Principles 990128 P F Reynolds CS655 1 MacLennan s Principles Abstraction Avoid requiring something to be stated more than once factor out the recurring pattern Subprograms user defined types inheritance Automation Automate mechanical tedious or error prone activities Garbage collection looping structures 990128 P F Reynolds CS655 2 1 MacLennan s Principles 2 Defense in Depth Have a series of defenses so that if an error isn t caught by one it will probably be caught by another Array bound being part of type definite loops Information Hiding The language should permit modules to be designed so that 1 the user has all of the information needed to use the module correctly and nothing more and 2 the implementor has all of the information needed to implement the module correctly and nothing more Modules packages objects 990128 P F Reynolds CS655 3 MacLennan s Principles 3 Labeling Avoid arbitrary sequences more than a few items long Do not require the user to know the absolute position of an item in a list Instead associate a meaningful label with each item and allow the items to occur in any order Case statement position independent parameters Localized Cost Users should only pay for what they use avoid distributed costs Violations Algol60 loops dynamic type binding 990128 P F Reynolds CS655 4 2 MacLennan s Principles 4 Manifest Interface All interfaces should be apparent manifest in the syntax Module specifications function prototypes Orthogonality Independent functions should be controlled by independent mechanisms Algol68 types Ada parameter passing 990128 P F Reynolds CS655 5 MacLennan s Principles 5 Portability Avoid features or facilities that are dependent on a particular machine or a small class of machines Ada prohibition of aliasing C Algol60 I O Preservation of Information The language should allow the representation of information that the user might know and that the compiler might need Definite looping structures 990128 P F Reynolds CS655 6 3 MacLennan s Principles 6 Regularity Regular rules without exception are easier to learn use describe and implement Violations strings in most langs Pascal functions Security No program that violates the definition of the language or its own intended structure should escape detection Strong typing in Algol60 Pascal Ada C 990128 P F Reynolds CS655 7 MacLennan s Principles 7 Simplicity A language should be as simple as possible There should be a minimum number of concepts with simple rules for their combination Pascal Ada name equivalence Structure The static structure of the program should correspond in a simple way to the dynamic structure of the corresponding computations Single entry single exit violation Pascal s upsidedown procedures first form 990128 P F Reynolds CS655 8 4 MacLennan s Principles 8 Syntactic Consistency Similar things should look similar different things different Violations in Algol68 blocks vs compound statements Ada private limited private Zero One Infinity The only reasonable numbers are zero one and infinity Array dimensions identifier sizes function nesting 990128 P F Reynolds CS655 9 More Principles Ghezzi Jazayeri Efficiency translation execution Readability Writability Reliability rigorous semantics clear distinction between static and dynamic checks modularity 990128 P F Reynolds CS655 10 5 More Principles Predictability Learnability Maintainability Verifiability Must it end in ility 990128 P F Reynolds CS655 11 More Principles Yemini Berry Wide horizon Whenever the semantics of a construct C in a language for concurrent programming implies the delay of a process task executing in C C should be able to have other alternatives and all such constructs should be able to serve as alternatives to each other Not satisfied by semaphores monitor calls or Ada select statement 990128 P F Reynolds CS655 12 6 More Principles Yemini Berry 2 Closure under binding For every concurrent program there exists a sequential possibly non deterministic program with an equivalent semantics No unitask program in Ada can simulate declare task t1 is s1 e1 end t1 task t2 is s2 e2 end t2 begin end s1 and s2 are called in arbitrary order 990128 P F Reynolds CS655 13 What s this violate COMMON a b lab 3 WRITE 6 5 5 FORMAT ASSIGN 3 TO lab CALL sub2 END SUBROUTINE sub2 COMMON c d lab GOTO lab 3 i 1 0 RETURN END 990128 P F Reynolds CS655 14 7 What s this violate 10 i 0 call doThing i i i 1 IF i n THEN GOTO 10 990128 P F Reynolds CS655 15 What s this violate i 10 s 10 s this is a string IF i 10 THEN someFunc i s 3 14159 IF s pi THEN 990128 P F Reynolds CS655 16 8 What s this violate val 7 a val 2 val 15 a val 18 990128 P F Reynolds CS655 17 What s this violate GOTO i branch to i th statement statement 1 statement 2 statement i statement n 990128 P F Reynolds CS655 18 9 What s this violate 990128 P F Reynolds CS655 19 What s this violate DIMENSION a 20 CALL subr a END SUBROUTINE subr n DIMENSION n 25 IF n 22 END 990128 P F Reynolds CS655 20 10 What s this violate i 1 WHILE i n DO BEGIN WRITE exp i i i 1 END i 1 REPEAT WRITE exp i i i 1 UNTIL i n FOR i 1 TO n 1 DO WRITE exp i 990128 P F Reynolds CS655 21 What s this violate FOR curr NULL curr temp temp curr next FREE char curr 990128 P F Reynolds CS655 22 11 What s this violate IF I 0 IF I 0 990128 P F Reynolds CS655 23 What s this violate 10 IF n 0 THEN GOTO 20 n 0 GOTO 22 20 IF n 0 THEN GOTO 30 IF m 0 Then GOTO 25 n infinity GOTO 40 25 n m 2 30 IF n 1000 THEN GOTO 40 n n 1729 GOTO 10 40 CONTINUE 990128 P F Reynolds CS655 24 12 What s this violate GOTO 10 ASSIGN 20 to n GOTO n 10 20 30 40 assigned GOTO n 3 GOTO 10 20 30 40 n computed GOTO 990128 P F Reynolds CS655 25 What s this violate WHILE inch DO READ infile inch i 1 WHILE inch DO BEGIN READ infile inch scannedText i inch i i 1 END WHILE inch DO READ infile inch 990128 P F Reynolds CS655 26 13 What s this violate I I I 1 I I 1 990128 P F Reynolds CS655 27 What s this violate TYPE stackType array 1 100 of INTEGER VAR stack stackType top integer 0 PROCEDURE PUSH object INTEGER END PUSH PROCEDURE POP VAR object INETGER END POP BEGIN main push 10 IF stack 1 END main 990128 P F Reynolds CS655 28 14 What s this violate PRINTF Address of x d n x Fails sometimes PRINTF Address of x ld n long x Succeeds 990128 P F Reynolds CS655 29 What s this violate VAR i INTEGER PROCEDURE ref VAR j INETGER VAR x INTEGER BEGIN j 2 x j i WRITELN x END ref BEGIN i 1 ref i END 990128 P F Reynolds CS655 30 15 What s this violate …