1. Software Engineering Lecture 4 Vladimir Safonov, Professor, head of laboratory St. Petersburg University Email: v_o_safonov@mail.ru WWW: http://user.rol.ru/~vsafonov

2. (C) Vladimir O. Safonov, 20042 The role of the programming paradigms under consideration Structured programming – systematic design and development of (mainly) the control flow parts (statements) Stepwise refinement – convenient top-down technique of rapid application prototyping Modular programming – systematic program decomposition into relatively independent parts (modules) Also of great importance are techniques for design and development of data types and structures

3. (C) Vladimir O. Safonov, 20043 Abstract Data Types (ADT) Founders (1970s): - Sir Tony Hoare: Oxford University, MSR; - Prof. Barbara Liskov: Massachusetts Institute of Technology - Prof. David Parnas (U.S. and Canadian universities) Basic principles - Type = concrete representation + set of abstract operations - Direct access to concrete representation is not allowed - Handling values (or objects) of the type T is possible only via its abstract operations P1, … Pn, so the type T is encapsulated ADT can be regarded as a kind of modularity (name and set of operations – interface; concrete representation and bodies of operations – implementations Before ADT similar approach was applied to predefined (primitive) types: VAR i: INTEGER; BEGIN … i := i+1; … END

4. (C) Vladimir O. Safonov, 20044 ADT specifics “Types are not sets” (F. Morris) Encapsulation Formal specification of ADT semantics necessary R. Floyd’s – C.A.R. Hoare’s method: P { S } Q Denotational semantics of ADT (D. Scott, J. Donahue): program is a transformer of predicates the kind: P(X1, … Xn), where Xi – set of variables Algebraic specification of ADT (the ADJ group, lead – prof. J.A. Goguen, the author of OBJ specification language): type – multi-sorted algebra; S1, … Sk – sorts (carrier); P1, …, Pm – operations (signature); semantics – set of equates of the kind: T1 = T2 where T1 and T2 - terms

5. (C) Vladimir O. Safonov, 20045 ADT advantages Modularity Increasing reliability Formal specification and verification of ADT ADT libraries and reusability

6. (C) Vladimir O. Safonov, 20046 ADT issues and how to solve them “ADT are non-efficient” (each abstract operation is a routine!) -> using macros and inline routines; program optimization (the task of the compiler rather than user) “ADT are too complicated” (it seems simpler to code “from scratch” and to write explicitly something like: P^.X [index]^.Y := something ) -> such “coding technique” leads to buggy and non- readable programs. ADT is a comfortable technique of abstraction from details: SetMyProp(P, something);

7. (C) Vladimir O. Safonov, 20047 ADT languages FORTRAN & PASCAL - predefined types are regarded as ADT with infix: X+Y or functional: SIN(X) style of operations CLU (B. Liskov) – the first ADT language; played outstanding part in all subsequent development of programming languages – M.I.T., 1974 ALPHARD (M. Shaw, Carnegie-Mellon Univ.) – the“competitive project” to CLU; based on similar ideas, but Hoare-style specification and verification added to the language Ada (package, task); Modula-3 (interface); Java, C# … - contain elements of ADT concept

8. (C) Vladimir O. Safonov, 20048 References and home task to lecture 4 Please read the following papers and get acquainted to their approached to ADT: Liskov B, Guttag J. Using abstractions and specifications in program development. – Addison- Wesley, 1986 Hoare, C.A.R. Proof of correctness of data representation (article, 1972)