From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me X-Spam-Level: ** X-Spam-Status: No, score=2.1 required=5.0 tests=BAYES_00,REPLYTO_WITHOUT_TO_CC, TO_NO_BRKTS_PCNT autolearn=no autolearn_force=no version=3.4.4 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f7ded,1b5bd95f6fbb3cd,start X-Google-Attributes: gidf7ded,public X-Google-Thread: 100062,1b5bd95f6fbb3cd,start X-Google-Attributes: gid100062,public X-Google-Thread: 103376,1b5bd95f6fbb3cd,start X-Google-Attributes: gid103376,public From: Magnus.Kempe@di.epfl.ch (Magnus Kempe) Subject: Ada FAQ: Programming with Ada (part 2 of 3) Date: 1995/04/20 Message-ID: <3n609u$8gd@disunms.epfl.ch> X-Deja-AN: 101258328 distribution: world followup-to: poster content-type: text/plain; charset=iso-8859-1 summary: Ada Programmer's Frequently Asked Questions (and answers), organization: None keywords: advanced language, artificial languages, computer software, mime-version: 1.0 reply-to: Magnus.Kempe@di.epfl.ch (Magnus Kempe) newsgroups: comp.lang.ada,comp.answers,news.answers Date: 1995-04-20T00:00:00+00:00 List-Id: Archive-name: computer-lang/Ada/programming/part2 Comp-lang-ada-archive-name: programming/part2 Posting-Frequency: monthly Last-modified: 20 April 1995 Last-posted: 21 March 1995 Ada Programmer's Frequently Asked Questions (FAQ) IMPORTANT NOTE: No FAQ can substitute for real teaching and documentation. There is an annotated list of Ada books in the companion comp.lang.ada FAQ. Recent changes to this FAQ are listed in the first section after the table of contents. This document is under explicit copyright. This is part 2 of a 3-part posting. Part 3 begins with question 8.5; it should be the next posting in this thread. Part 1 should be the previous posting in this thread. 5.6: What do "covariance" and "contravariance" mean, and does Ada support either or both? (From Robert Martin) [This is C++ stuff, it must be completely re-written for Ada. --MK] R> covariance: "changes with" R> contravariance: "changes against" R> class A R> { R> public: R> A* f(A*); // method of class A, takes A argument and returns A R> A* g(A*); // same. R> }; R> class B : public A // class B is a subclass of class A R> { R> public: R> B* f(B*); // method of class B overrides f and is covariant. R> A* g(A*); // method of class B overrides g and is contravariant. R> }; R> The function f is covariant because the type of its return value and R> argument changes with the class it belongs to. The function g is R> contravariant because the types of its return value and arguments does not R> change with the class it belongs to. Actually, I would call g() invariant. If you look in Sather, (one of the principle languages with contravariance), you will see that the method in the decendent class actually can have aruments that are superclasses of the arguments of its parent. So for example: class A : public ROOT { public: A* f(A*); // method of class A, takes A argument and returns A A* g(A*); // same. }; class B : public A // class B is a subclass of class A { public: B* f(B*); // method of class B overrides f and is covariant. ROOT* g(ROOT*); // method of class B overrides g and is contravariant. }; To my knowledge the uses for contravariance are rare or nonexistent. (Anyone?). It just makes the rules easy for the compiler to type check. On the other hand, co-variance is extremely useful. Suppose you want to test for equality, or create a new object of the same type as the one in hand: class A { public: BOOLEAN equal(A*); A* create(); } class B: public A { public: BOOLEAN equal(B*); B* create(); } Here covariance is exactly what you want. Eiffel gives this to you, but the cost is giving up 100% compile time type safety. This seem necessary in cases like these. In fact, Eiffel gives you automatic ways to make a method covariant, called "anchored types". So you could declare, (in C++/eiffese): class A { public: BOOLEAN equal(like Current *); like Current * create(); } Which says equal takes an argument the same type as the current object, and create returns an object of the same type as current. Now, there is not even any need to redeclare these in class B. Those transformations happen for free! 5.7: What is meant by upcasting/expanding and downcasting/narrowing? (Tucker Taft replies): Here is the symmetric case to illustrate upcasting and downcasting. type A is tagged ...; -- one parent type type B is tagged ...; -- another parent type ... type C; -- the new type, to be a mixture of A and B type AC (Obj : access C'Class) is new A with ...; -- an extension of A to be mixed into C type BC (Obj : access C'Class) is new B with ...; -- an extension of B to be mixed into C type C is tagged limited record A : AC (C'Access); B : BC (C'Access); ... -- other stuff if desired end record; We can now pass an object of