Re: Practicalities of Ada for app development

[Georg Bauhaus <rm.dash-bauhaus@futureapps.de>]

On 14.06.12 09:35, Ludovic Brenta wrote:

> As Maciej rightly pointed out, regular expressions must be compiled
> at run time (GNAT.Regexp.Compile).

Before putting much emphasis on having to compile RE patterns
at run-time, I suggest considering what the cost actually is,
in use cases of substantial relevance.

Some use cases for pattern matching:

1) [static, special] Highest possible efficiency of matching
methods is desired in terms of both time and space. For example,
for some FPGA, or for a program that is started and run many many
times per second in limited environments.

I bet there is money for funding this kind of approach in its
market. Looks like it has been done.
Not sure that this approach should be the goal of an Ada package
for general use.

2) [source text] A convenient and effective method of
specifying regular languages in source is desired, to meet
the needs of ubiquitous applications.
For example, finding sequences of digits in date strings
to be split apart (strings from data bases, user input, documents);
matching parts of URLs REs such as file name patterns,
or hashed session identifiers; finding good bits in white noise.

3) [external] Regular expressions are external to
the program. As is the case for configuration files,
when using shells, when "programming" mail filters,
etc.

So, we have static, (1) specialized REs, 82) REs in source text
(either static or dynamic), and (3) REs external to the program.

In case 3, the program *must* be capable of compiling,
or otherwise processing, the external RE.

In case 2, the fraction of time spent on compiling
plain REs is very likely a small fraction of the programs'
running times, especially of server programs' running times.

In case 1, the cost of the alternatives mentioned
do not seem to be negligible at all. That is, the cost of
maintaining a bulky extra parser for some regular language,
or the cost of integrating a source generator that produces
automata.


It is worth mentioning that the source code "generator"
built into C++'s template system can achieve some of the
desired effects for static REs.