08/05/2013. Surfing Ada for ESP. C. Montangero Dipartimento d Informatica Corso di ESperienze di Programmazione a.a. 2012/13

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

2 ... per vincere alle corse dei cavalli

3 Dewer s talk at MIT (circa 2005) Robert B. K. Dewar is a professor at NYU, President of AdaCore, and one of the early figures in the development of Ada. In this lecture given at the Massachusetts Institute of Technology, Mr. Dewar gives an overview of the history of Ada, which includes the motivation for its conception, the story of its development, and the role Ada plays in present day programming. The lecture also covers the fundamental ideas behind Ada, its influence on other languages, and the truths and myths associated with the language. 3

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20 Ichbiah Barnes 20

21 Table of contents 21

22 Chapt3: Classical Programming Programs achieve actions by executing statements. These may contain expressions that are formulas defining the computation of values. The major entities involved in a program are declared by declarations The creation of the declared entities is achieved during program execution by a process that is called the elaboration of declarations Key: Syntax vs semantics 22

23 Constants constant does not mean that the initial value must be known at compilation time (that is, in Ada terminology, the initial value expression need not be static). Good programming style in Ada will systematically emphasize constants, like in the above example 23

24 Variables Ada semantics is based on entities Constant and variables are called objects Not in OO sense Rather because they are acted upon during execution 24

25 Aggregates For array/record types, Ada s aggregate combines component values into a composite value of the type positional notation ordered sequence of component values named notation explicit component-value pairings 25

26 The Ada Model of Time Ordine di valutazione indeterminato per assegnamento, espressioni, lista argomenti,indici... Libertà al compilatore di ottimizzare Programma erroneo vs scorretto OUT_ROW(FLOOR(X)) := IN_ROW(CEIL(Y)); vs U := FLOOR(X); -- guarantees that FLOOR is called before CEIL OUT_ROW(U) := IN_ROW(CEIL(Y)); 26

27 CHAPTER 4: Types A set of values and a set of (predefined) operations: Equality and inequality: = /= Ordering relations: < <= > >= The assignment basic operation: := Other basic operations called attributes type INT is range -2**24.. 2**24; -- integer type SCALE is (LOW, MEDIUM, HIGH); -- enumeration type MASS is digits 8 range E9; -- floating point type VOLT is delta 0.01 range _000.0; -- fixed point type LINE is array ( ) of CHARACTER; -- array type PAIR is record X, Y : INTEGER; end record; -- record type MY_INT is new INTEGER; -- derived type TEXT is access STRING; -- access type FILE is limited private; -- private 27

28 Type hierarchy PRED, SUCC, MAX, MIN POS, VAL Numerics 28

29 Type Equivalence When do two objects have the same type? two families name equivalence structural equivalence Name equivalence is used in Ada each type declaration declares a distinct type for two objects to have the same type, their declarations must refer to the same type name directly, or indirectly by a subtype Structural equivalence an equivalence rule is defined between types on the basis of their structural properties E.g two arrays with the same range and domain 29

30 Constraints and Subtypes An object has a certain type static property of the program Constraint: (from Pascal) restricts the values that may be assumed by an object Subtype: a type with an associated constraint An object can be declared to have a certain subtype Again a static property of the object. Not always possible to determine statically (at compilation time) whether or not a value satisfies a constraint: run-time checks Constraints and subtypes are concepts that are, in general, related to the dynamic behavior of programs. 30

31 Subtypes example 31

32 More examples Dynamic constraints Loop control Attributes Separate compilation 32

33 Ada 2012: Ada With Contracts By Rober Dewar, April 09, ada-with-contracts/ ?pgno=2 Besides preconditions and postconditions for subprograms, Ada 2012 supports several other kinds of contracts. One category involves predicates on types: conditions that must always be met by values of the type. One of the most important such predicates, ranges on scalar types, has been part of the language since Ada 83: Test_Score : Integer range 0 through 100; Distance : Float range ; This capability (sadly missing from C, C++, Java, and many other languages) is extremely important for program understandability and reliability. It is hardly new (it's been part of Pascal for more than 40 years), but for some reason, it did not catch on in many other languages. And the consequences are significant. Many buffer overruns in C could be avoided if the language supported range constraints. An important project at Microsoft is to add such range declarations to the Windows code base. Hundreds of thousands of such statements have been added, and have detected a large number of potential buffer overruns. 33