type C to anything that takes an A or B as follows (this presumes that Foobar and QBert are primitives of A and B, respectively, so they are inherited; if not, then an explicit conversion (upcast) to A and B could be used to call the original Foobar and QBert). XC : C; ... Foobar (XC.A); QBert (XC.B); If we want to override what Foobar does, then we override Foobar on AC. If we want to override what QBert does, then we override QBert on BC. Note that there are no naming conflicts, since AC and BC are distinct types, so even if A and B have same-named components or operations, we can talk about them and/or override them individually using AC and BC. Upcasting (from C to A or C to B) is trivial -- A(XC.A) upcasts to A; B(XC.B) upcasts to B. Downcasting (narrowing) is also straightforward and safe. Presuming XA of type A'Class, and XB of type B'Class: AC(XA).Obj.all downcasts to C'Class (and verifies XA in AC'Class) BC(XB).Obj.all downcasts to C'Class (and verifies XB in BC'Class) You can check before the downcast to avoid a Constraint_Error: if XA not in AC'Class then -- appropriate complaint if XB not in BC'Class then -- ditto The approach is slightly simpler (though less symmetric) if we choose to make A the "primary" parent and B a "secondary" parent: type A is ... type B is ... type C; type BC (Obj : access C'Class) is new B with ... type C is new A with record B : BC (C'Access); ... -- other stuff if desired end record; Now C is a "normal" extension of A, and upcasting from C to A and (checked) downcasting from C'Class to A (or A'Class) is done with simple type conversions. The relationship between C and B is as above in the symmetric approach. Not surprisingly, using building blocks is more work than using a "builtin" approach for simple cases that happen to match the builtin approach, but having building blocks does ultimately mean more flexibility for the programmer -- there are many other structures that are possible in addition to the two illustrated above, using the access discriminant building block. For example, for mixins, each mixin "flavor" would have an access discriminant already: type Window is ... -- The basic "vanilla" window -- Various mixins type Win_Mixin_1 (W : access Window'Class) is ... type Win_Mixin_2 (W : access Window'Class) is ... type Win_Mixin_3 (W : access Window'Class) is ... Given the above vanilla window, plus any number of window mixins, one can construct a desired window by including as many mixins as wanted: type My_Window is new Window with record M1 : Win_Mixin_1 (My_Window'access); M3 : Win_Mixin_3 (My_Window'access); M11 : Win_Mixin_1(My_Window'access); ... -- plus additional stuff, as desired. end record; As illustrated above, you can incorporate the same "mixin" multiple times, with no naming conflicts. Every mixin can get access to the enclosing object. Operations of individual mixins can be overridden by creating an extension of the mixin first, overriding the operation in that, and then incorporating that tweaked mixin into the ultimate window. I hope the above helps better illustrate the use and flexibility of the Ada 9X type composition building blocks. 5.8: How does Ada do "narrowing"? Dave Griffith said . . . Nonetheless, The Ada9x committee chose a structure-based subtyping, with all of the problems that that is known to cause. As the problems of structure based subtyping usually manifest only in large projects maintained by large groups, this is _precisely_ the subtype paradigm that Ada9x should have avoided. Ada9x's model is, as Tucker Taft pointed out, quite easy to use for simple OO programming. There is, however, no good reason to _do_ simple OO programming. OO programmings gains click in somewhere around 10,000 LOC, with greatest gains at over 100,000. At these sizes, "just declare it tagged" will result in unmaintainable messes. OO programming in the large rapidly gets difficult with structure based subtyping. Allowing by-value semantics for objects compounds these problems. All of this is known. All of this was, seemingly, ignored by Ada9x. (Tucker Taft answers) As explained in a previous note, Ada 9X supports the ability to hide the implementation heritage of a type, and only expose the desired interface heritage. So we are not stuck with strictly "structure-based subtyping." Secondly, by-reference semantics have many "well known" problems as well, and the designers of Modula-3 chose to, seemingly, ignore those ;-) ;-). Of course, in reality, neither set of language designers ignored either of these issues. Language design involves tradeoffs. You can complain we made the wrong tradeoff, but to continue to harp on the claim that we "ignored" things is silly. We studied every OOP language under the sun on which we could find any written or electronic material. We chose