[Jweb_Guru]

Oh, I didn't mean to say that S-expressions can't be used for configuration. It's that in general you don't want a Turing-complete language to do configuration management, because you want to be able to reason about things like rollbacks, dependencies and diffs.

[zeveb]

Yeah, except that inevitably one does end up wanting some element of Turing-compleness, hence the Jinja templates used in Saltstack & Ansible.

In a S-expression-based configuration language, one would either embed an S-expression-based programming language, or generate the S-expressions with a programming language which can manipulate S-expressions.

I don't think that it's that easy to get away from needing Turing-completeness in general. No reason you can't still support rollbacks, dependencies and diffs anyway.

[firstprimate]

Can you elaborate on this?

[TeMPOraL]

I think this is something along the lines of what a friend once told me: "Compare the grammar of Java and C++. C++ has a very complex definition, whereas Java is brain-dead simple. And that fact enables all the powerful transformations IDE can do."

Also there's this school of thought that attributes most security problems to people accidentally using Turing-complete languages where they meant to use something less powerful. Consider vulnerability to arbitrary code execution through user input injection, which could be interpreted by your program being a "parser" (so-called "shotgun parser" - it's implicitly distributed throughout your code base) for a Turing-complete superset of what was supposed to be a list of accepted inputs. There are pretty good talks about this line of thought and I personally find it pretty interesting.

But none of this affects the fact that in complicated enough programs, you need "configuration" to be more code than data, which leads people not knowing of Lisp to reinvent a subset of it in XML or JSON or something similar.

[firstprimate]

Right. That addresses the point about mangling YAML into something that looks like a language. It was this that made be curious: "It's that in general you don't want a Turing-complete language to do configuration management, because you want to be able to reason about things like rollbacks, dependencies and diffs."