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Re: sysinst split project - The Configuration File

At Wed, 16 Dec 2009 14:51:02 +0000, David Holland 
<> wrote:
Subject: Re: sysinst split project - The Configuration File
> Oh? The Towers of Hanoi thing was just a joke?

Indeed I believe it was.  :-)

> The basic reason configuration, which shouhld be data, should not b
> expressed in a Turing-complete way is that doing so makes it nearly
> impossible to write correctly-functioning configuration management
> tools.

Hold on a moment here.

The part of a programming language that is used to declare and define
values is not necessarily Turing Complete.  The whole language is, but
you don't necessarily use the whole language to declare and define data
structures and their values.

True, it could be more tempting in a scripting language to use
computation to adjust and manipulate the values being defined during
their "creation", but even doing that does not necessarily directly
prevent the creation of proper configuration management tools.  In fact
it could aid them as well.

> As in, you can have tools that select among canned configurations, but
> not tools that can interpret and edit an existing configuration.

Who says the configuration management tools won't also be able to
execute the language used to define configurations?

Indeed the obvious simple answer here is to use something like lisp or
scheme where all data and code are defined with the very same syntax.

If I'm not mistaken there's been lots of research, and development, of
configurations and configuration management tools using lisp and scheme.

I don't have enough of a grasp of lua yet to know if it would offer
similar benefits in this kind of context or not though.

> Yes, but that's not the point; the point is that people who want XML
> for some reason can get it via an automatic conversion step.

Sure, if someone defined an XML DTD for Sendmail configurations then I'm
someone could also write a converter that could express Sendmail
configurations in XML.  That alone though doesn't make any of it worth
doing, even if there was someone who wanted an XML representation of
Sendmail configurations.

> Parsers for anything suitable for representing configuration data are
> cheap. For that matter, even parsers for sane programming languages
> are cheap. Parsing C++ is not cheap, but that's a whole different
> problem.

Parsing full XML with DTDs is not cheap, and that is the problem!  :-)

I do agree that problib might be appropriate for some configuration
data, but only because it is now included by fiat in NetBSD (and
Darwin, for what it's worth), but that doesn't really solve the whole
problem for sysinst, and it doesn't even begin to address any of the
issues for potential use of bsdinstaller either.

                                                Greg A. Woods
                                                Planix, Inc.

<>       +1 416 218 0099

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