value-based semantics for what we believe are good reasons, based on reasonable tradeoffs. First of all, in the absence of an integrated garbage collector, by-reference semantics doesn't make much sense. Based on various tradeoffs, we decided against requiring an integrated garbage collector for Ada 9X. Secondly, many of the "known" problems with by-value semantics we avoided, by eliminating essentially all cases of "implicit truncation." One of the problems with the C++ version of "value semantics" is that on assignment and parameter passing, implicit truncation can take place mysteriously, meaning that a value that started its life representing one kind of thing gets truncated unintentionally so that it looks like a value of some ancestor type. This is largely because the name of a C++ class means differnt things depending on the context. When you declare an object, the name of the class determines the "exact class" of the object. The same thing applies to a by-value parameter. However, for references and pointers, the name of a class stands for that class and all of its derivatives. But since, in C++, a value of a subclass is always acceptable where a value of a given class is expected, you can get implicit truncation as part of assignment and by-value parameter passing. In Ada 9X, we avoid the implicit truncation because we support assignment for "class-wide" types, which never implicitly truncates, and one must do an explicit conversion to do an assignment that truncates. Parameter passing never implicitly truncates, even if an implicit conversion is performed as part of calling an inherited subprogram. In any case, why not either ignore Ada 9X or give it a fair shot? It is easy to criticize any particular design decision, but it is much harder to actually put together a complete integrated language design that meets the requirements of its user community, doesn't bankrupt the vendor community, and provides interesting fodder for the academic community ;-). _________________________________________________________________ 6: Ada Numerics 6.1: Where can I find anonymous ftp sites for Ada math packages? In particular where are the random number generators? ftp.rational.com Freeware version of the ISO math packages on Rational's FTP server. It's a binding over the C Math library, in public/apex/freeware/math_lib.tar.Z archimedes.nosc.mil Stuff of high quality in pub/ada The random number generator and random deviates are recommended. These are mirrored at the next site, wuarchive. wuarchive.wustl.edu Site of PAL, the Public Ada Library: math routines scattered about in the directories under languages/ada in particular, in subdirectory swcomps source.asset.com This is not an anonymous ftp site for math software. What you should do is log on anonymously under ftp, and download the file asset.faq from the directory pub. This will tell you how to get an account. ftp.cs.kuleuven.ac.be Go to directory pub/Ada-Belgium/cdrom. There's a collection of math intensive software in directory swcomps. Mirrors some of PAL at wuarchive.wustl.edu. sw-eng.falls-church.va.us Go to directory public/adaic/tools/atip/adar to find extended-precision decimal arithmetic (up to 18 digits). Includes facilities for COBOL-like formatted output. 6.2: How can I write portable code in Ada 83 using predefined types like Float and Long_Float? Likewise, how can I write portable code that uses Math functions like Sin and Log that are defined for Float and Long_Float? (from Jonathan Parker) Ada 83 was slow to arrive at a standard naming convention for elementary math functions and complex numbers. Furthermore, you'll find that some compilers call the 64-bit floating point type Long_Float; other compilers call it Float. Fortunately, it is easy to write programs in Ada that are independent of the naming conventions for floating point types and independent of the naming conventions of math functions defined on those types. One of the cleanest ways is to make the program generic: generic type Real is digits <>; with function Arcsin (X : Real) return Real is <>; with function Log (X : Real) return Real is <>; -- This is the natural log, inverse of Exp(X), sometimes written Ln(X). package Example_1 is ... end Example_1; So the above package doesn't care what the name of the floating point type is, or what package the Math functions are defined in, just as long as the floating point type has the right attributes (precision and range) for the algorithm, and likewise the functions. Everything in the body of Example_1 is written in terms of the abstract names, Real, Arcsin, and Log, even though you instantiate it with compiler specific names that can look very different: package Special_Case is new Example_1 (Long_Float, Asin, Ln); The numerical algorithms implemented by generics