develop libraries of generally usable packages

• separately compiled and made available in the program library and used

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Hierarchical Program Development...

• ... or stepwise refinement – Wirth, N., Program development by stepwise refinement, Comm. ACM 14,4 (1971) 221-227

• 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

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An example

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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

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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

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An example : sort

• ITEM qualunque

– purché abbia un’operazione ITEM x ITEM -> BOOL

• ROW array di ITEM, con qualunque indice

– purché discreto, naturalmente

-- possibile default

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Uso di SORT (animaz)

-- usa le procedura generica

-- il tipo degli elementi

-- struttura da ordinare

-- tipo dell’oggetto da ordinare

-- oggetto da ordinare

-- chiamata istanza di SORT

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mapcar

• mapcar restituisce la lista dei risultati di

ACTION su ogni elemento di T

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DATA STRUCTURE

(PLUS A B)

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IMPLEMENTATION

• Poco dinamismo, più efficienza a run-time

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CHAPTER 14: Exception Handling

• Eccezione predefinita:

– Funzione capitozzata

– Attributo predefinito

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Ada vs Java

• Possible effect: exception

• Not in the function spec

– cfr Java’s throws clause

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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

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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

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Inheritance

• with extension clause

– as extends

• and redefines Area

• how to be sure to call the right Area?

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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)

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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

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The Object class

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The Circle «class»

• .Cir is Obj’s public child

• .Cir implies with Obj

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More in general...

• piecewise private part

– private package Parent.Child

– to structure the implementation

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