34 CHAPTER 5: Numeric Types a numeric variable has an abstract value three anonymous types cannot be named in a program no variable of such a type can be declared numeric literals have those types universal_integer integer type with a range large enough to encompass every conceivable integer type of the implementation. Integer literals are of type universal_integer. universal_real real type with a precision that is high enough to encompass any implemented real type. Real literals are of type universal_real universal_fixed fixed type with a finer delta than any implemented fixed point type 36

35 Implementation-dependent numeric types Integer types There is an implementation-dependent type INTEGER defined in the (virtual) package STANDARD it reflects the properties of the underlying hardware: the most efficiently handled integer size is used From SYSTEM.MIN_INT to SYSTEM.MAX_INT Defined in the package SYSTEM Additionally, an implementation may provide types LONG_INTEGER and SHORT_INTEGER user defined integer types, by stating the required range subtypes of a type derived from one of these predefined types the predefined type being suitably selected by the implementation 37

36 Dichiarazioni in SYSTEM 38

37 Real types Floating point typesfloat, LONG_FLOAT (and so on) The representation has two independent parameters the length of the mantissa, for the relative accuracy the range of the exponent, for the range of the floating point values the Ada user defines only the required precision, as a number of decimal digits the language requires the implementation to choose an exponent range that is workable in relation to its mantissa length Fixed point types have an absolute accuracy, called the delta the user can define the required delta the range of magnitude Brown s rigorous axiomatic system to handle approximate computations 39

38 static vs dynamically allocated objects Static lifetime determined by the static structure of the program static: textual Dynamic objects created by allocators creation not directly related to the program structure not possible to define their names by declarations an allocator returns an internal name, not an identifier it cannot be used explicitly in the text to denote the new object indirect names to access the dynamic objects declaration of access types 40

39 Gestione della memoria Allocazione dello heap: Elaborazione della dichiarazione del tipo access Pragma per dimensionarlo e inibire gc Primitive di deallocazione esplicita Recupero dello heap: All uscita del blocco della dichiarazione del tipo

40 CHAPTER 7: Derived Types Need a type with the same characteristics as another one, and possibly some additional ones? MODEL is the parent type, COPY is a derived one The two types have similar characteristics but they are nevertheless distinct types 44

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42 A derived type -- implicitely derived -- behaves as Basic form of inheritance Not only procedures: attributes,... Another example: unit of measure (mass, force, velocity,...) 46

43 The Explanation of Numeric Types Is equivalent to The selection of the predefined type is done by the compiler the type chosen must support the precision and at least the range required this portability: the programmer need not know which floating point type is actually used the compiler may generate code that corresponds to one of the hardware floating point types 47

44 CHAPTER 8: Subprograms Algol60: The specificationprovides the information needed to call the subprogram it may be hard for the reader to find, in a program with a large number of subprograms spread over several pages of text an isolated variable declaration between two large subprograms is a well-known source of error Pascal: fixed sequence: types, variables, subprograms, Ada allows the subprogram declaration to be separated Specification : signature Body : signature stated again (for readability and overloading) Bodies at the end, but richer opportunities with modules Algol60 and Pascal: one big program 51

45 Parameter passing Mode Nature Rights Contract In Constant Only reading The caller must supply a value In out Variable Both reading and updating The caller must supply a value, and the subprogram must return a value Out Variable Only updating The subprogram must return a value Copy or reference? the execution of a program is erroneous if its effect depends on which mechanism is selected by the implementation (for composite types) 52

46 Parameter Passing Notations Equivalent calls Second one more expressive 53

0. Overview of this standard Design entities and configurations... 5

0. Overview of this standard Design entities and configurations... 5 Contents 0. Overview of this standard... 1 0.1 Intent and scope of this standard... 1 0.2 Structure and terminology of this standard... 1 0.2.1 Syntactic description... 2 0.2.2 Semantic description...