like Example_1 can usually be made to work for a range of floating point precisions. A well written program will perform tests on Real to reject instantiations of Example_1 if the floating points type is judged inadequate. The tests may check the number of digits of precision in Real (Real'Digits) or the range of Real (Real'First, Real'Last) or the largest exponent of the set of safe numbers (Real'Safe_Emax), etc. These tests are often placed after the begin statement of package body, as in: package body Example_1 is ... begin if (Real'Machine_Mantissa > 60) or (Real'Machine_Emax < 256) then raise Program_Error; end if; end Example_1; Making an algorithm as abstract as possible, (independent of data types as much as possible) can do a lot to improve the quality of the code. Support for abstraction is one of the many things Ada-philes find so attractive about the language. The designers of Ada 95 recognized the value of abstraction in the design of numeric algorithms and have generalized many of the features of the '83 model. For example, no matter what floating point type you instantiate Example_1 with, Ada 95 provides you with functions for examining the exponent and the mantissas of the numbers, for truncating, determining exact remainders, scaling exponents, and so on. (In the body of Example_1, and in its spec also of course, these functions are written, respectively: Real'Exponent(X), Real'Fraction(X), Real'Truncation(X), Real'Remainder(X,Y), Real'Scaling(X, N). There are others.) Also, in package Example_1, Ada 95 lets you do the arithmetic on the base type of Real (called Real'Base) which is liable to have greater precision and range than type Real. It is rare to see a performance loss when using generics like this. However, if there is an unacceptable performance hit, or if generics cannot be used for some other reason, then subtyping and renaming will do the job. Here is an example of renaming: with Someones_Math_Lib; procedure Example_2 is subtype Real is Long_Float; package Math renames Someones_Math_Lib; function Arcsin(X : Real) return Real renames Math.Asin function Log (X : Real) return Real renames Math. Ln; -- Everything beyond this point is abstract with respect to -- the names of the floating point (Real), the functions (Arcsin -- and Log), and the package that exported them (Math). ... end Example_2; I prefer to make every package and subprogram (even test procedures) as compiler independent and machine portable as possible. To do this you move all of the renaming of compiler dependent functions and all of the "withing" of compiler dependent packages to a single package. In the example that follows, its called Math_Lib_8. Math_Lib_8 renames the 8-byte floating point type to Real_8, and makes sure the math functions follow the Ada 95 standard, at least in name. In this approach Math_Lib_8 is the only compiler dependent component. There are other, perhaps better, ways also. See for example, "Ada In Action", by Do-While Jones for a generic solution. Here's the spec of Math_Lib_8, which is a perfect subset of package Math_Env_8, available by FTP in file ftp://lglftp.epfl.ch/pub/Ada/FAQ/math_env_8.ada --*************************************************************** -- Package Math_Lib_8 -- -- A minimal math package for Ada 83: creates a standard interface to vendor -- specific double-precision (8-byte) math libraries. It renames the 8 byte -- Floating point type to Real_8, and uses renaming to create -- (Ada 95) standard names for Sin, Cos, Log, Sqrt, Arcsin, Exp, -- and Real_8_Floor, all defined for Real_8. -- -- A more ambitious but perhaps less efficient -- package would wrap the compiler specific functions in function calls, and -- do error handling on the arguments to Ada 95 standards. -- -- The package assumes that Real_8'Digits > 13, and that -- Real_8'Machine_Mantissa < 61. These are asserted after the -- begin statement in the body. -- -- Some Ada 83 compilers don't provide Arcsin, so a rational-polynomial+ -- Newton-Raphson method Arcsin and Arccos pair are provided in the body. -- -- Some Ada 83 compilers don't provide for truncation of 8 byte floats. -- Truncation is provided here in software for Compilers that don't have it. -- The Ada 95 function for truncating (toward neg infinity) is called 'Floor. -- -- The names of the functions exported below agree with the Ada9X standard, -- but not, in all likelihood the semantics. It is up to the user to -- be careful...to do his own error handling on the arguments, etc. -- The performance of these function can be non-portable, -- but in practice they have their usual meanings unless you choose -- weird arguments. The issues are the same with most math libraries. --*************************************************************** --with Math_Lib; -- Meridian DOS