From: Christoph & Ursula Grein <[log in to unmask]> > Mark Lundquist schrieb: > > > ... This is precisely my answer to "Ada isn't portable because > > you don't even know how big an Integer is" (for instance). Portability > > doesn't mean identical behavior, it means that you can deploy under a > > different implementation of the language system without changes to the > > source text and the requirements of the system continue to be met. If I > > say (to continue the example) > > > > X: Integer; > > > > instead of using an implementation-independent integer type, this is an > > implicit statement that "I do not care about the range of values of this > > object"; that is, it has no bearing on the system requirements. You > > should be able to say that, because there are a lot of times when you > > really don't care (so for instance, the fact that you run out of > > integers sooner on some platform is no more of a "portability" concern > > than when you run out of memory or disk space! :-) > > > > > ... > > > It's all but impossible to avoid the use of Standard types; for example, > > > Integer is the index type for String (a bad decision made in Ada 83). > > > > Good point. > > > > To stray from the topic for a bit -- the low level of abstraction of > > String does seem to cause some problems, beginning in Ada95. > > > > I see a contradiction in these two statements. I do not care for the range of > Integer when I use Strings. Who would use indices such that a program runs > with 32 bit integers but doesn't for 16 bit? If such ranges are needed, it > seems to me that one would also run out of memory when using a machine > where one doesn't have 32 bits. Note that I wasn't arguing in the first statement that you shouldn't care about representations, nor in the second that you should. Sometimes you do, and sometimes you don't! It depends on the situation. Actually, what I wrote wasn't very clear, because what I had in mind was really not the index type issue Ben brought up. If you don't care about the index base type of Standard.String, that's fine. If you do care, you can define a string type with the index type you want. Literals, and all the predefined string type operations from RM 3.6.3, will be defined for it, and it will be type-convertible to Standard.String... > And where in Ada95 does the 'low level abstraction' lead to problems? > What would be a 'high level abstraction'? We have Ada.Strings.Fixed, > -Bounded, -Unbounded. Exactly! The operations defined in these packages are virtually the same, suggesting that they represent different implementations of the same abstraction -- But there's no type that expresses the abstraction itself. Instead, there are four different types expressing 4 different implementations of the abstraction (times two counting the Wide_ variants, which are all replicated again). Ada really has two families of string types: the predefined string type family of RM 3.6.3, and the Fixed/Bounded/Unbounded family (including the Wide_ variants). The two families are unrelated, except that they overlap on Standard.String/Fixed (and Standard.Wide_String/Wide_Fixed). One family defines the types in terms of their representations (low-level) and the other defines them in terms of their operations (high-level, but without any factoring). Ada95 was constrained by the Ada83 definition of String. But even aside from that, isn't it nice that String is publicly an array type, so you can index and slice it? Would that have been worth trading away to unify the string type families? I don't know. It's tempting to say Ada, like C++, should provide user-defined indexability (and slicability), maybe using an attribute-definition-clause. But then, you would have to invent new syntax for declaring the public view of the type to have array-like properties, so that the compiler knows an indexed component or slice when it sees one (in C++ it's not a problem because the '[]' operator is its own syntax). Worse yet, Ada would have to somehow follow C++ into the morass of function calls being lvalues, i.e. functions returning variables instead of values. I don't know if the current state of affairs is pragmatically that big of a deal, other than just cluttering up Annex A. You can always write statically polymorphic string methods using generics... > [To me, some people seem to be very paranoid about predefined types. > Don't you dare write "type Something is array (1..10);" - that's bad and > not portable; "type Something is array (Integer_8 range 1..10);" however > is very good and portable.] > Right, I totally agree with you! Dogmatism of that kind is often a workaround for cluelessness. The point is, know what you care about and why, and then know how to express that in language you're programming in. > --------- __o __o /\_ _ \\o (_)\__/o (_) > ------- _`\<,_ _`\<,_ _>(_) (_)/<_ \_| \ _|/' \/ > ------ (_)/ (_) (_)/ (_) (_) (_) (_) (_)' _\o_ > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > Mein Radunfall > My bike accident Yikes! > Christoph Grein -- Mark Lundquist Senior Software Engineer Rational Software Development Solutions Business Unit UNIX Suites Group Aloha, OR, USA