Партнёры:
Хостинг:
Archive-name: computer-lang/Ada/programming/part3
Comp-lang-ada-archive-name: programming/part3
Posting-Frequency: monthly
Last-modified: 22 May 1996
Last-posted: 23 April 1996
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 3 of a 4-part posting; part 1 contains the table of contents.
Part 2 begins with question 5.
Part 4 begins with question 9.
Parts 1 and 2 should be the previous postings in this thread.
Part 4 should be the next posting in this thread.
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/source-code/bindings/ADAR-bindings
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. But Ada --and
especially Ada 95-- was also designed to 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. Below the GNU
Ada 95 compiler is referred to several times. This compiler can be
obtained by anonymous FTP from cs.nyu.edu, and at mirror sites
declared in the README file of directory pub/gnat.
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 matrices of type 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 ([email protected]).
_________________________________________________________________
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.
7.2: How does Ada compare to other languages in efficiency of code?
Ada vs. C: An analysis at Tartan found that Ada and C had fairly
similar performance, with Ada having a slight edge. See "C vs. Ada:
Arguing Performance Religion" by David Syiek, ACM Ada Letters, Nov/Dec
1995 (Volume XV Number 6), pp. 67-69.
Ada vs. assembly language: There is a documented case where an Ada
compiler and a novice Ada programmer did better than experienced
assembly language programmers. See "Ada Whips Assembly" by Elam and
Lawlis, Crosstalk, March 1992. Published by the Software Technology
Support Center, Hill Air Force Base, Utah: Defense Printing Service.
_________________________________________________________________
8: Advanced Programming Techniques with Ada
8.1: How can I redefine the assignment operation?
The general answer is: use controlled types (RM95-7.6).
For detailed explanations, read the following papers:
* "Tips and Tidbits #1: User Defined Assignment" by Brad Balfour,
HTML at http://www.acm.org/~bbalfour/tips_no_1.html
* "Abstract Data Types Are Under Full Control with Ada 9X" by Magnus
Kempe, Postscript file at
http://lglwww.epfl.ch/Ada/Resources/Papers/OO/ADT_Control-revised.ps
8.2: Does Ada have automatic constructors and destructors?
Yes, controlled types have special, user-definable operations that
control the construction and destruction of objects and values of
those types (see question 8.1, above).
(Also: 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.
8.3: 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.
8.5: What about the "Ripple Effect"?
(Tucker Taft explains)
We have eliminated all remnants of the Beaujolais Effect, but we did
debate various instances of the "Ripple" effect during the language
revision process (apologies to Gallo Ripple Wine enthusiasts ;-).
In brief, the (undesirable) Ripple effect was related to whether the
legality of a compilation unit could be affected by adding or removing
an otherwise unneeded "with" clause on some compilation unit on which
the unit depended, directly or indirectly.
This issue came up at least twice. One when we were considering rules
relating to use of attributes like 'Address. In Ada 83 as interpreted
by the ARG, if a compilation unit contains a use of 'Address, then
there must be a "with" of package System somewhere in the set of
library unit specs "with"ed by the compilation unit (directly or
indirectly).
In Ada 9X, we have eliminated this rule, as it was for some compilers
an unnecessary implementation burden, and didn't really provide any
value to the user (if anything, it created some confusion). The rule
now is that the use of an attibute that returns a value of some
particular type makes the compilation unit semantically dependent on
the library unit in which the type is declared (whether or not it is
"with"ed).
The second place the Ripple effect came up was when we were trying to
provide automatic direct visibility to (primitive) operators.
Ultimately we ended up with an explicit "use type" clause for making
operators directly visible. For a while we considered various rules
that would make all primitive operators directly visible; some of the
rules considered created the undesirable "Ripple" effects; others
created annoying incompatibilities; all were quite tricky to implement
correctly and efficiently.
8.6: How to write an Ada program to compute when one has had too much alcohol
to legally drive?
Someone asked if there is an Ada archive of this sort of program. Each
drink has a number of units of alcohol, max legal level, etc.
(from Bob Kitzberger :-)
Oh, this is much to vague. Don't touch that whizzy development
environment until you fully analyze the problem domain (unless that
whizzy development environment includes Rose, in which case, you get
to avoid paper and pencil from the git-go).
Let's see, we have several classes to describe before we get to the
implementation:
Person
subclass Drinker
attributes: weight, age, timeline for amount consumed
Drink
attributes: percentage of alcohol, quantity of drink
Country
attributes: legal age to drink; max legal level of alcohol in
blood
Turn on the stereo, perhaps the Brandenburg Concertos. Then, flesh out
the domain classes. Then, have a Belgian beer and consider what to do
next. You decide on implementing these classes in a simple way,
leading to your first successful prototype. Then, have another beer
and decide what to do next. "Identify risk areas" you mutter to
yourself, and off you go...
If the beer wasn't too strong, you'd probably realize that the only
thing of any difficulty in this is the amount consumed / rate of
decay. Decide on investigating this aspect further. Create
implementation classes for this and include a reference from the
Drinker class to this new timeline/decay Class. Have another beer.
Implement your second prototype. Congratulate yourself for making
progress so quickly.
Have another beer. Wander over to the stereo and change the CD to
something more in the mood, maybe some Hendrix or Stevie Ray Vaughn.
Back in front of the computer; pop another beer. Decide that it would
be very cool if each drink was its own subclass of drink, and start
cataloguing every drink out of your "Pocket Bartender's Guide". Have a
slightly muddled epiphany that you really should create a class for
each kind of alcohol (vodka, tequila, etc.) and the individual drink
classes should each multiply inherit from all relevant Alcohol
classes. Ooh, this is going to be a bit rough, so you have another
beer. Draw a few of the hundreds of new class relationships needed,
put that on the back burner when you think "persistence! that's what's
missing!" Change the CD to Kraftwerk. Start your PPP connection, ask
the people on comp.object for recommendations on a good OODBMS to use
to keep track of all of those persistent objects. Make many many typos
in your posting; everyone ignores it. Fall asleep on the keyboard.
8.7: Does Ada have macros?
No, neither Ada 83 nor Ada 95 do. There was a Steelman requirement
that the language developed NOT have a macro capability. This was a
well thought-out requirement. What you see in a piece of Ada code is
what you get (within a debugger for example). This does not hold true
for macro languages.
General text-substitution macros like those in the C preprocessor are
thought to be too unsafe. For example, a macro can refer to a variable
X and depending where the macro is expanded X may or may not be
visible. Ada programs are supposed to be readable and in many cases C
macros are the main culprits in producing unreadable C programs.
Compile time macro facilities tend to be dreadfully over- and misused,
resulting in horrible maintenance problems. Furthermore, there is a
tendency to use macros to patch up glaring omissions in the language.
For example, C has no named constants, a very bad omission, but
#define is used to patch over this gap.
In C, three "legitimate" uses of macros are for defining compile-time
constants, types, and inline functions. Ada has all three of these
facilities, without macros.
If one wants macros to handle conditional compilation, the better way
to achieve the equivalent is in most instances to isolate the system
dependent parts and then put them in separate units with multiple
system-specific implementations.
|
Закладки на сайте Проследить за страницей |
Created 1996-2024 by Maxim Chirkov Добавить, Поддержать, Вебмастеру |