Ada. with Long_Float_Math_Lib; -- Dec VMS --with Ada.Numerics.Generic_Elementary_Functions; -- Ada9X package Math_Lib_8 is --subtype Real_8 is Float; -- Meridian 8-byte Real subtype Real_8 is Long_Float; -- Dec VMS 8-byte Real --package Math renames Math_Lib; -- Meridian DOS Ada package Math renames Long_Float_Math_Lib; -- Dec VMS --package Math is new Ada.Numerics.Generic_Elementary_Functions(Real_8); -- The above instantiation of the Ada.Numerics child package works on -- GNAT, or any other Ada 95 compiler. Its here if you want to use -- an Ada 95 compiler to compile Ada 83 programs based on this package. function Cos (X : Real_8) return Real_8 renames Math.Cos; function Sin (X : Real_8) return Real_8 renames Math.Sin; function Sqrt(X : Real_8) return Real_8 renames Math.Sqrt; function Exp (X : Real_8) return Real_8 renames Math.Exp; --function Log (X : Real_8) return Real_8 renames Math.Ln; -- Meridian function Log (X : Real_8) return Real_8 renames Math.Log; -- Dec VMS --function Log (X : Real_8) return Real_8 renames Math.Log; -- Ada 95 --function Arcsin (X : Real_8) return Real_8 renames Math.Asin; -- Dec VMS --function Arcsin (X : Real_8) return Real_8 renames Math.Arcsin; -- Ada 95 function Arcsin (X : Real_8) return Real_8; -- Implemented in the body. Should work with any compiler. --function Arccos (X : Real_8) return Real_8 renames Math.Acos; -- Dec VMS --function Arccos (X : Real_8) return Real_8 renames Math.Arccos; -- Ada 95 function Arccos (X : Real_8) return Real_8; -- Implemented in the body. Should work with any compiler. --function Real_8_Floor (X : Real_8) return Real_8 renames Real_8'Floor;-- 95 function Real_8_Floor (X : Real_8) return Real_8; -- Implemented in the body. Should work with any compiler. end Math_Lib_8; 6.3: Is Ada any good at numerics, and where can I learn more about it? First of all, a lot of people find the general Ada philosophy (modularity, strong-typing, readable syntax, rigorous definition and standardization, etc.) to be a real benefit in numerical programming, as well as in many other types of programming. Further, Ada--and especially Ada 95--was designed to also meet the special requirements of number-crunching applications. The following sketches out some of these features. Hopefully a little of the flavor of the Ada philosophy will get through, but the best thing you can do at present is to read the two standard reference documents, the Ada 95 Rationale and Reference Manual. 1. Machine portable floating point declarations. (Ada 83 and Ada 95) If you declare "type Real is digits 14", then type Real will guarantee you (at least) 14 digits of precision independently of machine or compiler. In this case the base type of type Real will usually be the machine's 8-byte floating point type. If an appropriate base type is unavailable (very rare), then the declaration is rejected by the compiler. 2. Extended precision for initialization of floating point. (Ada 83 and Ada 95) Compilers are required to employ extended-precision/rational-arithmetic routines so that floating point variables and constants can be correctly initialized to their full precision. 3. Generic packages and subprograms. (Ada 83 and Ada 95) Algorithms can be written so that they perform on abstract representations of the data structure. Support for this is provided by Ada's generic facilities (what C++ programmers would call templates). 4. User-defined operators and overloaded subprograms. (Ada 83 and Ada 95) The programmer can define his own operators (functions like "*", "+", "abs", "xor", "or", etc.) and define any number of subprograms with the same name (provided they have different argument profiles). 5. Multitasking. (Ada 83 and Ada 95) Ada facilities for concurrent programming (multitasking) have traditionally found application in simulations and distributed/parallel programming. Ada tasking is an especially useful ingredient in the Ada 95 distributed programming model, and the combination of the two makes it possible to design parallel applications that have a high degree of operating system independence and portability. (More on this in item 6 below.) 6. Direct support for distributed/parallel computing in the language. (Ada 95) Ada 95 is probably the first internationally standardized language to combine in the same design complete facilities for multitasking and parallel programming. Communication between the distributed partitions is via synchronous and asynchronous remote procedure calls. Good discussion, along with code examples, is found in the Rationale, Part III E, and in the Ada 95 Reference Manual, Annex E. See also "Ada Letters", Vol. 13, No. 2 (1993), pp. 54 and 78, and Vol. 14, No. 2 (1994), p. 80. (Full support for these features is provided by compilers that conform to the Ada 95 distributed computing Annex. This conformance is optional, but for instance GNAT, the Gnu Ada 95 compiler, will meet these requirements.) 7. Attributes of floating point types. (Ada 83 and Ada 95) For every floating point type (including user defined types), there are built-in functions that return the essential characteristics of the type. For example, if you declare "type Real is digits 15" then you can get the max exponent of objects of type Real from Real'Machine_Emax. Similarly, the size of the Mantissa, the Radix, the largest Real, and the Rounding policy of the arithmetic are given by Real'Machine_Mantissa, Real'Machine_Radix, Real'Last, and Real'Machine_Rounds. There are many others. (See Ada 95 Reference Manual, clause 3.5, subclause 3.5.8 and A.5.3, as well as Part III sections G.2 and G.4.1 of the Ada 95 Rationale.) 8. Attribute functions for floating point types. (Ada 95) For every floating point type (including user defined types), there are built-in functions that operate on objects of that type. For example, if you declare "type Real is digits 15" then Real'Remainder (X, Y) returns the exact remainder of X and Y: X - n*Y where n is the integer nearest X/Y. Real'Truncation(X), Real'Max(X,Y), Real'Rounding(X) have the usual meanings. Real'Fraction(X) and Real'Exponent(X) break X into mantissa and exponent; Real'Scaling(X, N) is exact scaling: multiplies X by Radix**N, which can be done by incrementing the exponent by N, etc. (See citations in item 7.) 9. Modular arithmetic on integer types. (Ada 95) If you declare "type My_Unsigned is mod N", for arbitrary N, then arithmetic ("*", "+", etc.) on objects of type My_Unsigned returns the results modulo N. Boolean operators "and", "or", "xor", and "not" are defined on the objects as though they were arrays of bits (and likewise return results modulo N). For N a power of 2, the semantics are similar to those of C unsigned types. 10. Generic elementary math functions for floating point types. (Ada 95) Required of all compilers, and provided for any floating point type: Sqrt, Cos, Sin, Tan, Cot, Exp, Sinh, Cosh, Tanh, Coth, and the inverse functions of each of these, Arctan, Log, Arcsinh, etc. Also, X**Y for floating point X and Y. Compilers that conform to the Numerics Annex meet additional accuracy requirements. (See subclause A.5.1 of the Ada 95 RM, and Part III, Section A.3 of the Ada 95 Rationale.) 11. Complex numbers. (Ada 95) Fortran-like, but with a new type called Imaginary. Type "Imaginary" allows programmers to write expressions in such a way that they are easier to optimize, more readable and appear in code as they appear on paper. Also, the ability to declare object of pure imaginary type reduces the number of cases in which premature type conversion of real numbers to complex causes floating point exceptions to occur. (Provided by compilers that conform to the Numerics Annex. The Gnu Ada 95 compiler supports this annex, so the source code is freely available.) 12. Generic elementary math functions for complex number types. (Ada 95) Same functions supported for real types, but with complex arguments. Standard IO is provided for floating point types and Complex types. (Only required of compilers that support the Numerics Annex, like Gnu Ada.) 13. Pseudo-random numbers for discrete and floating point types. (Ada 95) A floating point pseudo-random number generator (PRNG) provides output in the range 0.0 .. 1.0. Discrete: A generic PRNG package is provided that can be instantiated with any discrete type: Boolean, Integer, Modular etc. The floating point PRNG package and instances of the (discrete) PRNG package are individually capable of producing independent streams of random numbers. Streams may be interrupted, stored, and resumed at later times (generally an important requirement in simulations). In Ada it is considered important that multiple tasks, engaged for example in simulations, have easy access to independent streams of pseudo random numbers. The Gnu Ada 95 compiler provides the cryptographically secure X**2 mod N generator of Blum, Blum and Shub. (See subclause A.5.2 of the Ada 95 Reference Manual, and part III, section A.3.2 of the Ada Rationale.) 14. Well-defined interfaces to Fortran and other languages. (Ada 83 and Ada 95) It has always been a basic requirement of the language that it provide users a way to interface Ada programs with foreign languages, operating system services, GUI's, etc. Ada can be viewed as an interfacing language: its module system is composed of package specifications and separate package bodies. The package specifications can be used as strongly-type interfaces to libraries implemented in foreign languages, as well as to package bodies written in Ada. Ada 95 extends on these facilities with package interfaces to the basic data structures of C, Fortran, and COBOL and with new pragmas. For example, "pragma Convention(Fortran, M)" tells the compiler to store the elements of matrix M in the Fortran column-major order. (This pragma has already been implemented in the Gnu Ada 95 compiler. Multi- lingual programming is also a basic element of the Gnu compiler project.) As a result, assembly language BLAS and other high performance linear algebra and communications libraries will be accessible to Ada programs. (See Ada 95 Reference Manual: clause B.1 and B.5 of Annex B, and Ada 95 Rationale: Part III B.) 6.4: How do I get Real valued and Complex valued math functions in Ada 95? (from Jonathan Parker) Complex type and functions are provided by compilers that support the numerics Annex. The packages that use Float for the Real number and for the Complex number are: Ada.Numerics.Elementary_Functions; Ada.Numerics.Complex_Types; Ada.Numerics.Complex_Elementary_Functions; The packages that use Long_Float for the Real number and for the Complex number are: Ada.Numerics.Long_Elementary_Functions; Ada.Numerics.Long_Complex_Types; Ada.Numerics.Long_Complex_Elementary_Functions; The generic versions are demonstrated in the following example. Keep in mind that the non-generic packages may have been better tuned for speed or accuracy. In practice you won't always instantiate all three packages at the same time, but here is how you do it: with Ada.Numerics.Generic_Complex_Types; with Ada.Numerics.Generic_Elementary_Functions; with Ada.Numerics.Generic_Complex_Elementary_Functions; procedure Do_Something_Numerical is type Real_8 is digits 15; package Real_Functions_8 is new Ada.Numerics.Generic_Elementary_Functions (Real_8); package Complex_Nums_8 is new Ada.Numerics.Generic_Complex_Types (Real_8); package Complex_Functions_8 is new Ada.Numerics.Generic_Complex_Elementary_Functions (Complex_Nums_8); use Real_Functions_8, Complex_Nums_8, Complex_Functions_8; ... ... -- Do something ... end Do_Something_Numerical; 6.5: What libraries or public algorithms exist for Ada? An Ada version of Fast Fourier Transform is available. It's in journal "Computers & Mathematics with Applications," vol. 26, no. 2, pp. 61-65, 1993, with the title: "Analysis of an Ada Based Version of Glassman's General N Point Fast Fourier Transform" The package is now available in the AdaNET repository, object #: 6728, in collection: Transforms. If you're not an AdaNET user, contact Peggy Lacey (lacey@rbse.mountain.net). _________________________________________________________________ 7: Efficiency of Ada Constructs 7.1: How much extra overhead do generics have? If you overgeneralize the generic, there will be more work to do for the compiler. How do you know when you have overgeneralized? For instance, passing arithmetic operations as parameters is a bad sign. So are boolean or enumeration type generic formal parameters. If you never override the defaults for a parameter, you probably overengineered. Code sharing (if implemented and requested) will cause an additional overhead on some calls, which will be partially offset by improved locality of reference. (Translation, code sharing may win most when cache misses cost most.) If a generic unit is only used once in a program, code sharing always loses. R.R. Software chose code sharing as the implementation for generics because 2 or more instantiations of Float_Io in a macro implementation would have made a program too large to run in the amount of memory available on the PC machines that existed in 1983 (usually a 128k or 256k machine). Generics in Ada can also result in loss of information which could have helped the optimizer. Since the compiler is not restricted by Ada staticness rules within a single module, you can often avoid penalties by declaring (or redeclaring) bounds so that they are local: package Global is subtype Global_Int is Integer range X..Y; ... end Global; with Global; package Local is subtype Global_Int is Global.Global_Int; package Some_Instance is new Foo (Global_Int); ... end Local; Ada rules say that having the subtype redeclared locally does not affect staticness, but on a few occasions optimizers have been caught doing a much better job. Since optimizers are constantly changing, they may have been caught just at the wrong time. _________________________________________________________________ 8: Advanced Programming Techniques with Ada 8.1: Does Ada have automatic constructors and destructors? (Tucker Taft replies) At least in Ada 9X, functions with controlling results are inherited (even if overriding is required), allowing their use with dynamic binding and class-wide types. In most other OOPs, constructors can only be called if you know at compile time the "tag" (or equivalent) of the result you want. In Ada 9X, you can use the tag determined by the context to control dispatching to a function with a controlling result. For example: type Set is abstract tagged private; function Empty return Set is abstract; function Unit_Set(Element : Element_Type) return Set is abstract; procedure Remove(S : in out Set; Element : out Element_Type) is abstract; function Union(Left, Right : Set) return Set is abstract; ... procedure Convert(Source : Set'Class; Target : out Set'Class) is -- class-wide "convert" routine, can convert one representation -- of a set into another, so long as both set types are -- derived from "Set," either directly or indirectly. -- Algorithm: Initialize Target to the empty set, and then -- copy all elements from Source set to Target set. Copy_Of_Source : Set'Class := Source; Element : Element_Type; begin Target := Empty; -- Dispatching for Empty determined by Target'Tag. while Copy_Of_Source /= Empty loop -- Dispatching for Empty based on Copy_Of_Source'Tag Remove_Element(Copy_Of_Source, Element); Target := Union(Target, Unit_Set(Element)); -- Dispatching for Unit_Set based on Target'Tag end loop; end Convert; The functions Unit_Set and Empty are essentially "constructors" and hence must be overridden in every extension of the abstract type Set. However, these operations can still be called with a class-wide expected type, and the controlling tag for the function calls will be determined at run-time by the context, analogous to the kind of (compile-time) overload resolution that uses context to disambiguate enumeration literals and aggregates. 8.2: How can I redefine assignment operations? See "Tips and Tidbits #1: User Defined Assignment" by Brad Balfour (where is this located?) 8.3: Should I stick to a one package, one type approach while writing Ada software? (Robb Nebbe responds) Offhand I can think of a couple of advantages arising from Ada's separation of the concepts of type and module. Separation of visibility and inheritance allows a programmer to isolate a derived type from the implementation details of its parent. To put it another way information hiding becomes a design decision instead of a decision that the programming language has already made for you. Another advantage that came "for free" is the distinction between subtyping and implementation inheritance. Since modules and types are independent concepts the interaction of the facilities for information hiding already present in Ada83 with inheritance provide an elegant solution to separating subtyping from implementation inheritance. (In my opinion more elegant than providing multiple forms of inheritance or two distinct language constructs.) 8.4: What is the "Beaujolais Effect"? The "Beaujolais Effect" is detrimental, and language designers should try to avoid it. But what is it? (from Tucker Taft) The term "Beaujolais Effect" comes from a prize (a bottle of Beaujolais) offered by Jean Ichbiah during the original Ada design process to anyone who could find a situation where adding or removing a single "use" clause could change a program from one legal interpretation to a different legal interpretation. (Or equivalently, adding or removing a single declaration from a "use"d package.) At least one bottle was awarded, and if the offer was still open, a few more might have been awarded during the Ada 9X process. However, thanks to some very nice analysis by the Ada 9X Language Precision Team (based at Odyssey Research Associates) we were able to identify the remaining cases of this effect in Ada 83, and remove them as part of the 9X process. The existing cases in Ada 83 had to do with implicit conversion of expressions of a universal type to a non-universal type. The rules in Ada 9X are subtly different, making any case that used to result in a Beaujolais effect in Ada 83, illegal (due to ambiguity) in Ada 9X. The Beaujolais effect is considered "harmful" because it is expected that during maintenance, declarations may be added or removed from packages without being able to do an exhaustive search for all places where the package is "use"d. If there were situations in the language which resulted in Beaujolais effects, then certain kinds of changes in "use"d packages might have mysterious effects in unexpected places. (from Jean D. Ichbiah) It is worth pointing that many popular languages have Beaujolais effect: e.g. the Borland Pascal "uses" clause, which takes an additive, layer-after-layer, interpretation of what you see in the used packages (units) definitely exhibits a Beaujolais effect. Last time I looked at C++, my impression was that several years of Beaujolais vintage productions would be required. For component-based software development, such effects are undesirable since your application may stop working when you recompile it with the new -- supposedly improved -- version of a component.